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The intensity of sexual selection predicts weapon size in male bovids
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.
... In bovids, horns have several functions. In males, during intrasexual competition for mates, horns are either used as a weapon to attack the opponent, or as a defensive shield [14,15]. Females use their horns to defend themselves and their offspring and, like males, as weaponry in intrasexual competition for resources [14,16,17]. ...
... The In bovids, horns have several functions. In males, during intrasexual competition for mates, horns are either used as a weapon to attack the opponent, or as a defensive shield [14,15]. Females use their horns to defend themselves and their offspring and, like males, as weaponry in intrasexual competition for resources [14,16,17]. ...
... In males, during intrasexual competition for mates, horns are either used as a weapon to attack the opponent, or as a defensive shield [14,15]. Females use their horns to defend themselves and their offspring and, like males, as weaponry in intrasexual competition for resources [14,16,17]. Horns can also be used for self-grooming by scratching body parts that are difficult to reach [18,19]. ...
Simple Summary
The horns of dairy cows are typically removed at a young age. However, surprisingly little is known about the biological function and the role of horns in the regulation of body temperature, or about the potential effects of horn removal for the cow. Farmers reported that horns get warmer during rumination, and studies on goats indicate that horns help with the regulation of body temperature. To study the possible function of horns in the regulation of body temperature in dairy cows, we used infrared thermography to measure the superficial temperature of the horns, eyes, and ears of 18 cows on three different farms in the Netherlands. Social and non-social behaviours of these cows were registered as well. Based on environmental temperature, humidity, and wind speed, the heat load index (HLI) was calculated as a measure of the heat load experienced by a cow. The temperature of the horns rose by 0.18 °C per unit HLI, indicating that the horns serve to lose heat. Dehorned cows had higher eye temperatures than horned cows, though this result may not be reliable due to the low sample size and experimental setup. We did not, however, find changes in horn temperature during rumination, nor with any other behaviours. Our study supports a role of horns in the regulation of body temperature, but not related to rumination. These results should be considered when measuring the potential effects of horn removal, a painful procedure.
Abstract
Dairy cattle are typically disbudded or dehorned. Little is known, however, about the biological function and role of horns during thermoregulatory processes in cattle, and thus about the potential physiological consequences of horn removal. Anecdotal evidence suggests that dairy cow horns increase in temperature during rumination, and few studies on other bovid species indicate that horns aid thermoregulation. The objective of this study was, therefore, to elucidate a possible thermoregulatory function of the horns in dairy cattle. Using non-invasive infrared thermography, we measured the superficial temperature of the horns, eyes, and ears of 18 focal cows on three different farms in a temperate climate zone under various environmental circumstances. Observations of social and non-social behaviours were conducted as well. Based on environmental temperature, humidity, and wind speed, the heat load index (HLI) was calculated as a measure of the heat load experienced by a cow. The temperature of the horns increased by 0.18 °C per unit HLI, indicating that horns serve the dissipation of heat. Dehorned cows had higher eye temperatures than horned cows, though this result should be interpreted with caution as the low sample size and experimental setup prevent casual conclusions. We did not, however, find changes in horn temperature during rumination, nor with any other behaviours. Our study thus supports a role of horns in thermoregulation, but not related to rumination. These results should be considered when assessing the potential consequences of horn removal, a painful procedure.
... What may explain these differences between species? Although previous research has demonstrated that dimorphism in body size and horns is linked primarily to the opportunity for sexual selection (Loison et al 1999;Bro-Jørgensen 2007), dimorphism in coloration and the presence of manes, beards, hair tufts, and other pelage appendages is less well understood. ...
... Conversely, males of nonterritorial species follow and defend females directly, particularly when they are in estrus (Gosling 1986;Isvaran 2005). Although molecular paternity studies remain few (Coltman et al. 2002;Festa-Bianchet et al. 2019), evidence from behavioral studies suggests that territoriality may reduce the potential for polygyny, as females often range over multiple territories, forcing males to share reproductive opportunities with rivals (Gosling 1986;Bro-Jørgensen 2007, 2011. By contrast, access to estrous females in non-territorial systems is generally controlled by males that attain top rank in dominance hierarchies and monopolize breeding groups by excluding subordinates from mating (Gosling 1986;Bro-Jørgensen 2007, 2011. ...
... Although molecular paternity studies remain few (Coltman et al. 2002;Festa-Bianchet et al. 2019), evidence from behavioral studies suggests that territoriality may reduce the potential for polygyny, as females often range over multiple territories, forcing males to share reproductive opportunities with rivals (Gosling 1986;Bro-Jørgensen 2007, 2011. By contrast, access to estrous females in non-territorial systems is generally controlled by males that attain top rank in dominance hierarchies and monopolize breeding groups by excluding subordinates from mating (Gosling 1986;Bro-Jørgensen 2007, 2011. For a given female group size, non-territorial strategies may therefore be expected to show a greater potential for male polygyny, which translates into heightened competition and stronger selection for male dimorphic traits than in territorial systems. ...
