Article

The intensity of sexual selection predicts weapon size in male bovids

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

As a classical example of a sexually selected trait, the horns of male bovids offer a prime opportunity to identify predictors of the intensity of sexual selection. Here I use the comparative method to quantify sexual and natural selection pressures behind interspecific variation in horn length. I show that male horn length depends on factors proposed to affect the mean mate number per mating male, correlating positively with group size and negatively with male territoriality. This suggests that whereas group size increases the opportunity for sexual selection, territoriality reduces it because territorial males are unable to follow and monopolize female groups as effectively as males in nonterritorial species. Sexual body size dimorphism also correlates positively with group size and negatively with territoriality, corroborating these factors as predictors of the intensity of sexual selection on males. Female horn length was unaffected by the factors related to mating system, suggesting that this trait is mainly under natural selection. Using female horn length as a proxy for forces of natural selection revealed a negative effect on male horn length. Thus where natural selection favors female horns, possibly as effective weapons against predators, a similar selection pressure on males might prevent them from evolving too elaborate horns through sexual selection. There was no correlation found between horn length and latitude, thus providing no support for the hypothesis that horns have a thermoregulatory function.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

... For example, horns account for up to 15% of body mass in male bighorn sheep (Ovis canadensis) [41], and moose (Alces alces) increase their energy requirements up to 20% while developing their antlers [42]. Furthermore, the intensity of intrasexual selection predicts weapon size across both bovids [43] and cervids [44]. Precopulatory competitive traits such as these cranial protrusions and body size play a dual role in ungulate contests, being used to convey individual quality in displays and as competitive tools in direct physical confrontations (reviewed in [45]). ...
... Given the allometric relationship between weapon and body size, the relative role of these traits in securing competitive success has been difficult to separate, although rare cases of individuals suffering a break in their weapon but still retaining their relative success in competitive bouts highlight the importance of body size in successful contest outcomes [46]. Consequently, many bovids and cervids show male-biased sexual size dimorphism [47,48], and female defence (which facilitates male attempts to monopolize access to females) is present in about half of all species examined [43,49]. ...
... Bovids and cervids also exhibit varying levels of sperm competition. In territorial species, for example, males cannot prevent females from moving between territories and remating [43,50]. Even in species with female-defence polygyny (e.g., red deer, Cervus elaphus), females may change harems and copulate with several males during a reproductive cycle [40]. ...
Article
Full-text available
In polyandrous species, males face reproductive competition both before and after mating. Sexual selection thus shapes the evolution of both pre- and postcopulatory traits, creating competing demands on resource allocation to different reproductive episodes. Traits subject to strong selection exhibit accelerated rates of phenotypic divergence, and examining evolutionary rates may inform us about the relative importance and potential fitness consequences of investing in traits under either pre- or postcopulatory sexual selection. Here, we used a comparative approach to assess evolutionary rates of key competitive traits in two artiodactyl families, bovids (family Bovidae) and cervids (family Cervidae), where male–male competition can occur before and after mating. We quantified and compared evolutionary rates of male weaponry (horns and antlers), body size/mass, testes mass, and sperm morphometrics. We found that weapons evolve faster than sperm dimensions. In contrast, testes and body mass evolve at similar rates. These results suggest strong, but differential, selection on both pre- and postcopulatory traits in bovids and cervids. Furthermore, we documented distinct evolutionary rates among different sperm components, with sperm head and midpiece evolving faster than the flagellum. Finally, we demonstrate that, despite considerable differences in weapon development between bovids and cervids, the overall evolutionary patterns between these families were broadly consistent.
... The shape and presence of horns are sexually dimorphic for some species and populations of bovids; shape differences between males and females are most predictable at the smallest and largest body sizes and considerably more variable at middle size ranges (Geist, 1971;Jarman, 1983;Packer, 1983;Rice, 1984). There is a strong correlation between the shape of horns and the animal's fighting style (Caro et al., 2003;Lundrigan, 1996) and between horn size and the intensity of sexual selection within a species (Bro-Jørgensen, 2007). Horn shape is also important because animals assess size of horns visually when deciding to engage in intrasexual competition, and females appear capable of recognizing conspecifics based on horn shape (Bubenik, 1990;Coulon et al., 2007). ...
... Recent investigations of horn shape evolution used horn length or qualitative binary variables (e.g., smooth vs. crenulated horns, straight vs. twisted horns) rather than geometric shape to test for correlations between aspects of shape and ecology, habitat, behavior, and life history (Bro-Jørgensen, 2007;Calamari, 2016;Caro et al., 2003). Other attempts to quantify horn shape have relied on complex classification systems (Mloszewski, 1981), or cross-sectional areas, single curves, or silhouettes (Kitchener, 1985;Lundrigan, 1996). ...
Article
The bony cranial structures of even-toed hoofed mammals are important for understanding ecology and behavior of ruminants. Horns, the cranial appendages of the family Bovidae, are covered in a layer of keratin that is often not preserved in the fossil record; however, this keratin sheath is intimately involved in the processes that influence horn shape evolution. To understand the relationship between these two components of horns, we quantified both core and sheath shape for four extant species using three-dimensional geometric morphometric analyses in separate, core- and sheath-specific morphospaces as well as a combined morphospace. We assessed correlations between the horn and sheath morphospaces using two-block partial least squares regression, a Mantel test of pairwise distances between species, and Procrustes ANOVA. We measured disparity in the combined morphospace as Procrustes distances between mean shapes of cores and sheaths within and between species and as Procrustes variance. We also tested whether core and sheath shapes could be discriminated by taxon with a canonical variate analysis. Results show that horn core and sheath morphospaces are strongly correlated. The differences in shape between a species' core and sheath were statistically significant, but not as great as those between the cores and sheaths of different species when close relatives were not considered, and core and sheath Procrustes variances are not significantly different within species. Cores and sheath shapes were highly identifiable and were assigned to the correct clade 93% of the time in the canonical variate analysis. Based on these tests, horn cores are distinguishable in geometric morphometric analyses, extending the possibility of using geometric morphometrics to study the ecology and evolution of bovid horns to the fossil record.
... The evolution of offensive weaponry, such as horns, tusks and tail clubs, has often been attributed to sexual selection (McCullough et al., 2014;Arbour & Zanno, 2018), with strong dimorphism often being present between the sexes (Bro-Jørgensen, 2007;Emlen, 2008;Smith & Fisher, 2013;McCullough et al., 2015). In contrast, sexual dimorphism in defensive body armour is a less understood phenomenon, frequently overlooked because of the prevailing idea that these traits are shaped by predator-prey interactions, as pointed out by Broeckhoven et al. (2017a). ...
... While sexual differences in offensive weaponry are well established (Bro-Jørgensen, 2007;Emlen, 2008;McCullough et al., 2015), the phenomenon of sexual dimorphism in defensive body armour has frequently been overlooked. To investigate this, we tested for sexual differences in osteoderm expression in four species of cordyline lizards that can be further subdivided into two phenotypes: an intermediate phenotype characterized by isolated osteoderms and an Fig. 2A; Tables 1, 2). ...
Article
While sexual dimorphism in offensive weaponry is a well-established phenomenon, few studies have looked at sexual differences in defensive body armour. In this study, we investigate sexual dimorphism in the expression of osteoderms-bony elements embedded in the skin-in four species of cordyline lizards by using high-resolution micro-computed tomography. Our results unambiguously show that only in the species with an intermediate phenotype characterized by isolated osteoderms without any clear ecological function do males have a higher degree of osteoderm expression than females. By contrast, no significant sexual differences are present in the species with an armoured phenotype consisting of a continuous layer of imbricating osteoderms. These findings suggest that sexual selection acting on dermal armour might be ubiquitous, but that in the armoured phenotype, its effect is weak because of a stronger selection pressure towards improved defence or physiological function. ADDITIONAL KEYWORDS: body armour-defensive morphology-micro-computed tomography-osteoderm-sexual selection.
... Weaponry is a common adaptation fundamental to understanding the evolutionary ecology of extant and extinct species [1,2]. Weapons may evolve via sexual selection, when males engage in intraspecific combat over reproductive resources [2,3], or through natural selection, as has been suggested for the horns of female bovids, which are used both as antipredator defences and in intraspecific combat for food resources [2,4]. They can also be co-opted from locomotor or feeding traits (e.g. ...
... The great phenotypic diversity of weapons in living species is known to be influenced by multiple factors including mating system, intensity of sexual selection, fighting style, body size and mechanical constraints [4,[7][8][9][10][11][12]. However, rigorous studies attempting to define common ecological or anatomical correlates of weaponry within a broad phylogenetic framework are lacking and no research to our knowledge has attempted to study weaponry broadly in extinct vertebrates. ...
Article
Full-text available
Weaponry, for the purpose of intraspecific combat or predator defence, is one of the most widespread animal adaptations, yet the selective pressures and constraints governing its phenotypic diversity and skeletal regionalization are not well understood. Here, we investigate the evolution of tail weaponry in amniotes, a rare form of weaponry that nonetheless evolved independently among a broad spectrum of life including mammals, turtles and dinosaurs. Using phylogenetic comparative methods, we test for links between morphology, ecology and behaviour in extant amniotes known to use the tail as a weapon, and in extinct taxa bearing osseous tail armaments. We find robust ecological and morphological correlates of both tail lashing behaviour and bony tail weaponry, including large body size, body armour and herbivory, suggesting these life-history parameters factor into the evolution of antipredator behaviours and tail armaments. We suggest that the evolution of tail weaponry is rare because large, armoured herbivores are uncommon in extant terrestrial faunas, as they have been throughout evolutionary history.
... En los rumiantes silvestres los cuernos se utilizan como armas contra los predadores o en combates entre machos, quienes desde antes de la temporada reproductiva compiten por el acceso a los grupos de hembras (Preston et al., 2003;Bro-Jørgensen, 2007). Lo mismo se aplica a los animales domésticos, aunque al estar bajo condiciones de domesticación y ambiente controlado, particularmente en los sistemas intensivos de producción, algunas consecuencias difieren de lo que se observa en los rumiantes silvestres. ...
... Además de las funciones de combate, protección y sociales, los cuernos se usan para el autoacicalamiento de áreas del cuerpo que de otra forma resultan inaccesibles al animal (Taschke, 1995). Al mismo tiempo los cuernos pueden ser utilizados como herramientas táctiles y parecen tener funciones importantes en la termorregulación (Kiltie, 1985;Bro-Jørgensen, 2007). ...
Book
Full-text available
Este libro nace del interés de los autores por ofrecer una herramienta práctica para la toma de decisiones en el manejo de animales, particularmente respecto al ejercicio de determinadas prácticas que son dolorosas. En este trabajo se incluye información científicamente fundamentada para ayudar a determinar la necesidad de llevarlas a cabo o la posibilidad de aplicar otras opciones. El lector encontrará información sintetizada para la correcta elección de las técnicas; conocerá cuándo y cómo llevarlas a cabo y estará en condiciones de comparar las diferentes opciones a través de datos conductuales y fisiológicos. Indudablemente esta obra puede contribuir a fomentar, tanto en técnicos como en productores, una zootecnia moderna, tendiente a la erradicación del dolor innecesario en los animales y a favor de su bienestar.
... The evolution of offensive weaponry, such as horns, tusks and tail clubs, has often been attributed to sexual selection (McCullough et al., 2014;Arbour & Zanno, 2018), with strong dimorphism often being present between the sexes (Bro-Jørgensen, 2007;Emlen, 2008;Smith & Fisher, 2013;McCullough et al., 2015). In contrast, sexual dimorphism in defensive body armour is a less understood phenomenon, frequently overlooked because of the prevailing idea that these traits are shaped by predator-prey interactions, as pointed out by Broeckhoven et al. (2017a). ...
... While sexual differences in offensive weaponry are well established (Bro-Jørgensen, 2007;Emlen, 2008;McCullough et al., 2015), the phenomenon of sexual dimorphism in defensive body armour has frequently been overlooked. To investigate this, we tested for sexual differences in osteoderm expression in four species of cordyline lizards that can be further subdivided into two phenotypes: an intermediate phenotype characterized by isolated osteoderms and an armoured phenotype characterized by a continuous layer of adjoining or imbricating osteoderms. ...
Article
While sexual dimorphism in offensive weaponry is a well-established phenomenon, few studies have looked at sexual differences in defensive body armour. In this study, we investigate sexual dimorphism in the expression of osteoderms-bony elements embedded in the skin-in four species of cordyline lizards by using high-resolution micro-computed tomography. Our results unambiguously show that only in the species with an intermediate phenotype characterized by isolated osteoderms without any clear ecological function do males have a higher degree of osteoderm expression than females. By contrast, no significant sexual differences are present in the species with an armoured phenotype consisting of a continuous layer of imbricating osteoderms. These findings suggest that sexual selection acting on dermal armour might be ubiquitous, but that in the armoured phenotype, its effect is weak because of a stronger selection pressure towards improved defence or physiological function.
... SSD is particularly common in ungulates with males being at least 10% larger in two-thirds of species (unpublished data [9]). Weapon sizes are also generally more pronounced in males than females, especially among polygynous species [10]. These findings provide further support for the fitness benefits of being larger and better equipped to win contests and outcompete other males [11]. ...
... This is further supported by the fact that the post-canine teethpredominantly used for feeding-did not differ in size between the sexes [30]. Selection for weapons over body size is comparatively rare in male ungulates, which generally exhibit a positive allometric relationship between these characteristics [10,36]. In bovids, the rate of growth in weapons was actually found to decrease relative to body size, as horn size was either constrained or no longer the target of sexual selection [36]. ...