Among mammals, bovids provide some of the most striking examples of sexual dimorphism in colouration and pelage appendages, such as beards and manes. This dimorphism is usually assumed to have evolved through sexual selection on males in the context of intra- or intersexual communication. However, the sexes coloration and pelage appendages look similar between the two sexes in several bovid species thought to be characterized by large opportunities for sexual selection, hinting at fitness costs of dimorphic traits due to other selection pressures. This study applies the comparative method with phylogenetic control to identify the factors promoting and constraining the evolution of dimorphism in coloration and pelage appendages across bovids. We found that trait dimorphism correlated positively with large breeding group size, an indicator of the intensity of sexual selection, and negatively with male territoriality, which is also likely to affect the operation of sexual selection. The relative rarity of color and pelage dimorphism in species with territorial mating systems may be explained by weaker sexual selection due to difficulty in monopolizing females and/or sexual selection targeting other traits, such as territorial quality as an extended phenotype. We also found that dimorphism in color and pelage was reduced in species spending more time in mixed-sex groups outside the breeding season, possibly due to increased predation costs from non-uniformity. This suggests that benefits from integration into mixed-sex groups select against the extravagant male morphologies otherwise promoted by sexual selection.
... 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]. ...
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.
... /2025 to be driven by sexual selection. For example, in bovids, male horn length correlates negatively with male territoriality and positively with social group size, two variables that are suggested to impact the average number of mates per male (Bro-Jørgensen 2007). ...
... The correlation between body mass and armamented females in bovids is consistent with the hypothesis that heavier Stegosaurus species would show lesser magnitudes of sexual dimorphism than the lighter H. mjosi. Across stegosaur taxa, greater magnitudes of dimorphism in species like H. mjosi could be hypothesized to correlate with 1) lekking, 2) larger and less territorial social groups (Bro-Jørgensen 2007), and 3) perhaps polygyny, given the interspecific variation in thyreophoran osteoderms. Indeed, H. mjosi have been found in multi-individual mass death assemblages consistent with sociality (JRDI 5ES Quarry). ...
Stegosaurus is one of the most iconic organisms in Earth’s history due to its impressive dermal osteoderms. These consisted of throat ossicles, two pairs of posterior tail spikes, and an estimated 18 large plates arranged in two staggered rows along the neck, back, and tail. The function and evolution of these structures in stegosaurs is a quintessential question in dinosaur paleontology. Although sociosexual display has become a popular explanation of plate function, sexual dimorphism in dinosaurs has been a contentious topic. Spikes are meanwhile often described as defensive structures, but we consider an additional intra-sex combat function worthy of further study.
To investigate the questions surrounding stegosaur osteoderms, we reevaluate variation in North American stegosaur osteoderms, with attention to hypothesized sexual dimorphism in Hesperosaurus , a close relative of Stegosaurus with similar osteoderms. We correct errors in previous analyses, address challenges of the dimorphism hypothesis, and use outline analysis and effect size statistics to provide further statistical support that one sex possibly had larger, wide/broad plates (hypothesized male), while the other sex had smaller, tall/narrow plates (hypothesized female). Stegosaur plates are a difficult case study for sexual variation because the multiple plates borne by a single individual can result in datasets that violate assumptions of independence. Despite appreciable variation from head to tail along an individual, the variation in size and shape seen in Hesperosaurus plates is still consistent with the presence of sexual variation as evidenced by both size-independent, two-dimensional outline analysis as well as size-versus-shape regression analysis of principal component data, with high confidence in the latter.
We note pathologies on stegosaur caudal vertebrae, particularly in old adults of wide-morph Hesperosaurus , as well as in Stegosaurus tail spikes. These pathologies are consistent with not just predator–prey interactions but also intraspecific combat. There currently is no unambiguous indication of dimorphism in stegosaur thagomizers because alternative hypotheses, such as ontogenetic or interspecific variation, are not yet fully tested.
Finally, we present a hypothetical model of stegosaur osteoderm evolution and function based on fossil and extant evidence. Osteoderms adapted for defense in earlier thyreophorans likely became exapted for sexual display (i.e., plates) and intra-sex combat (e.g., tail spikes in stegosaurs, clubs in ankylosaurs). Therefore, sexual selection may have initiated the differentiation of plates from the terminal thagomizer and induced the unique asymmetry of staggered plates in Stegosaurus and Hesperosaurus . Osteoderm development in hypothesized males could have been driven to physiological upper limits by sexual selection, while the intensity of predation pressure dictated osteoderm development in hypothesized females. Increasing predation pressure might correlate with large body size, open habitats, spine-like plate processes, long tail spikes, and gular ossicles. Within this hypothetical evolutionary model, Hesperosaurus would represent a stegosaur taxon with a high magnitude of sexual dimorphism, consistent with its smaller body size, rounder plates, presumed lack of gular ossicles, and niche occupation within a wet, forested, vegetation-rich habitat in the northern Morrison foreland basin.
The question of stegosaur plate function requires us to look more broadly at thyreophoran osteoderm variation and evolution and to consider complex, multifactor models that incorporate many previously proposed hypotheses. We must sort between primary versus secondary functions of plates, spikes, and ossicles and how these functions changed over time.
... Finally, in supplementary text SIV we compare the power of our analyses with the power that would be achieved in family-level analyses of vertebrates. Our aim is to explicitly compare the statistical power in our phylogenetically overdispersed set of taxa with the power of typical clade-specific comparative analyses of sexual selection that would include most species within a taxonomic family (e.g., Bro-Jørgensen 2007;Ng et al. 2017;Beltrán et al. 2021). Taxonomic families are sometimes chosen by researchers because species within the same family share the same proxy traits and/or because they are a frequent target for dedicated phylogenetic studies. ...
... 6, S13; supplementary text SIV). This is important because clades with a few hundred species represent the sample size used for many family-level, trait-centered comparative analyses of sexual selection (Bro-Jørgensen 2007;Ng et al. 2017;Beltrán et al. 2021) and of speciation rate (Kraaijeveld et al. 2011;Cally et al. 2021;Anderson and Weir 2022;Helmstetter et al. 2023). ...
... Caro et al. (2003) found that different horn morphologies were associated with different social systems and fighting strategies in bovids and cervids (e.g., straight horns in monogamous, solitary species). Bro-Jørgensen (2007) found that male horn length in bovids was positively related to breeding-group size, a measure of the potential for polygyny. An analysis of weapon evolution in harvestmen (Arthropoda: Opiliones) found a correlation between sexual dimorphism and male dimorphism (i.e., variation in weapon size among males; Buzatto et al., 2014). ...
... There is a large literature that has compared the evolution of sexually selected traits in males and females (e.g., Baker & Wilkinson, 2001;Burns, 1998;Ord & Stuart-Fox, 2006;Wiens, 1999). However, this literature has rarely focused specifically on weapons (but see, e.g., Bro-Jørgensen, 2007;Emlen et al., 2005). Overall, we found that weapons were generally strongly related in their evolution between males and females across chameleons. ...
The evolution of sexually selected traits is a major topic in evolutionary biology. However, large-scale evolutionary patterns in these traits remain understudied, especially those traits used in male-male competition (weapons sensu lato). Here, we analyze weapon evolution in chamaeleonid lizards, both within and between the sexes. Chameleons are an outstanding model system because of their morphological diversity (including 11 weapon types among ~220 species) and a large-scale time-calibrated phylogeny. We analyze these 11 traits among 165 species using phylogenetic methods, addressing many questions for the first time in any group. We find that all 11 weapons have each evolved multiple times and that weapon origins are generally more frequent than their losses. We find that almost all weapons have each persisted for >30 million years (and some for >65 million years). Across chameleon phylogeny, we identify both hotspots for weapon evolution (10 types present per species) and coldspots (all weapons absent, many through loss). These hotspots are significantly associated with larger male body size, but are only weakly related to sexual-size dimorphism. We also find that weapon evolution is strongly correlated between males and females. Overall, these results provide a baseline for understanding large-scale patterns of weapon evolution within clades.
... Mountain ungulates, especially Caprinae, show a remarkable diversity of social organisation and morphological adaptations, thus offering the opportunity to compare and test the insights into sociality and mating systems developed by Jarman (1974) from Tropical antelopes. Furthermore, within mountain ungulates, some Caprinae are prized 'trophy' game animals for human hunters, because of the often spectacular male horns, which have evolved to assess dominance in ritualised fights and achieve mating rights (Coltman et al. 2002, Bro-Jørgensen 2007. In fact, insights into the behavioural mechanisms acting on male sexual dimorphism can provide important information to avoid anti-Darwinian effects of hunting regimes on hunted populations (e.g. ...
... The level of mate monopolisation can be linked to various forms of sexual dimorphism, such as difference in body size, colouring, shape and size of weaponry (Andersson 1994). In bovids, body mass and horn size are targets of sexual selection (Bro-Jørgensen 2007, Tidière et al. 2020; horn size, however, may also depend on combat style, so horn dimorphism is less likely to be related to polygyny than body mass. Therefore, sexual dimorphism in the Caprinae was categorised primarily on the basis of body mass dimorphism, into four classes: 'low' (<10%), 'mediumlow' (10-30%), 'medium-high' (30-50%) and 'high' (>50%). ...
1. Mountain ungulates of the subfamily Caprinae, including wild sheep, goats and goat-antelopes, show remarkable interspecific diversity in habitat preferences, social organisation and morphological features. We review how this diversity relates to their mating behaviour.
2. After introducing the ecology of mating systems and the evolution of the Caprinae, we investigate the pairwise, sequential relationships between habitat preferences, social behaviour, level of polygyny, and morphological features, and discuss the ecological processes underlying the patterns of mate monopolisation and acquisition.
3. From forest-dwelling, solitary, monogamous and monomorphic goat-antelopes, to highly dimorphic, polygynous and social wild sheep and goats inhabiting open landscapes, mountain ungulates reveal a close relationship between habitat openness and sexual dimorphism, through the level of sociality and that of mate monopolisation.
4. Although over the last few decades some information has been collected on the biology of Caprinae, our understanding of determinants of their mating systems is still hampered by limited data to estimate opportunities for sexual selection, as well as uncertainties over the occurrence and maintenance of alternative reproductive tactics, and lack of information on female mate choice.
5. The study of mating systems and that of the factors influencing them play a key role from an evolutionary and conservation standpoint. This is relevant to the Caprinae, whose main habitat is expected to be strongly affected by the ongoing climatic change, with potential effects on the phenology of their mating systems, and whose economic value is relevant for consumptive and nonconsumptive uses. A better understanding of the diversity and ecology of mating systems will require a wealth of additional field observations on male and female behaviour, as well as genetic assessments of reproductive success.
... 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). ...
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.
... 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]. ...
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.