Article
Full-text available
Sexual size dimorphism (SSD) is a common morphological trait in ungulates, with polygyny considered the leading driver of larger male body mass and weapon size. However, not all polygynous species exhibit SSD, while molecular evidence has revealed a more complex relationship between paternity and mating system than originally predicted. SSD is, therefore, likely to be shaped by a range of social, ecological and physiological factors. We present the first definitive analysis of SSD in the common hippopotamus ( Hippopotamus amphibius ) using a unique morphological dataset collected from 2994 aged individuals. The results confirm that hippos exhibit SSD, but the mean body mass differed by only 5% between the sexes, which is rather limited compared with many other polygynous ungulates. However, jaw and canine mass are significantly greater in males than females (44% and 81% heavier, respectively), highlighting the considerable selection pressure for acquiring larger weapons. A predominantly aquatic lifestyle coupled with the physiological limitations of their foregut fermenting morphology likely restricts body size differences between the sexes. Indeed, hippos appear to be a rare example among ungulates whereby sexual selection favours increased weapon size over body mass, underlining the important role that species-specific ecology and physiology have in shaping SSD.
... The predicted association between dimorphism and condition dependence for morphological traits has received great attention in vertebrate and invertebrate species with very conspicuous (sex-specific) armaments or ornaments (Cotton et al. 2004b;Tomkins et al. 2010). Such work was historically grounded in early comparative studies of solitary and social primates, ungulates, and birds attempting to understand the evolution of sexual dimorphism by their underlying developmental processes (Jarman 1983;Leigh 1992;Teather and Weatherhead 1994;Blanckenhorn et al. 2007;Bro-Jørgensen 2007). This was also of interest to developmental biologists and geneticists because sex-specific condition dependence represents a form of sex-limited epistasis, which could also resolve between-sex genetic correlations that would otherwise hamper the establishment of sexual dimorphisms (Bonduriansky 2007a(Bonduriansky , 2007b. ...
... Whereas the evidence on the relationship between condition dependence and trait dimorphism presented here and elsewhere (Wilkinson and Taper 1999;Cotton et al. 2004aCotton et al. , 2004bBonduriansky 2007aBonduriansky , 2007bBonduriansky , 2007c refers mostly to insects, we suspect it to be a general pattern in organisms where sexually antagonistic directional sexual selection drives dimorphism. After all, the underlying proximate causes of sexual size dimorphism in terms of growth and developmental mechanisms, both of which are strongly dependent on environmental conditions, are well established in many other vertebrate and invertebrate taxa (Jarman 1983;Leigh 1992;Teather and Weatherhead 1994;Blanckenhorn et al. 2007;Bro-Jørgensen 2007). Nevertheless, the causes and consequences of this phenomenon warrant further mechanistic scrutiny, particularly at the physiological and genetic levels (Tang et al. 2011;Emlen et al. 2012;Rohner et al. 2017;Shingleton and Frankino 2018). ...
Article
Sexual selection can displace traits acting as ornaments or armaments from their viability optimum in one sex, ultimately giving rise to sexual dimorphism. The degree of dimorphism should not only mirror the strength of sexual selection but also the net viability costs of trait maintenance at equilibrium. As the ability of organisms to bear exaggerated traits will depend on their condition, more sexually dimorphic traits should also exhibit greater sex differences in condition dependence. While this has been demonstrated among traits within species, similar patterns are expected across the phylogeny. We investigated this prediction within and across 11 (sub)species of sepsid flies with varying mating systems. When estimating condition dependence for seven sexual and nonsexual traits that vary in their sexual dimorphism, we not only found a positive relationship between the sex difference in allometric slopes (our measure of condition dependence) and relative trait exaggeration within species but also across species for those traits expected to be under sexual selection. Species with more pronounced male aggression further had relatively larger and more condition-dependent male fore- and midlegs. Our comparative study suggests a common genetic/developmental basis of sexual dimorphism and sex-specific plasticity that evolves across the phylogeny—and that the evolution of size consistently alters scaling relationships and thus contributes to the allometric variation of sexual armaments or ornaments in animals.
... Thresholds reported in bold are statistically different from the maximum body mass in the dataset. The sex-and population-specific maximum adult body mass (BM max ) have been obtained from the original study except in M. kirkii (collected from Bro-Jørgensen, 2007). From BM max and T BM , we calculated the proportional threshold mass (Tp) as T BM /BM max . ...
... Many factors affecting the evolution of horn size and shape in male and female bovids have been identified (Bro-Jørgensen, 2007;Caro, Graham, Stoner, & Flores, 2003;Geist, 1966;Lundrigan, 1996;Packer, 1983;Stankowich & Caro, 2009). In several instances, these factors covary with body size. ...
Article
Full-text available
Allometric relationships describe the proportional covariation between morphological, physiological, or life‐history traits and the size of the organisms. Evolutionary allometries estimated among species are expected to result from species differences in ontogenetic allometry, but it remains uncertain whether ontogenetic allometric parameters and particularly the ontogenetic slope can evolve. In bovids, the nonlinear evolutionary allometry between horn length and body mass in males suggests systematic changes in ontogenetic allometry with increasing species body mass. To test this hypothesis, we estimated ontogenetic allometry between horn length and body mass in males and females of 19 bovid species ranging from ca. 5 to 700 kg. Ontogenetic allometry changed systematically with species body mass from steep ontogenetic allometries over a short period of horn growth in small species to shallow allometry with the growth period of horns matching the period of body mass increase in the largest species. Intermediate species displayed steep allometry over long period of horn growth. Females tended to display shallower ontogenetic allometry with longer horn growth compared to males, but these differences were weak and highly variable. These findings show that ontogenetic allometric slope evolved across species possibly as a response to size‐related changes in the selection pressures acting on horn length and body mass.
... Horns and antlers of ungulates are among the most extravagant ornamentations seen in nature, and their large variation in form, size and function has intrigued natural historians for centuries (Gould 1992). Today, the evolution of horns and antlers in male ungulates is attributed to sexual selection (Bro-Jørgensen 2007;Clutton-Brock 1982;Geist 1966). In polygynous species, male reproductive success is limited by access to mates (Clutton-Brock et al. 1988). ...
... In polygynous species, male reproductive success is limited by access to mates (Clutton-Brock et al. 1988). Antlers are honest signals of body size, and potentially fighting ability, and are decisive for the outcome of male-male combats determining dominance rank and access to mates (Bro-Jørgensen 2007;Clutton-Brock et al. 1980, 1982. As expected for an honest signal of competitive ability, the production of antlers is costly and may account for as much as 1/3 of summer energy intake (Moen et al. 1999). ...
Article
Full-text available
The costs of reproduction are important in shaping individual life histories, and hence population dynamics, but the mechanistic pathways of such costs are often unknown. Female reindeer have evolved antlers possibly due to interference competition on winter-feeding grounds. Here, we investigate if variation in antler size explains part of the cost of reproduction in late winter mass of female reindeer. We captured 440 individual Svalbard reindeer a total of 1426 times over 16 years and measured antler size and body mass in late winter, while presence of a ‘calf-at-heel’ was observed in summer. We found that reproductive females grew smaller antlers and weighed 4.3 kg less than non-reproductive females. Path analyses revealed that 14% of this cost of reproduction in body mass was caused by the reduced antler size. Our study is therefore consistent with the hypothesis that antlers in female Rangifer have evolved due to interference competition and provides evidence for antler growth as a cost of reproduction in females. Antler growth was constrained more by life history events than by variation in the environment, which contrasts markedly with studies on male antlers and horns, and hence increases our understanding of constraints on ornamentation and life history trade-offs.
... It appears that the functional signal in female neck vertebrae is not as strong as in males, which is not surprising given that female ruminants do not engage in the prolonged or ritualized combat behaviours shaped by sexual selection in their male counterparts [56,57] and therefore probably experience less extreme forces in the horns, skull and neck. Sexual dimorphism in body size and weapon size and shape is linked to mating strategy and reproductive behaviour throughout the ruminant clade [58][59][60]. We can now suggest that dimorphism related to intraspecific competition is also present in the cervical spine. ...
Article
Cranial weapons of all shapes and sizes are common throughout the animal kingdom and are frequently accompanied by the evolution of additional traits that enhance the use of those weapons. Bovids (cattle, sheep, goats, antelope) and cervids (deer) within the mammal clade Ruminantia are particularly well known for their distinct and varied cranial appendages in the form of horns and antlers, which are used as weapons in intraspecific combat between males for access to mates. Combat in these species takes many forms, including head-on collisions (ramming); stabbing an opponent's head or body with horn tips (stabbing); rearing and clashing downwards with horns (fencing); or interlocking antlers or horns while vigorously pushing and twisting (wrestling). Some aspects of weapon and skull morphology have been linked to combat behaviours in bovid and cervid species, but the contribution of postcranial structures that support these weapons, such as the neck, has not been explored. To investigate the role of the neck in intraspecific combat, we quantified biomechanically relevant linear variables of the cervical vertebrae (C1-C7) from males and females of 55 ruminant species. We then used phylogenetic generalized least-squares regression to assess differences among species that display primarily ramming, stabbing, fencing and wrestling combat styles. In males, we found that wrestlers have longer vertebral centra and longer neural spines than rammers, stabbers or fencers, while rammers have shorter and wider centra and taller neural spine lever arms. These results suggest a supportive role for the cervical vertebrae in resisting forces generated by male-male combat in ruminant mammals and indicate that evolutionary forces influencing cranial weapons also play a role in shaping the supporting anatomical structures.
... Consequently, temporal changes in horn size have attracted the attention of evolutionary biologists and wildlife managers over the past decades. Long-term variations in male weapon size may entail evolutionary responses to sexual or natural selection (Geist, 1996;Bro-Jørgensen, 2007) as well as to artificial selection through hunting (McDougall et al., 2006;Festa-Bianchet and Mysterud, 2018). * Corresponding author. ...
Article
Full-text available
The development of horns in Caprinae can be largely influenced by food-limiting factors such as population density and climate, as well as by negative evolutionary responses to size-selective harvesting. In this study, we investigated the effects of population density, environmental covariates and trophy hunting on horn development in a population of European mouflon Ovis aries musimon introduced to the Mediterranean region of Croatia in the early 1980s. The study population was subject to commercial trophy hunting on males since the mid-1980s. This allowed to analyse the temporal trend in early horn growth in 341 rams legally culled. Cohort-based linear model and analysis of deviance (ANODEV) revealed a significant negative trend in early horn growth, with a decline of ca. 10% over only 15 cohorts (1993-2007). The increase in population density (by ca. 600%) and summer temperature, selected via LASSO regression, explained about 61% and 13% of the decline in early horn growth, as revealed by the ANODEV R2. Our results suggest a prominent role of food-limiting factors in the decline of weapon size. We also found a negative relationship between age at death and early horn growth in our study population, which suggests the occurrence of hunters’ selectivity towards large horns. The effect of trophy hunting on the decline in horn size, however, is difficult to quantify. Our analysis was largely influenced by the rapid increase of mouflon density after introduction, thereby limiting the possibility to detect potential effects of hunting selection, although the large number of rams shot before 5 years of age may possibly lead to undesirable consequences of trophy hunting on mating success. To clarify the consequences of commercial trophy hunting on the mouflon population, the long-term pattern of horn growth and the age-dependent male siring success should be further investigated.
... Group size, however, can influence the expression of secondary sexual characteristics in some species (e.g. horn length in bovids is positively correlated with breeding group size; Bro-jorgensen, 2007). If intrasexual selection is the driving force of colouration in yellow toads and if group size influences colour, we predicted that subjects held with three rival males would develop brighter colouration than subjects held with just one rival male. ...
Article
Dynamic sexual dichromatism occurs when males and females differ in colouration for a limited time. Although this trait has been primarily studied in cephalopods, chameleons, and fishes, recent analyses suggest that dynamic dichromatism is prevalent among anurans and may be mediated through sexual selection and sex recognition. Yellow toads, Incilius luetkenii, exhibit dynamic dichromatism during explosive breeding events at the onset of the rainy season: males change from a cryptic brown to a bright yellow and back again during the brief mating event. We tested the hypothesis that dynamic dichromatism in yellow toads is influenced by conspecific interactions and mediated through sex hormones and stress hormones. We placed male toads into one of four social treatments (with three other males, one male, one female, or no other toads). Immediately before and after each one-hour treatment, we quantified male colour with a reflectance spectrometer and we collected a blood sample to assess plasma concentrations of both testosterone and corticosterone. We found that males held with conspecific animals showed the brightest yellow colour and showed little or no change in their corticosterone levels. Across treatments, toads with duller yellow colour had higher levels of corticosterone. Male colour showed no association with testosterone. Interestingly, males showed substantial temporal variation in colour and corticosterone: toads were duller yellow and exhibited greater levels of corticosterone post-treatment across subsequent days at the onset of the rainy season. Our findings reveal that both conspecific interactions and corticosterone are involved in the dynamic colour change of yellow toads.
... As is typical for dung beetles (Emlen et al. 2005), there is considerable variation between horned species in the degree of investment in horns, and this variability in relative horn size is likely to be associated with differences in the intensity of male-male competition between species (Simmons & 220 Tomkins 1996;Bro-Jørgensen 2007). We therefore assessed the relationship between relative horn size and abundance of horned species. ...
Article
Full-text available
The effect of sexual selection on species persistence is unclear. The cost of bearing ornaments or armaments might increase extinction risk, but sexual selection can also enhance the spread of beneficial alleles and increase the removal of deleterious alleles, potentially reducing extinction risk. 35 Here we investigate the effect of sexual selection on species persistence in a community of thirty-four species of dung beetles across a gradient of environmental disturbance ranging from old-growth forest to oil palm plantation. Horns are sexually selected traits used in contests between males, and we find that both horn presence and relative size are strongly positively associated with species persistence and abundance in altered habitats. Testes mass, an indicator of post-copulatory 40 selection, is, however, negatively linked with the abundance of species within the most disturbed habitats. This study represents the first evidence from a field system of a population-level benefit from pre-copulatory sexual selection.