... Sexual size dimorphism (SSD) is a significant source of phenotypic variation in many species, driven by a combination of sexual selection, fecundity selection, and ecological factors, which leads to niche divergences between the sexes (Andersson, 1994;Arnold & Wade, 1984;Bro-Jørgensen, 2007;Herrel et al., 2012;Shine, 1989). SSD refers to both the direction of intersexual differences and the magnitude of the divergence (Lovich & Gibbons, 1992). ...
Allometry, the relationship between body size and the size of other body parts, explains a significant portion of morphological variation across biological levels, at the individual level, within and between species. We used external morphology measurements of 6 Triturus (sub)species, focusing on the T. marmoratus species group, to explore allometric parameters within and between taxa. We tested for allometry of sexual size dimorphism in body, head, and limb dimensions and examined whether intraspecific allometry directed evolutionary allometry, as described by Rensch’s rule. Our findings indicated that female-biased trunk and head dimensions exhibited positive allometry, whereas male-biased limb dimensions showed isometric relationships or weak correlations with body size. Morphological divergences between sexes occurred along common allometric slopes, most often through changes in the intercepts. Among taxon, comparisons revealed that (sub)species diverged in the direction of the allometric slopes. In line with Rensch’s rule, sexual size dimorphism in female-biased traits significantly decreased as overall body size increased. However, the observed intraspecific allometric parameters deviated from theoretical expectations because the steepest allometric slopes for female-biased traits were recorded in the larger species. Our results contribute to understanding the dynamics of allometric relationships and sexual dimorphism in amphibians and provide a robust baseline for future comparative analyses.
... Een belangrijk wapen tegen predatoren waarover (veel) volwassen runderen beschikken zijn horens (Stankowich & Caro, 2009;Bro-Jørgensen, 2007 ...
A desk study on strengthening the wolf-resistant capacity of cattle in natural grazing. After years of absence, the wolf has also settled in the Netherlands. Since then, the protection of sheep against predation by wolves has received considerable attention and various protective measures are available for application in practice. However, these measures are not a solution for the protection of cattle used for natural grazing, because these cattle often independently traverse large areas. The question of how these cattle can be better equipped to defend themselves against wolf attacks is the reason for this desk study. In this report, recommendations are made that are based on the relevant scientific literature and on contacts with (inter)national (practice) experts.
... Among terrestrial mammals, artiodactyls show striking variation in SSD in body mass and horn/antler size (Bro-Jørgensen, 2007); however, the latter may also depend on fighting style, and hereafter SSD will refer to differences in body mass. From the evolutionary standpoint, male-biased SSD in ungulates is thought to be driven primarily by sexual selection acting on males (Andersson, 1994;Darwin, 1871;Loison et al., 1999). ...
Male‐biased sexual size dimorphism (SSD) is common in ungulates. The dominant scenario for the evolution of ungulate SSD suggests that habitat openness leads to greater SSD by increasing group size and thus sexual selection through male–male competition for mates. At a more proximate level, adaptive changes in SSD may result from the plastic response of individuals to environmental variation. In this study, we used 161,948 body mass data from a seasonally size‐dimorphic species, the northern chamois Rupicapra rupicapra, to examine the role of forest cover and other environmental variables in the expression of SSD. Data were collected from individuals hunted in the Austrian Alps, grouped into 28 mountain ranges with different forest cover, geological substrate and population density. Population‐specific growth curves were fitted using monomolecular models, and SSD was calculated as the log‐transformed ratio of male to female asymptotic body mass. A path model in which environmental factors indirectly influenced SSD via male or female body mass suggested that SSD increased with increasing density via reduced female body mass and decreased on siliceous substrates via reduced male body mass. Forest cover was negatively associated with body mass in both sexes, but not with variation in SSD.
... En rumiantes silvestres, se ha evidenciado que los cuernos, además de servirles para su defensa ante otros animales, también funcionan como signos de atracción para las hembras en la temporada reproductiva (Bro-Jørgensen, 2007). Los cuernos son proyecciones óseas y huecas del hueso frontal, recubiertas por una capa de queratina externa (Picard et al. 1996;Algra et al. 2023) Debido a que son estructuras en constante crecimiento, se encuentran densamente inervadas y provistas de vasos sanguíneos. ...
... The secondary sexual traits, such as antlers or horns, have evolved through sexual selection (Clutton-Brock et al. 1980;Coltman et al. 2002;Bro-Jørgensen 2007). Males utilize horns and antlers to demonstrate their quality when selecting mates or displaying fighting ability towards other males, serving as weapons during combat (Clutton-Brock et al. 1980;Hoem et al. 2007;Bergeron et al. 2008;Vanpé et al. 2007). ...
Hunting directly impacts the population dynamics of ungulates and can have a significant effect on the quality of phenotypic traits such as horns or antlers. In Poland, following a demographic collapse in the 1990s and the introduction of a hunting ban in 2001, the population of moose (Alces alces) has increased from 1,800 to over 20,000 individuals, recolonising its former range. As the moose is a charismatic species and a popular subject for nature photography, we analysed changes in antler size and shape in this cervid between 2005 and 2021 based on photos of male moose and antler casts provided by photographers or available in social media. Our findings indicate that during the hunting ban, the probability of observing the cervina antler type significantly decreased over time, from 47% in 2012 to 28% in 2021. Meanwhile, the probability of observing the intermediate and palmate antler types significantly increased from 44 to 53% and from 9 to 19%, respectively. The mean number of tines significantly increased from 3.2 in 2005 to 4.7 in 2021, and the antler size index significantly increased from 3.4 to 3.9. The most likely mechanism behind the observed changes could be the ageing of a population released from hunting pressure. We also observed regional variation in antler size, which is likely related to differences in environmental conditions. Our study serves as an example of how passive citizen science can contribute to our understanding of ecological trends and the quantification of population patterns. It also has important implications for management of species affected by trophy hunting.
... Based on the horn's possible role in thermoregulation, it was hypothesized (Baars et al., 2019) that the differences found would reflect a greater cold stress of horned cows due to heat loss via the horns. However, the role of the horn in thermoregulation was not confirmed in other works (Bro-Jørgensen, 2007;Kiltie, 2008;Wohlers and Stolz, 2022;Reiche et al., 2023). Taken together and with several limitations (small sample sizes, pooled samples and confounding factors, lack of evidence), these studies do not provide robust evidence for horn status-related effects on milk. ...
... This may, in turn, select for larger appendages. For example, the size of male weapons has been found to increase with group size in both bovids and pseudoscorpions (Zeh, 1986;Bro-Jørgensen, 2007). Euphrynichus bacillifer, the species with the longest pedipalps relative to body length in this study, is also one of the few Amblypygi species that can successfully be kept communally in captivity (M.Seiter, pers obvs). ...
The link between form and function is key to understanding the evolution of unique and/or extreme morphologies. Amblypygids, or whip spiders, are arachnids that often have highly elongated spined pedipalps. These limbs are used to strike at, and secure, prey before processing by the chelicerae. Amblypygi pedipalps are multifunctional, however, being used in courtship and contest, and vary greatly in form between species. Increased pedipalp length may improve performance during prey capture, but length could also be influenced by factors including territorial contest and sexual selection. Here for the first time, we use high-speed videography and manual tracking to investigate kinematic differences in prey capture between amblypygid species. Across six morphologically diverse species, spanning four genera and two families, we create a total dataset of 86 trials (9-20 per species). Prey capture kinematics varied considerably between species, with differences being expressed in pedipalp joint angle ranges. In particular, maximum reach ratio did not remain constant with total pedipalp length, as geometric scaling would predict, but decreased with longer pedipalps. This suggests that taxa with the most elongated pedipalps do not deploy their potential length advantage to proportionally increase reach. Therefore, a simple mechanical explanation of increased reach does not sufficiently explain pedipalp elongation. We propose other factors to help explain this phenomenon, such as social interactions or sexual selection, which would produce an evolutionary trade-off in pedipalp length between prey capture performance and other behavioural and/or anatomical pressures.
... Comparatively, bovids achieve mature headgear in a similar length of time; however, their horn sizes continue to increase with age (Geist, 1971;Hall, 2015). Given that sexually selective pressures have often been attributed to the evolution of headgear in other male pecorans (Bro-Jørgensen, 2007;Geist, 1966), it is possible that osteohistological signals of rapid growth in A. americana headgear, such as fibrolamellar bone at the periosteal surface, could indicate similar sexually selective pressures driving what Mitchell and Maher (2006) refer to as "precocial [prong]horn growth" in A. americana. The high density of secondary osteons at the periosteal surface in the female core suggest a comparatively reduced rate of growth, which is in-line with the assumption that females would not experience as strong sexually selective pressures as males for headgear. ...
Cranial bony projections (“headgear”) have diverse forms and functions, such as defense, species recognition, mate selection, and thermoregulation. Most commonly, they are associated with the artiodactyl infraorder, Pecora. All pecoran headgear—antlers, horns, ossicones, and pronghorns—are osseous protrusions of the frontal or parietal bone with an integumentary covering, although there is taxonomic, developmental, and compositional variation. However, compared with other pecorans, there is a dearth of literature addressing extant antilocaprids—Antilocapra americana. This study provides a foundational osteohistological description of A. americana pronghorn cores in order to start building a framework to better understand the complex interplay among microanatomy, development, behavior, environment, and phylogenetic history of pronghorn headgear. Osteohistological analysis of adult A. americana pronghorn cores reveal the inner medullary region is composed of trabecular bone. Based on similar studies in bovids, we propose that these trabeculae may function to reduce the effects of repeated loading incurred by intraspecific combat. The deep aspect of the outer region was found to be composed of compacted coarse cancellous bone and primary bone remodeled to dense Haversian bone, in both male and female specimens, respectively, and superficially composed of highly vascularized fibrolamellar bone. The presence of fibrolamellar bone may indicate that the bone is fast‐growing, and its presence at the periosteal surface suggests protracted growth of the pronghorn core beyond sexual maturity.
... Sexual selection is a major force driving the evolution of morphological and behavioral diversity. Studying factors that affect the strength of sexual selection can contribute to understanding patterns of this diversity, as well as of other evolutionary consequences of sexual selection, such as trade-offs associated with the development and maintenance of sexual traits (Rowe and Houle 1996;Bonduriansky and Chenoweth 2009), the degree of sexual dimorphism (Bro-Jorgensen 2007;Kelly 2008;Grath and Parsch 2016;Hamalainen et al. 2018), sexual conflict (Chapman et al. 2003;Bonduriansky and Chenoweth 2009), speciation (Maan and Seehausen 2011), and ecological processes (Giery and Layman 2019). ...