... Regardless of its initial evolutionary drive, a structure being used in conspecific fights is expected to be effectively deployed in defence against predators, as well (Fig. 1C,D) (Bro-Jørgensen, 2007;Emlen, 2008;Stankowich, 2012). Furthermore, the late ontogenetic appearance of skeletal and integumentary defensive/offensive structures, including weapons, does not exclude their importance in defence and/or agonistic behaviour. ...
Article
Ankylosaurian fossils are usually standard elements of Cretaceous continental vertebrate localities; however, bone-yielding horizons including more than one individual are extremely rare. Here, we present a unique assemblage of 12 partial, articulated or associated ankylosaurian skeletons and thousands of isolated bones and teeth discovered from the Santonian Iharkút vertebrate locality, western Hungary. Collected from an area of 600 m2 and from a single bone bed, this material is one of the richest ankylosaurian accumulation worldwide. The 12 skeletons are not monospecific but mostly based on the pelvic armour composition: six of them are from Hungarosaurus, two are referred to Struthiosaurus and four can be assigned to Nodosauridae indet. Sedimentological and taphonomical examinations revealed a single mass mortality event as the cause of the death and accumulation of these quadruped animals that are described here. The ankylosaur assemblage from Iharkút suggests at least a temporarily gregarious behaviour of these animals and also shows that Hungarosaurus and Struthiosaurus might live in the same moist habitat or at least preferred relatively close environments.
... The species is not territorial [26] and has among the largest group sizes of the Tragelaphus genus [27]. These two features promote competition for mates and have been linked with the intensity of sexual selection [28]. Thus, the biology of the eastern bongo and its conservation status make it an excellent model to test these novel hypotheses with basic (ecological) and applied (conservation) implications. ...
Article
Full-text available
Parent sex ratio allocation has consequences for individual fitness, population dynamics, and conservation. Theory predicts that parents should adjust offspring sex ratio when the fitness returns of producing male or female offspring varies. Previous studies have assumed that only mothers are capable of biasing offspring sex ratios, but have neglected fathers, given the expectation of an equal proportion of X-and Y-chromosome-bearing (CBS) sperm in ejaculates due to sex chromosome segregation at meiosis. This assumption has been recently refuted and both paternal fertility and paternal genetic quality have been shown to bias sex ratios. Here, we simultaneously test the relative contribution of paternal, maternal, and individual genetic quality, as measured by inbreeding, on the probability of being born a son or a daughter, using pedigree and lifelong offspring sex ratio data for the eastern bongo (Tragelaphus eurycerus isaaci). Our models showed first, that surprisingly, as individual inbreeding decreases the probability of being born male increases, second, that paternal genetic effects on sex ratio were stronger than maternal genetic effects (which were absent). Furthermore, paternal effects were opposite in sign to those predicted; father inbreeding increases the probability of having sons. Previous paternal effects have been interpreted as adaptive due to sex-specific inbreeding depression for reproductive traits. We argue that in the eastern bongo, the opposite sign of the paternal effect on sex ratios results from a reversed sex-specific inbreeding depression pattern (present for female but not male reproductive traits). We anticipate that this research will help stimulate research on evolutionary constraints to sex ratios. Finally, the results open a new avenue of research to predict sex ratio allocation in an applied conservation context. Future models of sex ratio allocation should also include the predicted inbreeding level of the offspring and paternal inbreeding levels.
... Moreover, the occurrence of multiple mating systems is common (around 11% of the species) and phenotypic sex differences were observed in 50% of the species, suggesting that social systems in cooperative species may be more complex than initially assumed. Different studies have demonstrated associations between mating system and sex dimorphism, suggesting that individuals exhibit greater variation in reproductive success under specific mating systems which may result in the evolution of sexually selected traits (Clutton-Brock et al. 1977, Kleiman 1977, Owens & Hartley 1998, Bro-Jorgensen 2007. Traditionally, the extent of sex differences exhibited by species was expected to be a clue to the strength of sexual selection (but see, Soulsbury et al. 2014). ...
Article
Sexual selection is generally thought to be weak in cooperative breeding species, largely because polygamous mating patterns that drive sexual selection can erode the kin-selected benefits of cooperation. Social selection, on the other hand, is expected to be strong among cooperative species especially because of the intense competition over status and resource access. In support of this view, several studies have shown monogamous mating and little sex difference in cooperative species. However, most previous studies have focused on species with relatively simple social systems and few studies have examined how mating patterns, social organization and ecological attributes have influenced the evolution of ornamentation in cooperative species. Here I used secondary data to examine several hypotheses and shed some light on how social and sexual selection influenced the evolution of phenotypic sex traits in cooperatively breeding birds. Despite the broad assumption that cooperative breeding species are monomorphic, results demonstrate that sex differences and the presence of ornamentation are widely spread in the group. Stable environments with higher precipitation are associated to the strongest differences between sexes. Results indicate that although extrapair matings and environment attributes are determinant to the evolution of sex differences, males and females of cooperative species seem to be more alike than their non-cooperative counterparts. The extent of mutual ornamentation found in cooperative species indicates that the combination of both sexual and social selection are imperative to determine how evolution has shaped phenotypic attributes in cooperative species.
... Regardless of the potential different origins of the large and spiral horns, this phenotype is favored by the local Tibetan people because of its important religious and cultural roles (e.g., totem and trophy) throughout history (Fan 1131; Huang 55 1989; He 2000). Additionally, the large horn size is an advan- tageous phenotype for sheep to obtain more food resources through intraspecific or interspecific competitions in general and for rams to outcompete for more courtships in particular ( Preston et al. 2003;Bro-Jorgensen 2007 fig. 6A) known to be important components of the HIF-1 (hypoxia-induced factors) pathway (Frede and Fandrey 2013), indicating potential roles for these genes in high- altitude hypoxia adaptation in Tibetan sheep. ...
Article
Full-text available
Tibetan sheep are the most common and widespread domesticated animals on the Qinghai-Tibetan Plateau (QTP), and have played an essential role in the permanent human occupation of this high-altitude region. However, the precise timing, route and process of sheep pastoralism in the QTP region remain poorly established, and little is known about the underlying genomic changes that occurred during the process. Here, we investigate the genomic variation in Tibetan sheep using whole-genome sequences, SNP arrays, mitochondrial DNA and Y-chromosomal variants in 986 samples throughout their distribution range. We detect strong signatures of selection in genes involved in the hypoxia and ultraviolet signaling pathways (e.g., HIF-1 pathway and HBB and MITF genes) and in genes associated with morphological traits such as horn size and shape (e.g., RXFP2). We identify clear signals of argali (Ovis ammon) introgression into sympatric Tibetan sheep, covering 5.23% - 5.79% of their genomes. The introgressed genomic regions are enriched in genes related to oxygen transportation system, sensory perception and morphological phenotypes, in particular the genes HBB and RXFP2 with strong signs of adaptive introgression. The spatial distribution of genomic diversity and demographic reconstruction of the history of Tibetan sheep shows a stepwise pattern of colonization with their initial spread onto the QTP from its northeastern part c. 3,100 years ago, followed by further southwest expansion to the central QTP c. 1,300 years ago. Together with archeological evidence, the date and route reveals the history of human expansions on the QTP by the Tang-Bo Ancient Road during the late-Holocene. Our findings contribute to a depth understanding of early pastoralism and the local adaptation of Tibetan sheep as well as the late-Holocene human occupation of the QTP.
... These structures are used for both prey capture (Smilodon saber teeth) and sexual combat (deer antlers, beaked whale tusks), but among extant species, sexually selected weaponry is most prevalent among the artiodactyls in the form of cranial appendages and elongated tusks. Most studies focus on understanding the ecological, social, and phylogenetic underpinnings of horns (Bovidae) and antlers (Cervidae) (Clutton-Brock et al. 1980;Packer 1983;Estes 1991b;Lundrigan 1996;Caro et al. 2003;Bro-Jørgensen 2007;Stankowich and Caro 2009;Goss 2012;), while the factors promoting the evolution, retention, and elaboration of tusks have received little attention (but see Geist 1971;Raia et al. 2015). Here, we investigate the patterns of tusk evolution in artiodactyls while exploring specific ecological factors that might favor their use over cranial weapons (e.g., antlers, horns). ...
Article
Full-text available
Combat weaponry, including elaborate horns and antlers and complex dentition, evolved independently several times among mammals. While it is evident that tusk and tusk-like dentition have emerged primarily among males for intrasexual combat, it is unclear what ecological factors favor the retention or re-evolution of tusks. We investigated patterns of tusk evolution in artiodactyls while exploring specific ecological factors that might favor their use over other cranial weapons (e.g., antlers, horns). We show that among males, small (<15 kg), solitary species tend to retain well-developed canines, and more solitary species live in more closed habitats. These results suggest that tusks are a better weapon option for smaller, slinking artiodactyls in forested environments with low visibility, whereas larger taxa living in more open environment can bear the cost of elaborate headgear and are better served by communicating across distances an honest signal of fighting ability. Small species in dense habitats may also be more likely to be ambushed by predators and have a need to defend themselves; small, slicing daggers may be a better defensive weapon and allow more maneuverability and faster escape than cumbersome headgear in densely vegetated habitats.
... Regardless, for both horns and antlers, competition for mates is the primary driver of weapon evolution in males. This is evidenced by larger weapon size among ungulate species with bigger breeding group sizes 11,12 , and higher within-population annual reproductive success for males with larger weapons 13,14 . ...
Article
Full-text available
Sexually selected weapons evolved to maximize the individual reproductive success of males in many polygynous breeding species. Many weapons are also retained outside of reproductive periods for secondary reasons, but the importance of these secondary functions is poorly understood. Here we leveraged a unique opportunity from the predator-prey system in northern Yellowstone National Park, WY, USA to evaluate whether predation by a widespread, coursing predator (wolves) has influenced a specific weapon trait (antler retention time) in their primary cervid prey (elk). Male elk face a trade-off: individuals casting antlers early begin regrowth before other males, resulting in relatively larger antlers the following year, and thus greater reproductive success, as indicated by research with red deer. We show, however, that male elk that cast their antlers early are preferentially hunted and killed by wolves, despite early casters being in better nutritional condition than antlered individuals. Our results run counter to classic expectations of coursing predators preferring poorer-conditioned individuals, and in so doing, reveal an important secondary function for an exaggerated sexually selected weapon-predatory deterrence. We suggest this secondary function played a key evolutionary role in elk; uniquely among North American cervids, they retain their antlers long after they fulfil their primary role in reproduction.
... The evolution of defensive morphologies in males, particularly weaponry, is conventionally ascribed to sexual selection, due to the apparent nature of male-male competition for mates (Bro-Jørgensen, 2007;Tobias et al., 2012) and the absence of exaggerated structures in females. If the evolution of weaponry is hypothesized to be driven by sexual selection, why do females of the same species still possess weapons? ...
Chapter
Animals are exposed to many threats and dangers in nature, with predation being among the most important factors. In response, prey have evolved an array of morphological defenses to protect themselves against predators, ranging from offensive weaponry to defensive armor and body size. In this article, I outline the morphological diversity in defensive traits and provide some examples that illustrate the role of these morphological defenses in a predator-prey context. As I discuss the different types, it is important to bear in mind that it is exceedingly difficult to test the functional significance of defensive morphologies. Although an antipredator function is often taken for granted, defensive morphologies might be subjected to other selective pressures (e.g., sexual selection) or might possess other functionalities besides protection (e.g., thermoregulation). Hence, whenever possible I attempted to discriminate between traits that primarily function as defensive tools and those that more likely function for another purpose but clearly have a (secondary) antipredator benefit.
... Domesticated and wild male bovids, which defend a harem of females rather than a given territory, have longer horns than males which do not defend a harem. Bovid horn length is thus associated with mating behavior and, in turn, with sexual selection (Bro-Jørgensen, 2007). This theory is supported by the observation that horns in male bovids are not primarily used for predator defence. ...
... Examples of exaggerated structures are found in many animal lineages (e.g., Gould, 1974) but seem to be particularly common in insects, such as stalk-eyed flies (Vasconcelos et al., 2019), lucanid beetles (Romiti et al., 2015), and several ant lineages (e.g., Blanchard et al., 2020;Boudinot et al., 2021;Sarnat et al., 2017). Although exaggerated traits are most likely to be associated with males and be under sexual selection (e.g., weapons used for fights and/or display; see Emlen, 2008 for further discussion), several cases of such morphologies are present in females (e.g., Matsuura, 2006), as well as not being sexually selected (e.g., Bro-Jørgensen, 2007). Termites (Miura & Maekawa, 2020), ants (Oster & Wilson, 1978), social aphids (Stern & Foster, 1997), and thrips (Crespi et al., 1997) are some of the lineages in which individuals within the colony may possess exaggerated morphological structures that were not driven by sexual selection. ...
Article
The division of labor into sterile and reproductive castes in social insects is often reflected in marked morphological differences, which might have played an important role in the remarkable adaptive success of these organisms. Some ant lineages have undergone further morphological differentiation, with the evolution of differences within the worker caste. In this study, we characterize morphological diversity in the head of Pheidole ants by comparing differences in size and shape among species and between minor and major worker subcastes. To this end, we integrate data from high‐resolution images, geometric morphometrics, and phylogenetic comparative methods. Our results indicated differences in morphological variation of each subcaste with respect to their geographical distribution, with distinct morphological patterns and evolutionary routes related to head shape. Allometry was shown to be a crucial element for the differentiation within and between each subcaste, corroborating the role of size in their morphological evolution. Additionally, we observed that closely related species often diverge considerably in morphospace, whereas convergence in their morphospace occupation characterizes some West and East Hemisphere species. Finally, although multiple shifts in the rate of morphological evolution occurred during the Miocene, the timing and position of these shifts were independent of size and shape, suggesting that their evolution has been decoupled throughout Pheidole evolution. The division of labor into sterile and reproductive castes in social insects is often reflected in marked morphological differences, which might have played an important role in the remarkable adaptive success of these organisms. Some ant lineages have undergone further morphological differentiation, with the evolution of differences within the worker caste. In this study, we characterize morphological diversity in the head of Pheidole ants by comparing differences in size and shape among species and between minor and major worker subcastes. To this end, we integrate data from high‐resolution images, geometric morphometrics, and phylogenetic comparative methods.