Higher male:female operational sex ratio (OSR) is often assumed to lead to stronger sexual selection on males. Yet, this premise has been directly tested by very few studies, with mixed outcomes. We investigated how OSR affects the strength of sexual selection against two deleterious alleles, a natural ebony mutant and a transgenic GFP insertion, in Drosophila melanogaster. To this end, we estimated the relative paternity share of homozygous mutant males competing against wild-type males under different OSRs (1:2, 1:1, 2:1). We also manipulated the mating pool density (18, 36, or 54 individuals) and assessed paternity over three consecutive days, during which the nature of sexual interaction changed. The strength of sexual selection against the ebony mutant increased with OSR, became weaker after the first day, and was little affected by density. In contrast, sexual selection against the GFP transgene was markedly affected by density: at the highest density, it increased with OSR, but at lower densities, it was strongest at 1:1 OSR, remaining strong throughout the experiment. Thus, while OSR can strongly affect the strength of sexual selection against “bad genes,” it does not necessarily increase monotonically with male:female OSR. Furthermore, the pattern of relationship between OSR and the strength of sexual selection can be locus-specific, likely reflecting the specific phenotypic effects of the mutation.
... However, this finding is not unexpected 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 (e.g. Preston et al. 2003;Bro-Jørgensen 2007;Knierim et al. 2015). The implications of the findings by Marquette et al. (2021) were 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 too large that disbudding could lead to tissue trauma. ...
Disbudding, removal of the horn buds, is performed for economic and practical reasons: to prevent bullying and injury to other animals (with implications for productivity and carcass damage, respectively) and human safety during handling. Mitigation of pain associated with the disbudding of calves is necessary to limit the pain-stress response that induces altered behavioural and physiologic states. The most common recommendation in Animal Welfare Guidelines is to disbud calves before they attain 2 months of age. From birth to 2 months of age the horn bud is not attached to the skull, which makes the removal of the horn bud and adjacent cells easier. The objectives of this review are to describe (1) the different methods of disbudding, (2) the pain responses associated with each of those methods and (3) how age and pain mitigation strategies affect those responses.
... [3][4][5][6] For many weapons, this means increases in length or overall weapon size. 2,[7][8][9][10][11] Longer weapons permit a male to touch, strike, grab, or flip an opponent before that rival can do the same. [12][13][14][15][16] Longer weapons may also function as agonistic signals-deterrentssettling contests before they escalate into dangerous battles. ...
... We inferred whether the preopercular spines function primarily offensively or defensively by characterizing the diversity, development, and evolution of spine shape among sculpins of the subfamily Oligocottinae, a group of 16 species that inhabit nearshore and intertidal habitats along the Pacific coast of North America. These fishes have diverse preopercle spine shapes and sizes, and they engage in a range of behaviors linked to sexual selection including territoriality, intraspecific combat, and ritualized posturing and threat displays, making this a useful group to test the potential role of sexual selection in the evolution of the shape of the preopercle (Atkinson, 1939;Morris, 1952;Ragland and Fischer, 1987;Bro-Jørgensen, 2007;Emlen, 2014). We use 3D geometric morphometrics to quantify the degree of sexual dimorphism, fluctuating asymmetry, and allometric shape change within each species. ...
From dissuading predators to gaining an edge on intraspecific rivals, animals have evolved weapons to meet various needs. Those with the most extreme weapons often use them to battle conspecifics, but some weapons defend against predation and others signal prowess to prospective mates and rivals. Many fishes have evolved armaments, but humans rarely observe these structures in action due to the inaccessibility of many weapon-bearing fish species. For example, how sculpins use the diverse horn-like spines that project from their head remains a mystery. We deduced the function of the weaponized preopercle in the 16 species of sculpins in the subfamily Oligocottinae by determining whether they exhibit three well-documented hallmarks of offensive weapons in terrestrial animals: ontogenetic change, sexual dimorphism, and fluctuating asymmetry. Geometric morphometrics of micro-computed tomography (lCT) scans show no sexual dimorphism in preopercular spine shape but reveal phylogenetically widespread ontogenetic shape change. Fluctuating asymmetry is low to moderate across species. Taken together, these results suggest that despite their varied reproductive habits, frequent territoriality, and possession of weapons that resemble bovid horns, oligocottine sculpins evolved their spines primarily to defend against predators.
... A positive evolutionary allometry will also result if larger species have, on average, larger weapons (e.g. Bovidae: Bro-Jørgensen, 2007;Tidière et al., 2017;porcelain crabs, Petrolisthes: Baeza & Asorey, 2012). This may occur if sexual selection on weapon size is stronger in larger species and/or selection for larger weaponry creates a concomitant increase in body size (Summers & Ord, 2022). ...
Across the animal kingdom, exaggerated weaponry is frequently used by one sex to contest access for potential mates. Within species, if disproportionate investment in weaponry confers an advantage to larger individuals, this may result in positive static allometry. It is predicted that the same selective pressures may also lead to positive evolutionary allometry, where larger species bear disproportionately large weapons on average, compared with smaller species. Furthermore, in species with stronger sexual selection, the static allometries of those weapons are expected to steepen. All adult males across the New Zealand sheetweb spider genus Cambridgea bear exaggerated chelicerae, which are used to compete for control of females' webs. Here, we characterize the distribution of chelicera lengths within each sex of 12 Cambridgea species to show that chelicerae almost always exhibit positive static allometry in males while female chelicera lengths are consistently isometric. We use comparative phylogenetic methods to demonstrate that the slopes of static allometries steepen in males of larger species but that the ratio of average chelicera length to cephalothorax width is tightly conserved across taxa, leading to an isometric evolutionary allometry. While static allometries indeed steepen in larger species, possibly due to stronger sexual selection, this conservation of relative trait size suggests that chelicera length is subject to other stabilizing selective pressures. Changes to species body plans might be constrained, while still allowing for disproportionate investment in weapon traits in the upper range of intraspecific body sizes.
... 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. ...
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.
... 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). ...
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. ...
Abstract 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 0.05) 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
... 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. ...
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). ...
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.
... In contrast, the main function of the tusks in the modern dugong is most likely social/reproductive, given their persistence in males and eruption at puberty (Domning, 2001). The function and evolution of tusks as secondary sexual characteristics has been examined in a number of terrestrial mammals (e.g., Bro-Jørgensen, 2007;Emlen, 2008;Geist, 1966;Weissengruber et al., 2005), and in some marine mammals, e.g., walrus Odobenus rosmarus (Emlen, 2008;Fay, 1982;Sjare & Stirling, 1996) and narwhal Monodon monoceros (Best, 1981;Gerson & Hickie, 1985;Silverman & Dunbar, 1980). Tusks as secondary sexual characteristics that aid in the success of male mating opportunities can function as ornaments for sexual display and/or weapons for intra-and/or intersexual interactions (Bro-Jørgensen, 2011;Darwin, 1871;Geist, 1966). ...
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.
... 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). ...
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.
... 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). ...
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.
... 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. ...
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.
... 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. ...
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.
Sexual selection is often invoked to explain the evolution of extravagant morphologies, such as antlers and horns. While the focus is typically on the process of exaggeration of these traits, the functional impact of exaggeration remains a topic of debate. One aspect that has been largely overlooked is how exaggerated structures might impact thermal biology. For example, as a hollow (i.e., non-vascularized or non-perfused) structure increases in size, its surface area and volume change, potentially impacting its ability to obtain and dissipate heat passively. However, if the exaggerated structure is vascularized, or in the case of arthropods, has hemolymph perfusion, then it may be actively used as a thermal radiator to avoid overheating in instances of thermal stress. Based on these and additional examples, we propose that morphological exaggeration may influence how arthropods manage heat exchange with the environment. Ultimately, individuals that bear exaggerated structures may develop ecological innovations that, due to selection or as a corollary effect, maximize effectiveness of thermoregulation. Our essay is divided into four sections. First, we delve on how exaggerated structures, particularly animal weapons, may impact how organisms exchange heat with the environment, and the implications for whole-organism thermoregulation. Second, we use beetles and fiddler crabs to provide experimental evidence of how structural exaggeration may influence thermal biology. Third, we examine macroecological data from arthropods to explore how the size of sexually-selected morphologies varies with changes in environmental temperature. Finally, we synthesize these pieces of evidence to identify significant ecological implications and gaps in knowledge. Through this essay, we aim to ignite discussion on how morphological changes driven by sexual selection can lead to innovations not only in the functional role of morphologies but also in the thermal biology of individuals.
In goitered gazelles, the agonistic behavior of territorial males and bachelors differs not only in intensity, but also in repertoire. The intensity of this behavior is also markedly different in adult and young males. Females largely repeat the behavior of males, up to butting each other, although females are usually hornless.
Phenotypes reflect how organisms adapt to their environments. Hong Kong (HK) feral cattle, a crossbreed of Bos taurus taurus and Bos taurus indicus, present an opportunity to study these adaptations in one of the very few global cattle populations not directly controlled by humans. These cattle are free-ranging since their release from farms in the 1970’s. HK has a subtropical climate, characterized by high humidity and temperatures during the wet season, and scarce precipitation during the dry season. We studied seasonal coat colour changes in HK feral cattle, and sexual dimorphism in body size and horn length. We provide the first evidence of seasonal changes in coat colour in cattle, with paler coats being more common in the wet season, while darker coats prevailed in the dry season. These seasonal changes were influenced by temperature, humidity, solar radiation, and body condition. We found that males were larger and had longer horns than females. Our results show a male-biased sex dimorphism in the HK feral cattle. Additionally, our findings suggest that thermoregulation costs drive colouration in these cattle. The phenotypic plasticity we demonstrate in these subtropical feral cattle improves our knowledge of the adaptations of ungulates to their habitat.
Ruminants are the only group of contemporary mammals that possess headgear in the form of bony appendages, with many species having a keratinous sheath covering them. Headgear can be found in representatives of four out of the six currently existing ruminant families. The structure of these appendages varies among families, resulting in four distinct types of headgear: antlers in the cervid family, permanent horns in bovids, shed horns in the American pronghorn, and ossicones in giraffids. Antlers are bony, branched structures that are shed and regenerated annually. They are exclusively found in males, except in reindeer, where both males and females possess them. Horns, on the other hand, are present in both sexes of bovids and are composed of a bony core covered by a keratinous sheath. They continue to grow throughout an animal's life, remain unbranched, and are not shed. The American pronghorn exhibits unique horn structures where the keratinous sheath is shed and regenerated annually. Ossicones are specific to giraffes and okapis, appearing as bony protrusions covered by skin. It is worth noting that there are ruminants that lack head appendages. These include mouse-deer, which diverged early from the rest of the ruminants before the development of bony protrusions, as well as water deer and musk deer, which lost them secondarily. Despite the anatomical differences between horns and antlers, gene expression analysis suggests similarities in the development of these structures, indicating their homology. It is likely that the bony head protrusions originated more than 20 million years ago in ruminants and underwent extensive transformations during the evolution of these mammals.