... Sexual selection is thought to be the primary cause of sexual dimorphism in mammals (Ralls, 1977), a proposition consistent with hypotheses concerning the role of male-male competition in promoting differences in the body size of sexes among ungulates (Bro-Jørgensen, 2007). Trivers (1972) proposed, however, that parental investment was the fundamental factor driving sexual selection. ...
Article
Full-text available
Ungulates exhibit diverse mating systems that range from monogamous pair territories to highly polygynous leks. We review mating systems and behaviors across ungulates and offer a new approach synthesizing how interacting factors may shape those mating systems. Variability exists in mating systems among and within species of ungulates and likely is affected by predation risk, availability of resources (food and mates), habitat structure, and sociality. Ungulate mating systems may be labile as a consequence of the varying strength of those interacting factors. In addition, degree of polygyny and sexual dimorphism in size are associated with the evolution of mating systems. Neither male–male combat nor paternal care, however, can completely explain differences in sexual size dimorphism for ungulates, a necessary component in understanding the development of some mating systems. Whatever the evolutionary pathway, sexual segregation limits paternal care allowing more intense male–male competition. Selection of habitat structure, because it modifies risk of predation, is a major determinant of sociality for ungulates. Likewise, ruggedness and steepness of terrain limit the types of mating systems that can occur because of limitations in group size and cohesiveness, as well as the ability of males to herd even small groups of females effectively. The quality and defensibility of resources affect mating systems, as does the defensibility of females. Population density of females also may be a critical determinant of the types of mating systems that develop. Size of groups likewise constrains the types of mating tactics that males can employ. Our aim was to use those relationships to create a broad conceptual model that predicts how various environmental and social factors interact to structure mating systems in ungulates. This model provides a useful framework for future tests of the roles of both ecological and social conditions in influencing the social systems of ungulates.
... Large-scale (e.g., family-level) comparative studies suggest that changes in the monopolizability of females (e.g., harem size) and fighting style (Kitchener 1991;Lundrigan 1996;Caro et al. 2003;Bro-Jørgensen 2007), and changes in the types of costs incurred from weapon expression (Emlen 2001;Emlen et al. 2005b), can drive evolutionary changes in weapon form; and biomechanical modeling suggests that changes in fighting style can drive changes in weapon form as well Klinkhamer et al. 2019). ...
Article
Exaggerated weapons of sexual selection often diverge more rapidly and dramatically than other body parts, suggesting that relevant agents of selection may be discernible in contemporary populations. We examined the ecology, reproductive behavior, and strength of sexual selection on horn length in five recently diverged rhinoceros beetle (Trypoxylus dichotomus) populations that differ in relative horn size. Males with longer horns were better at winning fights in all locations, but the link between winning fights and mating success differed such that selection favored large males with long horns at the two long‐horned populations, but was relaxed or nonexistent at the populations with relatively shorter horns. Observations of local habitat conditions and breeding ecology point to shifts in the relative abundance of feeding territories as the most likely cause of population differences in selection on male weapon size in this species. Comparisons of ecological conditions and selection strength across populations offer critical first steps toward meaningfully linking mating system dynamics, selection patterns, and diversity in sexually selected traits.
... Extreme sexual dimorphism is often associated with mating systems. When males physically compete for access to females, the males tend to be the larger sex and may develop weaponry such as horns or antlers (e.g., Bro-Jørgensen, 2007;Lindenfors et al., 2007; but note that in ~70% of bovids, females also have horns: Lundrigan, 1996). When females choose mates among males, male ornamentation in plumage, horns, vocalizations, or bright colors often occurs (Zuk & Simmons, 2018). ...
Article
Full-text available
Sex differences in aging occur in many animal species, and they include sex differences in lifespan, in the onset and progression of age‐associated decline, and in physiological and molecular markers of aging. Sex differences in aging vary greatly across the animal kingdom. For example, there are species with longer‐lived females, species where males live longer, and species lacking sex differences in lifespan. The underlying causes of sex differences in aging remain mostly unknown. Currently, we do not understand the molecular drivers of sex differences in aging, or whether they are related to the accepted hallmarks or pillars of aging or linked to other well‐characterized processes. In particular, understanding the role of sex‐determination mechanisms and sex differences in aging is relatively understudied. Here, we take a comparative, interdisciplinary approach to explore various hypotheses about how sex differences in aging arise. We discuss genomic, morphological, and environmental differences between the sexes and how these relate to sex differences in aging. Finally, we present some suggestions for future research in this area and provide recommendations for promising experimental designs. Sex difference in aging occurs across the animal kingdom, but there is considerable variation and they are not universal. The processes leading to sex‐specific aging are poorly understood and might originate in sex‐specific genome architecture, organismal biology, or environmental interactions. Here, we take a comparative approach to review the various hypotheses and suggest promising areas of research for further study.
... It is possible that some of the variation, at least for the suckler-bred calves, may be attributed to calf sex since male suckler-bred calves had a greater horn bud height (29.1%) and diameter (8.80%) compared with female suckler calves at time of disbudding. However, this finding is unsurprising for male calves as there is general agreement that the main evolutionary benefit of males having larger horns than females relates to intra-sexual competition for mates [7,31,32]. In the present study, while no dairy-bred females were included in the study, it would be of interest to quantify the horn bud measurements of male and female dairy-bred calves at time of disbudding. ...
Article
Full-text available
Background: Hot-iron disbudding is a common management procedure to prevent horn growth in calves. The study objective was to examine effect of age, breed and sex on horn bud size of dairy-bred and suckler-bred calves at time of disbudding. Results: The left and right horn bud size (diameter and height in mm) of 279 calves, including dairy-bred Holstein-Friesian (Male (M) = 88) and 191 suckler-bred (86 Charolais, CH; (M = 39, Female (F) = 47), 67 Limousin, LM; (M = 32, F = 35) and 38 Simmental, SI; (M = 22, F = 16) sired)) was measured using a digital calliper at time of disbudding. Calves were retrospectively assigned to two age categories at time of disbudding: 1), 14 to 28 days (d) old and 2), 29 to 60 d old. Holstein-Friesian M calves had a greater horn bud diameter (16.97 v.14.45 mm) and height (7.79 v. 5.00 mm) compared to suckler-bred M calves (P < 0.01), with no difference (P > 0.05) among the suckler-bred calves. Suckler-bred M calves had a greater horn bud diameter (14.46 vs 13.29 mm) and height (5.01 vs 3.88 mm) compared to suckler-bred F calves (P < 0.05). The slopes of the lines of best fit show that horn bud diameter and height increased with age (P < 0.05) for HF, SI male and CH female calves while there was no relationship with age (P > 0.05) for CH and LM male calves, or for SI and LM female calves. Linear regression of age with diameter and with height for each breed and sex showed high variability in the data as indicated by R-squared values ranging from 0.003-0.41 indicating that in the case of the diameter and the height, the weight of the fitting effect was poor. Conclusions: Calf age is not a good predictor of horn bud size and recommendations for the disbudding of calves should be based on horn bud size and not on age. The implications of these findings are that calves should be disbudded while horn development is still at the bud stage and when the bud is large enough to be easily palpable/visible, but not so large that disbudding could lead to severe tissue trauma.
... Furthermore, obtaining highly similar results when considering only individuals from the AdV site, with limited geological age and the presence of many individuals (> 20), it is possible to rule out possible geographical or temporal variations that could bias the results. In addition, given the wide presence of sexual dimorphism in mammals and the clear tendency for males to have a larger size or more complex morphologies linked to exhibition or struggles (Isaac 2005;Bro-Jørgensen 2007), but see the work of Ralls (1976) on mammals in which the largest individuals are female, it would seem reasonable to tentatively consider L. armatus of large caniniform as males. ...
Article
Mylodontidae (Mammalia, Xenarthra) is a family of ground sloths widely distributed in the South American fossil record, with members also present in Central and North America. Within the Mylodontidae, Lestodon armatus is the largest species, with an estimated body mass of more than three tonnes. This work focuses on the enlarged lower caniniforms of L. armatus as possibly exaggerated sexually dimorphic structures. Lower caniniforms from the late Pleistocene of Argentina, Uruguay, and Bolivia were studied using specimens from seven palaeontological collections. The possible sexual dimorphism in the caniniforms and its implications regarding the existence of sexual selection was assessed through morphometric analyses. The results support the existence of sexual dimorphism in L. armatus. Sexual dimorphism in an exaggerated structure in a large mammal suggests the existence of sexual selection, via competition between males or female mate choice, resulting in the evolution of the dimorphic structure. In L. armatus, the enlarged caniniforms would correspond to males and could have functioned as armaments in intraspecific fights or ornaments for sexual display. Based on observations in extant mammals, a polygynous mating system is proposed as highly probable in L. armatus, although the existence or composition of social groups cannot be certainly determined.
... For male animals, ornaments, armaments, and intense aggressive behavior are thought to be primarily driven by mating competition. Indeed, variation in competitive traits in males largely maps on to interspecific variation in sexual selection and mating systems (Bro-Jørgensen, 2007;Cooney et al., 2019;Emlen and Oring, 1977;Göran, 1998;Miles et al., 2018). For females, early hypotheses considered female competitive traits as byproducts of correlated selection on male traits (Darwin, 1871;Lande, 1980). ...
Article
Our understanding of the proximate and ultimate mechanisms shaping competitive reproductive phenotypes primarily stems from research on male-male competition for mates, even though competition is widespread in both sexes. We evaluate the hypothesis that the restricted nature of a resource required for reproduction, i.e. nest site, is a key variable driving territorial competition and testosterone secretion in female and male birds. Obligate secondary cavity-nesting has evolved repeatedly across avian lineages, providing a useful comparative context to explore how competition over limited nest cavities shapes aggression and its underlying mechanisms across species. Although evidence from one or another cavity-nesting species suggests that territorial aggression is adaptive in both females and males, this has not yet been tested in a comparative framework. We predicted that cavity-nesting generates more robust territorial aggression, in comparison to close relatives with less restrictive nesting strategies. Our focal species were two obligate secondary cavity-nesting species and two related species with more flexible nesting strategies in the same avian family: tree swallow (Tachycineta bicolor) vs. barn swallow (Hirundo rustica); Eastern bluebird (Sialia sialis) vs. American robin (Turdus migratorius). We assayed conspecific aggression using simulated territorial intrusion and found that cavity-nesting species displayed greater territorial aggression than their close relatives. This pattern held for both females and males. Because territorial aggression is often associated with elevated testosterone, we also hypothesized that cavity-nesting species would exhibit higher testosterone levels in circulation. However, cavity-nesting species did not have higher testosterone in circulation for either sex, despite some correlative evidence that testosterone is associated with higher rates of physical attack in female tree swallows. Our focus on a context that is relevant to both sexes – competition over essential breeding resources – provides a useful framework for co-consideration of proximate and ultimate drivers of reproductive competition in females and males.
... The degree in which fighting influences reproductive success might be an important selective pressure acting on weapons (Eberhard et al. 2018). In species in which reproductive success is heavily dependent on winning fights, the selective pressure on weapons should be higher (e.g., Bro-Jørgensen 2007). This should occur because the greater the importance of fights to reproductive success, the greater should be the intensity of fights (Arnott and Elwood 2008;Peixoto et al. 2014). ...
Article
Full-text available
In many species, individuals contest resources using specialized morphologies to overpower rivals, hereafter referred to as weapons. Despite their importance in fights, little is known about the selective forces affecting weapon evolution. This may be particularly important to understand why weapons are highly variable among species. Due to their role during fighting, we expect that whenever fighting becomes more important for individual fitness so should the intensity of selection on weapon strength and morphology (which affect the efficiency of a weapon during combat). If true, we expect species that fight more intensely to have stronger and more mechanically efficient weapons. We tested this idea using males of three species of Aegla crabs (A. longirostri, A. abtao, and A. denticulata) that vary in their fight intensity. We compared the muscle size, the mechanical advantage (a proxy for the efficiency of the movable finger of the claw), and the correlation between weapon biomechanics and overall weapon shape (a proxy for the efficiency of the entire claw) among the species. We found that species with more intense fights presented stronger claws, higher mechanical advantage, and less variation in the regression between biomechanics and overall shape. Interestingly, the species with the largest claws were not the most mechanically efficient, suggesting that weapon size is not the sole factor behind weapon evolution. We conclude that fight intensity might be an important factor affecting weapon biomechanics, which ultimately might lead to a better understanding of weapon evolution. Significance statement Animals fight using specialized morphologies to overpower rivals—termed weapons. Given the importance of fighting on leaving descendants to the next generation, weapon features related to winning fights are probably under selection. If true, then species in which fighting is more important should have stronger and more mechanically efficient weapons. Our results suggest that this might be true: Aegla crabs that fight more intensely have stronger and more efficient weapons (their claws). Interestingly, we also show that size is not the sole predictor of a better claw—muscle mass and mechanical efficiency might be higher in smaller claws when compared to larger claws. Thus, weapon evolution might not be solely tied to weapon size, but also to weapon morphology and mechanical efficiency.
... When genetic, behavioral, or life-history data were not available, we assessed the likelihood of male attempts to monopolize access to females based on the distribution of females during the breeding season, group size, and social structure, with the general idea that monopolization is not likely when females are solitary or widely dispersed during the breeding season or part of large, mixed sex groups (Table S2, Connor et al. 1998;Boness et al. 2002;Gowans et al. 2007;May-Collado et al. 2007;Moeller 2012). In bats and rodents, for example, sperm competition is more common in large groups (Hosken 1997;Dean et al. 2006), therefore monopolization is less likely, although this pattern was not observed in bovids (Bro-Jørgensen 2007). ...
Preprint
Full-text available
The Caribbean island biota is characterized by high levels of endemism, the result of an interplay between colonization opportunities on islands and effective oceanic barriers among them. A relatively small percentage of the biota is represented by ‘widespread species’, presumably taxa for which oceanic barriers are ineffective. Few studies have explored in detail the genetic structure of widespread Caribbean taxa. The cobweb spider Spintharus flavidus Hentz, 1850 (Theridiidae) is one of two described Spintharus species and is unique in being widely distributed from northern N. America to Brazil and throughout the Caribbean. As a taxonomic hypothesis, Spintharus “flavidus ” predicts maintenance of gene flow among Caribbean islands, a prediction that seems contradicted by known S. flavidus biology, which suggests limited dispersal ability. As part of an extensive survey of Caribbean arachnids (project CarBio), we conducted the first molecular phylogenetic analysis of S. flavidus with the primary goal of testing the ‘widespread species’ hypothesis. Our results, while limited to three molecular loci, reject the hypothesis of a single widespread species. Instead this lineage seems to represent a radiation with at least 16 species in the Caribbean region. Nearly all are short range endemics with several distinct mainland groups and others being single island endemics. While limited taxon sampling, with a single specimen from S. America, constrains what we can infer about the biogeographical history of the lineage, clear patterns still emerge. Consistent with limited overwater dispersal, we find evidence for a single colonization of the Caribbean about 30 million years ago, coinciding with the timing of the GAARLandia landbridge hypothesis. In sum, S. “flavidus” is not a single species capable of frequent overwater dispersal, but rather a 30 my old radiation of single island endemics that provides preliminary support for a complex and contested geological hypothesis.
... Males have evolved traits that promote access to females (precopulation), such as larger body size, ornaments, weapons and aggressive behaviour (Bro-Jørgensen, 2007), as well as fertilization (postcopulation), such as large testes and fast spermatogenesis rates to increase sperm production (Ramm & Stockley, 2010). Intense sexual selection drives the production and maintenance of these costly secondary sexual traits and behaviours, resulting in polygynous males allocating a disproportionate amount of resources towards reproduction (Crocker, Houser, & Webb, 2012). ...
Article
Life history trade‐off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding seasons, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade‐offs are present in the highly polygynous male southern elephant seal (Mirounga leonina) using 34‐years of individual‐level data. We compare age‐specific survival, recruitment and future breeding success probabilities of pre‐breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre‐breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre‐breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age‐specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population‐level trade‐off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade‐off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.
... Shape and size of male weapons are largely driven by selection operating through male-male competition over females (Geist 1966;Andersson 1994;Caro et al. 2003). In female ungulates, the primary function of weapons, when present, is linked to antipredator defence (Stankowich and Caro 2009) rather than sexual competition (Bro-Jørgensen 2007), although intrasexual selection can occasionally occur (Robinson and Kruuk 2007;Stankowich and Caro 2009). Given that symmetry in antlers and horns may be related to fitness (i.e., individuals with high survival rate and dominance status are characterized by low levels of FA and preferably selected as mates, see Møller et al. 1996;Pélabon and van Breukelen 1998) and thus carry evolutionary consequences, it is important to disentangle all factors that may affect the stable development of weapons and influence individual life histories and population dynamics in large herbivores. ...
Article
Full-text available
Developmental stability of an individual is often evaluated by means of fluctuating asymmetry (FA) in bilaterally paired morphological characters. Even though FA has been widely investigated in ungulates, its connection with the condition of individuals and their environment is still debated. In this study we investigated factors contributing to FA in horn length in the sexually monomorphic Alpine chamois. We measured right and left horn length of 1682 Alpine chamois (Nfemales = 734; Nmales = 948) shot during 2 consecutive hunting seasons (2015 and 2016) in 7 neighbouring districts in Central-Eastern Alps (Italy). We found no consistent left or right bias. Within our study population, FA values were normally distributed around a mean value that was not significantly different from zero (Skewness = − 0.107, SE = 0.06; Kurtosis = − 0.055, SE = 0.119). We also found that absolute FA in horn length was affected by environmental and climatic conditions experienced by the individuals during their first year and half of life. Statistically significant differences between right and left horn length were found with higher local population density and lower forage quality (i.e., siliceous substrate). Moreover, snow cover duration during the individuals’ first winter increased horn length asymmetry. No individual characteristics played a role in promoting horn length asymmetry. The associations between exposure to stressors and deviations from bilateral symmetry suggest that absolute FA can be used to identify populations whose individuals experienced stressful conditions early in life. We found in this relatively monomorphic species that both male and female horns were equally affected by climate, substrate, and local population density, thus showing that large male secondary sexual characters, such as the antlers of deer stags, are not the only traits which can be influenced by a negative environment and exhibit increasing FA.
... Regardless of its initial evolutionary drive, a structure being used in conspecific fights is expected to be effectively deployed in defence against predators, as well (Fig. 1C,D) (Bro-Jørgensen, 2007;Emlen, 2008;Stankowich, 2012). Furthermore, the late ontogenetic appearance of skeletal and integumentary defensive/offensive structures, including weapons, does not exclude their importance in defence and/or agonistic behaviour. ...
Article
Gregarious behaviour of large bodied herbivorous dinosaurs, such as ceratopsians, hadrosaurs and sauropods, has received much attention due to their iconic mass death assemblages (MDAs). Yet, social lifestyle of ankylosaurs, a highly specialized group of armoured herbivores that flourished predominantly during the Cretaceous Period, remains largely ambiguous. Whereas most ankylosaurs are found as isolated individuals, which may suggest a dominantly solitary lifestyle, the few examples of ankylosaur MDAs indicate that some members of this clade could have been gregarious. In this review, we assess taphonomic history, ontogenetic composition of the MDAs, defence system and other comparative anatomical attributes, and inferred habitat characteristics of ankylosaurs; aspects that may indicate and/ or influence group formation in extant herbivores and can also be studied in fossils. We show that the ankylosaurian gross anatomy, such as their heavy armour, barrel-shaped body and usually stocky limbs, combined with the rarity of their MDAs and multiple parallel trackways, all suggest a solitary adult life with efficient anti-predator defence system, limited agility, and confined foraging range. However, characteristics of the known MDAs of Pinacosaurus, Gastonia, and the Iharkút nodosaurids evaluated in this study imply that at least some ankylosaurs formed groups. Nevertheless, we found no common and consistent set of features to explain why these particular ankylosaurs were gregarious. While inefficient anti-predator defence along with likely higher agility of juvenile Pinacosaurus living in open habitats could account for their gregarious behaviour, such ontogenetic, anatomical and habitat features are not combined either in Gastonia or in the Iharkút nodosaurid MDAs. Instead, members of each MDA likely had their own specific conditions driving them to form relatively small herds, indicating a more complex social structuring in ankylosaurs than previously acknowledged. Studying morphological and functional disparity within Ankylosauria may help explain the repertoire of their social behaviour. Our holistic approach shows that combining palaeontological and biological information is essential and can provide new insights into the behavioural ecology of long extinct vertebrates.
Article
Full-text available
Animals sometimes have prominent projections on or near their heads serving diverse functions such as male combat, mate attraction, digging, capturing prey, sensing or defence against predators. Some butterfly larvae possess a pair of long frontal projections; however, the function of those projections is not well known. Hestina japonica butterfly larvae have a pair of long hard projections on their heads (i.e., horns). Here we hypothesized that they use these horns to protect themselves from natural enemies (i.e., predators and parasitoids). Field surveys revealed that the primary natural enemies of H. japonica larvae were Polistes wasps. Cage experiments revealed that larvae with horns intact and larvae with horns removed and fitted with horns of other individuals succeeded in defending themselves against attacks of Polistes wasps significantly more often than larvae with horns removed. We discuss that the horns counter the paper wasps’ hunting strategy of first biting the larvae’s ‘necks’ and note that horns evolved repeatedly only within the Nymphalidae in a phylogeny of the Lepidoptera. This is the first demonstration that arthropods use head projections for physical defence against predators.
Article
Full-text available
Elucidating the genetic basis of fitness-related traits is a major goal of molecular ecology. Traits subject to sexual selection are particularly interesting, as non-random mate choice should deplete genetic variation and thereby their evolutionary benefits. We examined the genetic basis of three sexually selected morphometric traits in bighorn sheep ( Ovis canadensis ): horn length, horn base circumference, and body mass. These traits are of specific concern in bighorn sheep as artificial selection through trophy hunting opposes sexual selection. Specifically, horn size determines trophy status and, in most North American jurisdictions, if an individual can be legally harvested. Using between 7,994–9,552 phenotypic measures from the long-term individual-based study at Ram Mountain (Alberta, Canada), we first showed that all three traits are heritable ( h² = 0.15–0.23). We then conducted a genome-wide association study (GWAS) utilizing a set of 3,777 SNPs typed in 76 individuals using the Ovine Infinium ® HD SNP BeadChip. We found suggestive association for body mass at a single locus (OAR9_91647990). The absence of strong associations with SNPs suggests that the traits are likely polygenic. These results represent a step forward for characterizing the genetic architecture of fitness related traits in sexually dimorphic ungulates.
Article
We propose a practical concept that distinguishes the particular kind of weaponry that has evolved to be used in combat between individuals of the same species and sex, which we term intrasexually selected weapons (ISWs). We present a treatise of ISWs in nature, aiming to understand their distinction and evolution from other secondary sex traits, including from ‘sexually selected weapons’, and from sexually dimorphic and monomorphic weaponry. We focus on the subset of secondary sex traits that are the result of same‐sex combat, defined here as ISWs, provide not previously reported evolutionary patterns, and offer hypotheses to answer questions such as: why have only some species evolved weapons to fight for the opposite sex or breeding resources? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that are only present or enlarged in adults of one of the sexes, and are used as weapons during intrasexual fights. Because of the absence of behavioural data and, in many cases, lack of sexually discriminated series from juveniles to adults, we exclude the fossil record from this review. We merge morphological, ontogenetic, and behavioural information, and for the first time thoroughly review the tree of life to identify separate evolution of ISWs. We found that ISWs are only found in bilateral animals, appearing independently in nematodes, various groups of arthropods, and vertebrates. Our review sets a reference point to explore other taxa that we identify with potential ISWs for which behavioural or morphological studies are warranted. We establish that most ISWs come in pairs, are located in or near the head, are endo‐ or exoskeletal modifications, are overdeveloped structures compared with those found in females, are modified feeding structures and/or locomotor appendages, are most common in terrestrial taxa, are frequently used to guard females, territories, or both, and are also used in signalling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only a few species possess better‐developed weapons than males, that the cases of independent evolution of ISWs are not evenly distributed across the phylogeny, and that animals possessing the most developed ISWs have non‐hunting habits (e.g. herbivores) or are faunivores that prey on very small prey relative to their body size (e.g. insectivores). Bringing together perspectives from studies on a variety of taxa, we conceptualize that there are five ways in which a sexually dimorphic trait, apart from the primary sex traits, can be fixed: sexual selection, fecundity selection, parental role division, differential niche occupation between the sexes, and interference competition. We discuss these trends and the factors involved in the evolution of intrasexually selected weaponry in nature.
Preprint
Full-text available
Sexual size dimorphism is biased toward males in most mammalian species. The most common explanation is precopulatory intramale sexual selection. Large males win fights and mate more frequently. In artiodactyls, previous tests of this hypothesis consisted of in-terspecific correlations of sexual dimorphism with group size as a surrogate for the intensity of sexual selection (Is). However, group size is not a proper measure of sexual selection for several reasons as is largely recognized in other mammalian taxa. I conducted an interspecific test on the role of sexual selection in the evolution of sexual dimorphism using the variance in genetic paternity as a proxy for the Is. I reviewed the literature and found 17 studies that allowed estimating Is = V/(W 2), where V and W are the variance and mean number of offspring per male, respectively. A phylogenetic generalized least squares analysis indicated that dimorphism (Wm/Wf) showed a significant positive regression with the intensity of sexual selection but not group size (multiple r2= 0.40; F 3,17 = 12.78, P = 0.002). This result suggests that sexual selection may have played a role in the evolution of sexual size dimorphism in Artiodactyla. An alternative hypothesis based on natural selection is discussed. Lay Summary: In most animals, females are larger than males. Paradoxically, sexual size dimorphism is biased toward males in most mammalian species. An accepted explanation is that sexual dimorphism in mammals evolved by intramale sexual selection. I tested this hypothesis by correlating sexual dimorphism with male reproductive success-measured using genetic paternity-in different Artiodactyl species. I found support for the sexual selection hypothesis; however, I also propose an alternative explanation based on natural selection.
Chapter
This chapter deals with a discussion of sexual selection followed by a definition of mating systems and their determinants. It describes male–male competition and its consequences, analyses female choice and the adaptive and nonadaptive bases for mate choice, and discusses evolutionary conflicts between the sexes and their consequences. The chapter investigates the cost of secondary sexual characters and its ecological and evolutionary consequences and examines the reasons for the presence of multiple secondary sexual characters and their significance. It explains the sex ratio theory and how it relates to sexual selection. Mating system is the label used for describing the way in which males and females are distributed in reproductive units, and the consequences of the distribution for reproductive behavior and parental care. Intrasexual selection is a very important force resulting in the evolution of armament and greater body size in one sex compared with the other. Sexual conflict may in fact be running sexual selection.
Article
Full-text available
This review describes the formation, structure, and function of bony compartments in antlers, horns, ossicones, osteoderm and the os penis/os clitoris (collectively referred to herein as AHOOO structures) in extant mammals. AHOOOs are extra‐skeletal bones that originate from subcutaneous (dermal) tissues in a wide variety of mammals, and this review elaborates on the co‐development of the bone and skin in these structures. During foetal stages, primordial cells for the bony compartments arise in subcutaneous tissues. The epithelial–mesenchymal transition is assumed to play a key role in the differentiation of bone, cartilage, skin and other tissues in AHOOO structures. AHOOO ossification takes place after skeletal bone formation, and may depend on sexual maturity. Skin keratinization occurs in tandem with ossification and may be under the control of androgens. Both endochondral and intramembranous ossification participate in bony compartment formation. There is variation in gradients of density in different AHOOO structures. These gradients, which vary according to function and species, primarily reduce mechanical stress. Anchorage of AHOOOs to their surrounding tissues fortifies these structures and is accomplished by bone–bone fusion and Sharpey fibres. The presence of the integument is essential for the protection and function of the bony compartments. Three major functions can be attributed to AHOOOs: mechanical, visual, and thermoregulatory. This review provides the first extensive comparative description of the skeletal and integumentary systems of AHOOOs in a variety of mammals.
Article
Full-text available
Sexual selection is generally held responsible for the exceptional diversity in secondary sexual traits in animals. Mating system evolution is therefore expected to profoundly affect the covariation between secondary sexual traits and mating success. While there is such evidence at the interspecific level, data within species remain scarce. We here investigate sexual selection acting on the exaggerated male fore femur and the male wing in the common and widespread dung flies Sepsis punctum and S. neocynipsea (Diptera: Sepsidae). Both species exhibit intraspecific differences in mating systems and variation in sexual size dimorphism (SSD) across continents that correlates with the extent of male‐male competition. We predicted that populations subject to increased male‐male competition will experience stronger directional selection on the sexually dimorphic male foreleg. Our results suggest that fore femur size, width and shape were indeed positively associated with mating success in populations with male‐biased SSD in both species, which was not evident in conspecific populations with female‐biased SSD. However, this was also the case for wing size and shape, a trait often assumed to be primarily under natural selection. After correcting for selection on overall body size by accounting for allometric scaling, we found little evidence for independent selection on any of these size or shape traits in legs or wings, irrespective of the mating system. Sexual dimorphism and (foreleg) trait exaggeration is therefore unlikely to be driven by direct precopulatory sexual selection, but more so by selection on overall size or possibly selection on allometric scaling.
Preprint
In the Drosophila lineage, both sperm and the primary female sperm storage organ, the seminal receptacle (SR), may reach extraordinary lengths. In D. melanogaster, long SRs bias fertilization toward long sperm during the displacement stage of sperm competition. This sperm-SR interaction, together with a genetic correlation between the traits, suggests that the coevolution of exaggerated sperm and SR lengths may be driven by Fisherian runaway selection. To further understand the costs and benefits of long sperm and SR genotypes in both sexes, we measured male and female fitness in inbred lines of D. melanogaster derived from four populations previously selected for long sperm, short sperm, long SRs, or short SRs. We specifically asked: do long SRs impose costs or benefits on the females that bear them? Do genotypes that generate long sperm in males impose a fitness cost on females sharing those genotypes? Is long sperm an honest indicator of male viability and associated with increased fitness? And finally, are the benefits of long sperm restricted to competitive fertilization success, or do long-sperm males also have increased mating success and fecundity in single matings? We found that both sexes have increased longevity in long sperm and long SR genotypes, with fewer reproduction-related benefits and evidence for trade-offs in males, compared to females. Our results suggest that sperm length and SR length are both indicators of increased viability.
Article
Sexual size dimorphism is biased toward males in most mammalian species. The most common explanation is precopulatory intramale sexual selection. Large males win fights and mate more frequently. In artiodactyls, previous tests of this hypothesis consisted of interspecific correlations of sexual dimorphism with group size as a surrogate for the intensity of sexual selection (Is). However, group size is not a proper measure of sexual selection for several reasons as is largely recognized in other mammalian taxa. I conducted an interspecific test on the role of sexual selection in the evolution of sexual dimorphism using the variance in genetic paternity as a proxy for the Is. I reviewed the literature and found 17 studies that allowed estimating Is= V/(W2), where V and W are the variance and mean number of offspring per male, respectively. A phylogenetic generalized least squares analysis indicated that dimorphism (Wm/Wf) showed a significant positive regression with the intensity of sexual selection but not group size (multiple r2= 0.40; F3,17= 12.78, P = 0.002). This result suggests that sexual selection may have played a role in the evolution of sexual size dimorphism in Artiodactyla. An alternative hypothesis based on natural selection is discussed.
Article
The fully aquatic lifestyle of dugongs means that direct observation of social tusk use is not usually possible. This study used body scarring as an indicator of tusk function by males. Tusk rake scars on 298 live wild dugongs, of both sexes and all sizes, were categorized and counted in over 1,000 photographs, and examined in relation to maturity and reproductive activity over seasons. All dugongs had tusk scars, but adults were the main recipients. Sexually active adults acquired the greatest number of fresh tusk wounds during the mating season. Subadults received fresh rakes at similar numbers year‐round. Adult males had more scars on the mid and posterior dorsum, indicating that males direct combative force to these regions of the male body when competing for females. Adult females had heaviest scarring and more tusk puncture wounds on the anterior‐mid dorsum and head, suggesting that male dugongs use tusks in sexual coercion. Heavy scarring sustained by solitary calves compared to dependent ones, suggests that mothers afford some protection. Body scarring caused by tusks may serve as an indicator of reproductive contribution of the recipients, providing that successful males are involved in more reproductive competitions, and successful females in more mating events.
Article
Varying forms of polygyny are observed across many animal groups. In some species, a male defends a group of females from other males, and successful defence leads to greater reproductive success. This is often referred to as harem polygyny and is most observed in mammals. A female-biased sex ratio has been associated with harem polygynous species. In such populations, formation of ‘harems’ may be an inevitable consequence of the relative lack of available males, rather than multiple females actively choosing to mate with a subset of specific males. Although a rare mating system for insects, harem polygyny has been described in several orders, including tree wētā (Orthoptera: Anostostomatidae: Hemideina) in New Zealand. Aggregations of multiple females have been found with a single male in their diurnal roosts. We aimed to determine how the sex ratio and local density of Hemideina thoracica affected the formation of harems in laboratory conditions. ‘Harems’ were observed when the sex ratio was female biased, but no more than would be expected by chance arrangement of individuals, although females preferred to associate with males rather than be alone. Conversely, when the sex ratio was male biased, females preferred to be alone. The number of females associated with each male was lowest when the sex ratio was even and local density was low. However, males and females did not associate randomly: when local density was high aggregations of multiple males with multiple females were observed more often than expected by chance, suggesting that, in contrast to classic ‘harem’ behaviour, males accepted other males being present as long as females were also found in the roost. Additionally, females were more likely to be associated with males than in low-density conditions. Individuals in this study showed much more flexibility in their mating behaviour than what would be expected in a harem polygynous species.
Article
Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.
Article
The aims of this observational study were to (1) define which animal’s phenotypic characteristics determine social position in the context of a commercial organic farm with mixed herd (horned and non-horned cows) and (2) determine the influence of social position on the time at the feeder. We took the following measurements from 27 dairy cows in lactation: body mass, age, body condition score, body length, withers height, distance between horns, horn circumference and length. Replacement and time at the feeder were recorded for 1 h at the time of supplementation. Dominance values for each animal were calculated and the herd was divided into three social categories: dominant (D), intermediate (I) and subordinate (S). Age, body length and body mass influenced (p < 0.001) dominance value of all animals. The presence of horn influenced (p = 0.034) the dominance value of the I and S animals because it was a unique characteristic of these categories. Dominant (84.3%) and intermediate (75.2%) animals spend more time (p < 0.05) at the feeder than the subordinate (59.5%); however, dominant animals tended (p = 0.093) to spend more time at the feeder than the intermediate animals. The social position of an animal was influenced by its age, body mass and body length, and its social position influenced the time at the feeder.
Article
A critical part of the sexual selection process in animals is the genetic mating system. Quantifying mating systems, especially in species with cryptic life histories can be challenging. One approach is to use genotypic markers and accurate parentage analysis, along with methods to account for bias when sampling natural populations, to calculate sexual selection metrics derived from Bateman's principles. In this study, 3 microsatellites were used to genotype 48 adults (23 female and 25 male) and 342 offspring from known mothers of live-bearing bluntnose klipfish. Parentage analysis was performed to interpret mating and reproductive success for both sexes. Metrics quantified were the opportunity for selection (I), the opportunity for sexual selection (Is), absolute (βss), and standardized (β'ss) Bateman gradients and the maximum intensity of precopulatory sexual selection (s'max). Multiple mating by both sexes were revealed by parentage analysis. However, females did not show significant Bateman gradients or a significant maximum intensity of precopulatory sexual selection (s'max), whereas male sexual selection metrics were all significantly greater than 0. These results suggest a polygynandrous mating system for this species. There is an opportunity for sexual selection to act on males but not females in this population, which is evolutionary tied to anisogamy, parental investment, and sex roles.
Article
In contrast to males, which compete with other males for access to mates, females compete with each other for forage-rich sites, birthing grounds, comfortable resting places, and access to sources of water and salt licking locations. This behavior has been observed in many species. However, many agonistic interactions between females occur where resources are not immediately at stake, and the reasons for their rivalry are often unclear. Therefore in this paper, I want to analyze the main causes of female-female aggression in the yearly cycle of goitered gazelles. I found that adult females had conflicts moistly with sub-adult females and less with other adult females; and these behaviors were observed mainly in May, with less in June, and only a few cases displayed during the rest of the year. The months of May-June had the most abundant and highest quality forage of the year, when competition for resources would seem to be least expected. Struggles for resting places occurred throughout the entire year, with only some bias for May that did not represent a primary level of aggression. In reality, the high rate of female-female aggressive interactions was related to the protection of birthing grounds, where mothers isolated themselves to give births, establish a strong selective mother-young bonds, keep their hiding fawns separated from alien offspring (having initial problems with distant visual recognition), and protect them against disturbance from all other females, which can undermine a fawn's hidden status and make it more vulnerable to predation.
Thesis
Full-text available
This integrative dissertation explores the ultimate (evolutionary) as well as proximate (i.e. mechanistic developmental) drivers of life-history traits in insects. The focus lies on the evolution of body size, sexual size dimorphism (SSD) and sex-specific body size plasticity, which are studied on different levels of biological organization. The following six chapters integrate experimental and quantitative genetic studies with comparative approaches and aim at broadening our current knowledge on how the astonishing phenotypic variation observed across the tree of life came about and how it is maintained. Chapter 1 explores global patterns of body size, sexual size dimorphism, relative wing size and geographic range size among 151 species of fruit flies (Diptera: Drosophilidae). In vertebrates, these traits accord fairly predictably with prominent ecogeographic “rules” (Bergmann’s, Rensch’s, Allen’s, Rapoport’s rules). However, the predictive power of these rules in invertebrates — and insects in particular — is very poor, at least in part due to lack of a mechanistic understanding of the drivers of such variation. As these traits are to some extent evolutionarily or ecologically interdependent, possible confounding effects between macroecological patterns are expected and might explain some of the apparent idiosyncrasy. Such interrelations are rarely considered. Here, I test the predictions of Bergmann, Rensch, Allen and Rapoport for a large number of drosophilids across the globe to investigate potential confounding effects between patterns. Although there is limited evidence for any confounding effects, I nevertheless demonstrate the usefulness of studying several macroecological patterns simultaneously, as it allows for a deeper, mechanistic understanding of ecogeographic variation. In chapter 2, I assess quantitative genetic latitudinal differentiation in life history traits in the widespread sepsid fly Sepsis fulgens (Diptera: Sepsidae) across 13 populations spanning 20 degrees latitude from southern Italy to Estonia. Despite very short generation times, I found a converse Bergmann cline (smaller size at higher latitudes). As development time did not change with latitude (flat cline), integral growth rate thus likely declines towards the pole. At the same time, early fecundity, but not egg size, increased with latitude. Rather than being mediated by seasonal time constraints, the body size reduction in the northernmost flies from Estonia could suggest that these are marginal, edge populations, as when omitting them the body size cline became flat as well. Most of the other sepsid species investigated to date also show flat body size clines, a pattern that strikingly differs from Drosophila. I conclude that S. fulgens life history traits appear to be shaped by similar environmental pressures and selective mechanisms across Europe, be they adaptive or not. This reiterates the suggestion that body size clines can result as a secondary consequence of selection pressures shaping an entire life history syndrome, rendering them inconsistent and unpredictable in general. Chapter 3 focusses on the evolution of sexual size dimorphism (SSD) and sex-specific body size plasticity. In insects, females are usually the larger and more plastic sex. However, because females are larger than males in most species, it is difficult to assess whether their greater plasticity is driven by selection on size or represents an effect of the female reproductive role per se. I here estimate sex-specific body size plasticity of populations and species that vary in the direction and extent of SSD, and show that males are typically more plastic than females if they are the larger sex. Hence, my findings indicate that primarily selection on size, rather than the reproductive role per se, drives the evolution of sex-specific body size plasticity. However, sepsid flies, and possibly Diptera in general, show a clear sexual asymmetry with greater male than female plasticity related to SSD, likely driven by strong sexual selection on males. Although further research controlling for phylogenetic and ecological confounding effects is needed, the patterns are congruent with theory suggesting that condition dependence plays a pivotal role in the evolution of sexual size dimorphism. In chapter 4, I investigate the potential link between the extent of sexual dimorphism and sex-specific condition dependence among traits and species. Sexual selection can displace traits acting as ornaments or armaments from their viability optimum in one sex, ultimately giving rise to sexual dimorphism. The degree of dimorphism should hence not only mirror the strength of sexual selection, but also the net viability costs and benefits of trait maintenance at equilibrium. The ability of organisms to bear exaggerated traits will depend on their condition. More sexually dimorphic traits should therefore also exhibit greater sex differences in condition dependence. While this has been shown to apply among traits within species, condition dependence and sexual dimorphism are also expected to correlate across the phylogeny. I investigated and quantified this prediction within and across 11 (sub)species of black scavenger flies that vary in their mating system. When estimating sex-specific condition dependence for seven sexual and non-sexual traits that vary in their sexual dimorphism, we not only found a positive relationship between the sex difference in allometric slopes (as our measure of condition dependence) and relative trait exaggeration among traits within species, but also across species for those traits expected to be under sexual selection in males. I additionally show species with more pronounced male aggression to have relatively larger and more condition-dependent male fore and mid legs. My comparative study suggests a common genetic/developmental basis of sexual dimorphism and sex-specific plasticity that apparently evolves across the phylogeny, and that the evolution of trait size consistently alters scaling relationships and thus contributes to the allometric variation of sexual armaments or ornaments in animals. In chapter 5, I investigate the physiological basis of adaptive size variation in the yellow dung fly Scathophaga stercoraria, which shows pronounced male-biased sexual size dimorphism and strong body size plasticity. I estimate variation of a major physiological threshold, the critical weight, which is the mass at which a larva initiates pupariation. Critical weight is associated with sexual size dimorphism and sex-specific plasticity, and is thus a likely target of selection on adult size. Detailed larval growth trajectories derived from individuals raised at two food and temperature treatments further reveal that sex-specific size plasticity is mediated by faster initial growth of males that later becomes reduced by greater male weight loss during the wandering stage. Hence, I illustrate the importance of detailed assessments of ontogenetic growth trajectories for the understanding of adaptive size variation and discuss the mechanistic basis of size determination in shaping sex-specific phenotypic plasticity. Chapter 6 is devoted to the effect temperature on the evolution of insect wings. Given its profound effect on biological systems, temperature is often held responsible for eliciting phenotypic plasticity as well as quantitative genetic differentiation. If genetic and plastic responses to temperature are adaptive, they should be related in magnitude and form, a pattern that should evolve repeatedly in different lineages. I quantified this putative relationship between quantitative genetic latitudinal variation in wing loading and wing shape and their thermal plasticity in two closely related sepsid flies with contrasting sexual size dimorphism. Common garden rearing revealed decreasing wing loading with latitude independently in both species, likely driven by selection for increased dispersal capacity in the cold. Thermal plasticity for wing loading was however non-linear, suggesting that the relationship between plasticity and genetic differentiation is more complex. Although both species showed similar patterns of wing shape allometry, sexual dimorphism and thermal plasticity, latitudinal differentiation only mirrored thermal plasticity in one but not the other species. Arguing that such discrepancies may be driven by variation in gene flow and demography, these results reiterate the notion that genetic wing shape differentiation may be complex and idiosyncratic even among ecologically similar closely related species. In summary, by integrating studies on different insect systems at multiple levels of biological organization (from single genotypes to population differentiation within species to global interspecific variation), this dissertation provided insights into the evolution of life histories and hence contributes to the understanding of diversity and disparity in the broadest sense.
Book
Full-text available
This book presents the first unified conceptual and statistical framework for understanding the evolution of reproductive strategies. Using the concept of the opportunity for sexual selection, the authors illustrate how and why sexual selection, though restricted to one sex and opposed in the other, is one of the strongest and fastest of all evolutionary forces. They offer a statistical framework for studying mating system evolution and apply it to patterns of alternative mating strategies. In doing so, they provide a method for quantifying how the strength of sexual selection is affected by the ecological and life history processes that influence females' spatial and temporal clustering and reproductive schedules. Directly challenging verbal evolutionary models that attempt to explain reproductive behavior without quantitative reference to evolutionary genetics, this book establishes a more solid theoretical foundation for the field. Among the weaknesses the authors find in the existing data is the apparent ubiquity of condition-dependent mating tactics. They identify factors likely to contribute to the evolution of alternative mating strategies--which they argue are more common than generally believed--and illustrate how to measure the strength of selection acting on them. Lastly, they offer predictions on the covariation of mating systems and strategies, consider the underlying developmental biology behind male polyphenism, and propose directions for future research. Informed by genetics, this is a comprehensive and rigorous new approach to explaining mating systems and strategies that will influence a wide swath of evolutionary biology.
Article
Full-text available
Article
Full-text available
I examined influence of body size and mating systems on sexual-size dimorphism by summarizing characteristics and testing for associations among the most dimorphic mammalian taxa-Macropodidae, Primates, Mustelidae, Pinnipedia, Elephantidae, Ruminantia. The most dimorphic taxa were seals in Otariidae. On average, males were three times larger than females, and all otariids displayed extensive dimorphism. Except for the Strepsirhini, most taxa had dimorphism ratios (mass of males:mass of females) between 1.2-1.8. Extent of dimorphism increased with body size but the effect was slight (power function between masses of males and females, 1.04-1.05) for most taxa. Phocid seals and macropodid marsupials had power functions of ca. 1.2. Mating systems were associated with size dimorphism in simian primates and ruminants. Monogamous simian primates were less dimorphic than simians that had polygynous mating systems. Ruminants with tending and harem mating systems were more dimorphic than those with territorial polygynous and monogamous mating systems. Polygyny and how it was conducted were associated with the extent of sexual size dimorphism.
Article
Full-text available
The tamarao, Bubalus mindorensis HEUDE, 1888, is an endemic bovine species of Mindoro in the Philippine Island Archipelago. For 2002 the IUCN assumed 30–200 individuals still living on the island. In this paper postnatal changes of the skull and tooth wear are described. The observations are based on 7 skulls in the Museum für Tierkunde Dresden and 3 skulls in the Zoological Museum Berlin. Postnatal changes in the skull involve the subsequent fusion of individual sutures between bones, starting at the back and base of the skull and progressing more towards the facial region and inner orbital region with advancing age. Comparisons in fusion of sutures with the bovids Bison bonasus (Bovidae, Bovinae) and in tooth eruption and wear with the domestic cattle Bos taurus (Bovidae, Bovinae) and Ovis aries (Bovidae, Caprinae) are used to estimate the age of the speci-mens of Bubalus mindorensis, giving a range of about 6 months to at least 14 years of age for the studied specimens. More substantial material, including 6 stuffed animals and 4 skeletons, of the tamarao had been in the Museum für Tierkunde Dresden but was destroyed during the second World War. The remain-ing mounted skeleton is briefly described. Kurzfassung. Der Mindorobüffel, Bubalus mindorensis HEUDE, 1888, ist eine endemische Boviden-Art von Mindoro im Philippinischen Inselarchipel. 2002 hat die IUCN 30–200 noch auf der Insel lebende Tiere angenommen. In dieser Veröffentlichung werden die postnatalen Veränderungen im Schädel und die Zahnab-nutzung beschrieben. Die Beobachtungen basieren auf 7 Schädeln im Museum für Tierkunde Dresden und 3 Schädeln aus dem Zoologischen Museum Berlin. Postnatale Veränderungen im Schädel äußern sich in der Fusion von Knochen, beginnend am Hinterhaupt und an der Schädelbasis und dann mit zunehmendem Alter weiter nach vorne in die Gesichtsregion ziehend. Vergleiche mit Fusionen der Knochennähte bei Bison bonasus (Bovidae, Bovinae) und mit Zahnwechsel und -abnutzung bei Hausrind Bos taurus (Bovidae, Bovinae) und Ovis aries (Bovidae, Caprinae) werden genutzt um das Alter der Exemplare von Bubalus mindorensis zu schätzen. Dies gibt ein Alter von etwa 6 Monaten bis mindestens 14 Jahre für die analysierten Exemplare. Umfangreicheres Material, darunter 6 Dermoplastiken und 4 Skelette, vom Tamarao waren im Museum für Tierkunde, sind aber im zweiten Weltkrieg zerstört worden. Das noch existierende auf-gestellte Skelett wird kurz beschrieben.
Article
Full-text available
We studied long-term cohort effects on chest girth and horn length in a recently established population of alpine ibex (Capra ibex ibex). Environmental conditions of the year of birth affected chest girth and first-annual increment of horns of males but did not affect chest girth and horns of females. Females compensated for a slow horn growth during their Ist year of life, whereas males did not. Level of polygyny of the species and environmental conditions experienced by the population could account for the occurrence of long-term cohort effects in male growth and its absence in female growth. Abundance of food resources throughout the study period allowed females to show compensatory growth. However, evolutionary constraints on growth that may exist in males of polygynous species may have prevented males from showing compensatory growth.
Article
Full-text available
It has been hypothesized that the evolution of sexual dimorphism could lead to sexual dimorphism in trophic structures, mainly the mouthparts, through inter-sexual niche partitioning. This hypothesis is based on the assumption that females select habitats on the basis of their requirements for diets with high nutrient concentrations (due to pregnancy and lactation), whereas males select for habitats with abundant resources (due to their larger body size and higher absolute nutrient requirement). We analysed a data set of the morphological traits of the mouth and teeth, which have been proposed as being functionally related to food selection ability (muzzle width, incisor protrusion), food comminution (molar occlusal surface area) and intake (incisor breadth), in males and females of species from the mammalian order Artiodactyla. Our analyses showed that all of the morphological traits studied covaried isometrically with body mass. The effect of sharing common ancestors did not have a significant effect on oral morphology, which indicates that oral morphology evolved in parallel in both sexes. We detected differences in body mass between the sexes and these differences remained when phylogeny was taken into account. Our results demonstrate that the dimensions of the oral traits result primarily from differences in body mass between the sexes rather than differences in niche adaptation between the sexes. The relationship between sexual dimorphism in body mass and differences in niche partitioning between the sexes in the Artiodactyla is discussed.
Book
The Japanese serow (Capricornis crispus) has been protected by law since 1955 in Japan, because it was becoming rarer and approaching extinction. Thereafter, the serow population has increased gradually. The Japanese serow is thought to be a primitive relict species on the islands of Japan, and the geographical range of the serow has retracted upwards into the moun­ tain forests to avoid contact with humans. Little was therefore known about these animals. However, increasing losses of forest habitat due to exploit­ ation of the mountain forests or expanding cultivation by local foresters have driven the Japanese serow back into the lowlands of Japan. Since then, complaints of damage to trees and other vegetation have accumulated against the serow. In some prefectures the shooting of Japanese serow was allowed in order to prevent damage to forests. The animals killed were taken for research by the Departments of the Environment and by universities. was set up at the summit of Mt. Gozaisho, The Japan Serow Center Komono-cho, Mie Prefecture, in 1962 and has made a great effort to breed the serow and its related species in captivity. In addition, the International Studbook of Capricorn is crispus in captivity was established in Japan, and the state of breeding of the Japanese serows is now reported annually. However, without detailed scientific research, it is impossible to conduct sensible protection, conservation or management of the serow in captivity or in the wild.
Article
The savanna ungulate faunas of the North American Miocene were broadly similar to those of present-day East Africa in terms of overall morphological and taxomic diversity. However, the predominant ungulates of the African faunas are bovids, which possess bony horns that are primitively sexually dimorphic in their occurrence. The predominant ungulates of the North American Tertiary were equids, camelids and oreodonts, which all lacked horns. The limited number of horned ruminants were largely Miocene immigrants from Eurasia. Horns were also absent from the large-bodied herbivores in the endemic faunas of South America and Australia. Studies on living ungulates show that a strong correlation exists between habitat type, feeding behaviour, social behaviour and morphology. The importance of the post-Eocene climatic changes to the history of mammalian evolution is stressed. The primitive condition in eupecorans and protoceratids is the absence of horns. The first horned members of these divisions had horns in the males only. Small present-day antelope, where horns may also be present in the females of the species, are probably secondarily small. Horns were acquired independently in ruminant artiodactyls at least 3 times; a maximum number of 7 times is not unlikely. In each case, horns first appeared at a critical body weight of about 18kg, and in correlation with a change in habitat from closed to open woodland. Horns in living ruminants are associated with territorial defence by males holding exclusive feeding and reproductive territories in woodland habitats. Such behaviour in present-day antelope is correlated with a body size of greater than 15 kg and a folivorous diet. Perissodactyls never evolved sexually dimorphic bony horns of the type seen in ruminant artiodactyls because their foraging and digestive strategies necessitate a larger daily intake of food. Study of the morphology and paleoecology of oreodonts suggest that they were woodland herd-forming browsers with exclusively folivorous diets. Studies of the behaviour and morphology of of living members of the Ruminantia, and of the morphology and paleoecology of their fossil ancestors, suggest that they were primitively tree browsers living in closed woodland habitats. The radiation of the Bovidae into open grassy habitats in the Pliocene may have been dependent on the immigration of grazing equids into the Old World. During the Tertiary, the food resources in North America were more widely dispersed; this may have been the result of the trees being more widely spaced. A possible causal mechanism for this was the stable land mass of North American continent during the Tertiary resulting in a more continental climate, with more severe effect of the post-Eocene seasonality on the vegetation. Thus most endemic North American ruminants did not evolve horns because, at the critical combination of body size and diet seen in the evolution of horns in the Old World ruminants, the dispersal of the food resources within the vegetation was too great for an effective home range to be maintained as an exclusive territory. -from Author
Article
A minimum of 51 tamarao (Bubalus mindorensis) occurred in a 20-km$^2$ study area at the Mt. Iglit Game Refuge and Bird Sanctuary, Occidental Mindoro, the Philippines. Juvenile bull tamarao formed groups similar to those in juvenile water buffalo (Bubalus bubalis), but adult tamarao did not form clans or aggregations like buffalo.
Article
The physical and morphological characteristics of red-fronted gazelles Gazella rufifrons kanuri Gray 1846 were determined in Waza National Park between September 1989 and December 1993 by describing body colour code and coloration, measuring body weight, body length, ear length, head length, horn length, hip height, shoulder height, tail length, and counting orbital glands from 141 carcasses. Mean body weight of red-fronted gazelles ranged from 7.8 kg for the young to 29.7 kg for the adult, while shoulder height varied from 38.7 cm in the young to 68.7 cm in the adult. Irrespective of age and sex, the shoulder height was lower than hip height by a fixed ratio of c. 1.04. Regardless of age and sex, orbital glands, though deeper in the adults, numbered nine with the highest concentration of four in the inter-digital fossae. The relationship between body weight and body length and horn length were significantly (P < 0.05, r= 0.8) positively correlated. The spatial appearance of horns, when viewed from the muzzle direction, is the most obvious physical and morphological characteristic for age and sex differenation of red-fronted gazelles in the field.
Article
Why do females across a wide range of taxa mate with more than one male? We suggest that a better understanding of polyandry may be gained by considering the implications of intragenomic conflict for female reproductive success. Here, we revisit the literature on cellular endosymbionts, transposable elements, segregation distorters, maternal-effect lethals and genomically imprinted genes to show that each of these selfish genetic elements can modify maternal and paternal haplotypes in ways that render them incompatible within the developing embryo. We propose that the cumulative threat to female reproductive success of genetic incompatibility arising from intragenomic conflict may be an important force driving the evolution of polyandry. By mating with more than one male, females can potentially exploit post-copulatory mechanisms for minimizing the risk and/or cost of fertilization by genetically incompatible sperm. This hypothesis differs fundamentally from other genetic benefit models of polyandry in that the fitness consequences of intragenomic conflict depend on an interaction between parental genomes and are thus non-additive. Reciprocal evolutionary change between selfish genetic elements and their suppressors, combined with the capacity of these elements for horizontal transfer between species, is likely to ensure the persistence of genetic incompatibility as a threat to female reproductive success.
Article
Intraspecific variation in male mating behaviour is wide-spread in ungulates. Such variation is particularly dramatic when it takes distinct forms, and these discrete behavioural patterns are called alternative tactics. Alternative male mating tactics in ungulates include female-defence, resource-defence, and lekking. I review patterns and processes in variation in male mating tactics within (as separate from between) ungulate populations. Across ungulates, the greatest diversity of mating tactics is typically shown by lekking populations. Males rarely show irreversible patterns, but often switch between two or more mating tactics. Overall, variation in mating tactics is most likely maintained as a conditional strategy influenced by multiple internal factors (especially age, health, body size) and external factors (particularly density at small, local scales). Much work remains to be done on the costs and benefits associated with different tactics, proximate mechanisms, the role of frequency-dependent selection, and the evolution of female mating behaviour.
Article
This field guide begins with a checklist. The main part of the volume consists of entries for each species. Each entry provides information on common names, measurements, recognition, geographical distribution (plus map), habitat, diet, behaviour, adaptations and conservation status. Illustrations are also included. Brief notes are also provided on the African environment (physical, climate and vegetation) and palaeoecology (habitats and species). Finally a short section examines African wildlife conservation.
Article
In the current resurgence of interest in the biological basis of animal behavior and social organization, the ideas and questions pursued by Charles Darwin remain fresh and insightful. This is especially true of The Descent of Man and Selection in Relation to Sex, Darwin's second most important work. This edition is a facsimile reprint of the first printing of the first edition (1871), not previously available in paperback. The work is divided into two parts. Part One marshals behavioral and morphological evidence to argue that humans evolved from other animals. Darwin shoes that human mental and emotional capacities, far from making human beings unique, are evidence of an animal origin and evolutionary development. Part Two is an extended discussion of the differences between the sexes of many species and how they arose as a result of selection. Here Darwin lays the foundation for much contemporary research by arguing that many characteristics of animals have evolved not in response to the selective pressures exerted by their physical and biological environment, but rather to confer an advantage in sexual competition. These two themes are drawn together in two final chapters on the role of sexual selection in humans. In their Introduction, Professors Bonner and May discuss the place of The Descent in its own time and relation to current work in biology and other disciplines.
Book
Why have males in many species evolved more conspicuous ornaments and signals such as bright colours, enlarged fins, and feather plumes, as well as larger horns and other weapons than females? Darwin's explanation for such secondary sex traits, the theory of sexual selection, became his scientifically perhaps most controversial idea. It suggests that the traits are favoured by competition over mates. After a long period of relative quiescence, theoretical and empirical research on sexual selection has erupted during the last decades. This book describes the theory and its recent development, reviews models, methods, and empirical tests, and identifies many remaining open problems. Among the topics discussed are the selection and evolution of mating preferences; relations between sexual selection, species recognition, and speciation; constraints on sexual selection; the selection of secondary sex differences in body size, weapons, and in visual, acoustic, and chemical signals. The rapidly growing study of sexual selection in plants is also reviewed. Other chapters deal with alternative mating tactics, and with the relationships among sexual selection, parental roles, and mating systems. The present review of this very active research field will be of interest to students, teachers, and research workers in behavioural and evolutionary ecology, animal behaviour, plant reproductive ecology, and other areas of evolutionary biology where sexual selection is a potential selection factor. In spite of much exciting progress, some of the main questions in the theory of sexual selection yet remain to be answered.
Article
Hornlike organs evolved independently in a number of mammalian families. Though these organs assumed great diversity they did evolve into several general functional types. A short review of the structure and development of hornlike organs is given. Some views on horn function and evolution are critically discussed. The evolution of hornlike organs is visualised as follows: In primitive large mammals the head blow became effective as a fighting form due to increased mass and inertia of the heads. Some forms grasped this potential. Combats were carried out from the broadside while opponents delivered head blows on each others body. Skull protuberances now became adaptive. Concurrently, defensive mechanisms evolve, decreasing the effectiveness of these protuberances. Foremost among them is a thick, heavy hide or specialised dermal shield. These adaptive syndromes gave no impetus towards larger and more complex horns. This impetus arose with the appearance of a new method of defense - catching the opponent's blows with the horned head. This leads to the evolution of heavy skulls and horns capable of catching and holding the opponent's head. The target area of attack remained the body. Frontal engagements resulted from the opponents' attempts to control each others horned head. It is shown that bovids and suids followed similar evolutionary roads in their mode of combat. The tusks of the suidae fulfill the same function and were subject to similar selection as the horns of short horned bovids. Thus Sus and Oreaisinos, and Bos and Phacochoerus are entirely similar in their mode of combat, hornlike organs and defense mechanisms. The primitive frontal engagement gave rise to two different modes of combat, ramming and wrestling. The
Article
SYNOPSIS. Blood smears from 519 mammals of South Vietnam were examined for hematozoa. Trypanosomes were found in Rattus norvegicus, R. exulans, R. nitidus, and Rattus sp. Hepatocystis vassali occurred in squirrels Callosciurus flavimanus; erythrocytic stages and liver merocysts were seen. Fruit bats Cynopterus brachyotis and one insectivorous bat Hipposideros larvatus, harbored closely related but as yet unidentified hemosporidia. Piroplasms were found only in carnivores: ferret-badgers Melogale personata, palm civets Paradoxurus hermaphroditus, and mongooses Herpestes javanicus.
Male mammals show a diverse array of mating bonds, including obligate monogamy, unimale and group polygyny and promiscuity These are associated with a wide variety of different forms of mate guarding, including the defence of feeding and mating territories, the defence of female groups and the defence of individual receptive females. Female mating bonds include long-term monogamy, serial monogamy, polyandry and promiscuity Both male and female mating behaviour varies widely within species. Variation in male mating behaviour is related to the effect of male assistance in rearing young and to the defensibility of females by males. The latter is, in turn, related to female ranging behaviour and to the size and stability of female groups. Much of the variation in mammalian mating bonds and systems of mate guarding can be attributed to differences in these three variables.
Article
The relationship between fluctuating asymmetry in horns of gemsbok (Oryx g. gazella) and a number of fitness components was determined in a field study in Etosha National Park, Namibia. The length and width of horns and skull length demonstrated fluctuating asymmetry. Both males and females with asymmetric horns were in poorer condition than symmetric individuals. Individuals of both sexes widi symmetric horns more often won aggressive interactions at waterholes. Although symmetric individuals spent more time in dense vegetation, their vigilance rate was not higher than that of asymmetric individuals. Ter- ritorial, single males had more symmetric horns than males in herds, suggesting that mating success was inversely related to horn asymmetry. Females with symmetric horns more often had calves than asymmetric females. Horn asymmetry thus appears to reliably reveal phenotypic quality as demonstrated by a suite of fitness components. Key words: developmental stability, dominance, mating success, natural selection, predation, sexual selection. (Behav Ecol 7:247—253 (1996))
Article
Summary(1) The savanna ungulate faunas of the North American Miocene were broadly similar to those of present-day East Africa in terms of overall morphological and taxonomic diversity. However, the predominant ungulates of the African faunas are bovids, which possess bony horns that are primitively sexually dimorphic in their occurrence. The predominant ungulates of the North American Tertiary were equids, camelids and oreodonts, which all lacked horns. A limited number of horned ruminants were present, but these were largely Miocene immigrants from Eurasia. Horns were also absent from the large-bodied herbivores in the endemic faunas of South America and Australia.(2) The absence of horns in equids and tylopod artiodactyls is unlikely to be due to genetic insufficiency. Bony horns were present in brontotheres, which were closely related to equids, and in protoceratids, which were closely related to camelids. Nasal horns were present in one oreodont genus.(3) Studies on living ungulates show that a strong correlation exists between habitat type, feeding behaviour, social behaviour and morphology. It is possible to use the morphological remains of extinct ungulates to reconstruct the types of feeding and social behaviour, and to use the distribution of morphologies and body sizes in a community of mammals, in conjunction with geological and paleobotanical evidence, to reconstruct the type of habitat.(4) The importance of the post-Eocene climatic changes to the history of mammalian evolution is stressed. Continents at higher latitudes have become increasingly seasonal in terms of temperature and rainfall since the equable global conditions of the early Tertiary. Savanna mosaic were the predominant biome in North America by the early Miocene, and in Eurasia by the middle Miocene. Living temperate-latitude species of ungulates may not be a reliable guide for the assessment of the interrelationship between behaviour and morphology in an evolutionary perspective, as their behaviour may have been recently adapted to a habitat type that has only been in existence since the Pleistocene.(5) The primitive condition in eupecorans and protoceratids is the absence of horns, with the presence of large sabre-like canines in the males. The first horned members of these divisions had horns in the males only. Small present-day antelope, where horns may also be present in the females of the species, are probably secondarily small.(6) Horns were acquired independently in ruminant artiodactyls at least three times, and a maximum number of seven times is not unlikely. In each case, horns first appeared at a critical body weight of about 18 kg, and in correlation with a change in habitat from closed to open woodland.(7) Horns in living ruminants are associated with territorial defence by males holding exclusive feeding and reproductive territories in woodland habitats. Such behaviour in present-day antelope is correlated with a body size of greater than 15 kg and a folivorous diet. It is argued that horns evolved in ruminant artiodactyls on the adoption of this type of territorial behaviour once the critical combination of body size, diet and habitat type had been attained in their evolution from small, essentially frugivorous, forest-dwelling animals.(8) Perissodactyls never evolved sexually dimorphic bony horns of the type seen in ruminant artiodactyls. This is because their foraging and digestive strategies necessitate a larger daily intake of food. In a woodland habitat they were never able to adopt a feeding area small enough to make exclusive territory maintenance an economical proposition. Territory holding in male perissodactyls is seen, but under the opposite conditions of habitat to territorial behaviour in ruminant artiodactyls.(9) Study of the morphology and paleoecology of oreodonts suggests that they were woodland herd-forming browsers with exclusively folivorous diets. They probably had some forestomach fermentation, but did not chew the cud. Similar studies of Tertiary camelids suggest that they were predominantly selective browsers eating herbage at a low level in open country and formed mixed-sex feeding groups. These combinations of feeding and social behaviour suggest a more open structure of the mid-Tertiary habitat in North America than in Eurasia.(10) Studies of the behaviour and morphology of living members of the Ruminantia, and of the morphology and paleoecology of their fossil ancestors, suggest that they were primitively tree browsers living in closed woodland habitats. Such habitats were abundant in the Old World, but in limited supply in North America during the Oligocene, where the protoceratids were the only ungulates to parallel the eupecoran type of feeding and social behaviour. South America appears to have had an even more open habitat in the Oligocene than North America, and no parallel to the eupecorans was seen amongst the indigenous ungulates. The radiation of the Bovidae into open grassy habitats in the Pliocene may have been dependent on the immigration of grazing equids into the Old World.(11) I conclude that there was a difference in habitat structure between North America and the Old World during the Tertiary. The food resources in North America were more widely dispersed, and this may have been the result of the trees being more widely spaced. A possible causal mechanism for this was the stable land mass of the North American continent during the Tertiary, resulting in a more continental climate, with a more severe effect of the post-Eocene seasonality on the vegetation. The faunal record of the two continents also implied a greater density of trees in the Old World.(12) Thus most endemic North American ruminants did not evolve horns because, at the critical combination of body size and diet seen in the evolution of horns in the Old World ruminants, the dispersal of the food resources within the vegetation was too great for an effective home range to be maintained as an exclusive territory.(13) Attention is drawn to the dangers of constructing evolutionary stories about living animals without primary reference to the fossil record to see if the hypotheses are upheld, and of assuming that fossil animal communities can be made to fit models of existing communities.