Our understanding of the evolution of social mating systems is largely based on an atemporal ecological framework, whereas macroevolutionary and phylogenetic perspectives looking at the causes of mating systems variation are less developed. Here, we present analyses of the evolution of social mating systems in birds at an unprecedented scale, including 66% of the world’s birds and using trait-dependent speciation and extinction models. We found that lekking (no social bond between the sexes) is very rarely lost, in accordance with the hypothesis that a male shifting to investing in one rather than multiple mates would suffer a severe fitness cost. In contrast, resource-defense polygamous lineages (with a weak, transient socio-sexual bond) frequently revert back to monogamy (strong, durable socio-sexual bond) and have an elevated extinction fraction. We tentatively attribute this to the impossibility of females settling on an optimal parental care strategy under this system. Finally, we found that most gains of lekking have been directly from monogamy rather than through an intermediate stage of resource-defense polygamy.
Background: In polygynous species, the development of secondary sexual characters is usually decisive for male reproductive success. However, our understanding about the links between the growth of these traits and reproductive efficiency is still elusive. Most research efforts in this topic have been also focused on adult males, although the development of some secondary sexual characters, like bovid horns, typically starts after birth, continues during the puberty and in some species, such as the common eland, slows or even stops during adulthood. In this study, we investigated the relationships between horn size and testicular function during sexual development in common elands using a comprehensive approach that considers both spermatogenic and sperm parameters.
Methods: Twenty-two non-sexually mature common elands were used for the present study. Horn size, body mass, testes mass, and gonadosomatic index were assessed. Spermatogenic activity was determined by cytological and histological analyses. Sperm concentration, morphology, morphometry, and intramale variation in sperm size were evaluated on epididymal sperm samples. Cluster analysis was performed to explore the influence of age on relationships between horn size and reproductive function.
Results: We found that bigger horns are associated with increased Sertoli cell efficiency and reduced intramale variation in sperm size. Both parameters were not related to one another while they have shown to be associated with enhanced sperm quality in ungulates. Moreover, horn size was positively linked to the testis mass, sperm concentration, and testicular investment in the seminiferous epithelium. Spiral length and basal circumference were the horn traits most strongly correlated with spermatogenic and sperm parameters as well as those responsible for the sexual dimorphism in this species. Cluster analysis rendered two groups: the first one including males ≤30 months old, while the second one those >30 months old. Horn development and reproductive function were still correlated within age groups, with the strongest relationship found between horn size and sperm size homogeneity in males >30 months old.
Conclusion: Taken together, our results indicate that horn size can be regarded as a good index of male reproductive potential during sexual development and provide insights into the role of secondary sexual characters in sexual selection dynamics.
In North America, most ungulate species exhibit life‐history traits typical of long‐lived, iteroparous species wherein young males tend to prioritize essential life functions including body growth and maintenance that constrains allocation of resources to horn, antler, and pronghorn growth. As a result, males of most ungulate species require several years of growth before reaching asymptotic body size and thereafter, peak weapon size is attained. Unique among ungulate species in North America, pronghorn possess a suite of life‐history traits resulting in a precocious (i.e., unusually early development) pace of life relative to other North American ungulates. We tested the hypothesis that the fast pace of life of pronghorn extends to precocious development of large horns, and evaluated how horn size was affected by environmental conditions during the year they were grown and the potential for cohort effects associated with environmental signatures during the year of birth. We evaluated the influence of age and the environment on horn size of pronghorn using data collected from 1,789 male pronghorn harvested from 2019 to 2022 in Wyoming, USA. Pronghorn attained 95% of their peak horn size by 3.5 years old. Climatic conditions influenced horn growth through cohort effects and year of growth pathways. Snow depth during the year of birth positively influenced horn size, whereas the effects of environmental conditions during the year of horn growth were dependent on age. For young animals, snow depth and moderate drought positively influenced horn size during the year of horn growth, but the effect was negligible for prime aged and old animals. The precocious nature of pronghorn extended to their horn growth characteristics, resulting in early attainment of a large proportion of their peak horn size. The unique ecology of pronghorn and rapid attainment of size early in life can allow for greater flexibility to balance hunter opportunity and production of large‐horned males for pronghorn as compared with other ungulates.
Identifying the evolutionary drivers of sexual signal complexity is a key challenge in the study of animal communication. Among mammals, male bovids and cervids often perform elaborate gestural displays during courtship, consisting of ritualized movements of various parts of the body but the causes underlying interspecific variation in complexity of such displays remain poorly understood. Here we apply the comparative method to investigate which factors may have either promoted or constrained gestural repertoire size. We found that sexual selection was a strong predictor of gestural display complexity in male bovids and cervids. Repertoire size was positively correlated with breeding group size, an indicator of the intensity of sexual selection in males. Moreover, repertoires were larger in species adopting nonterritorial and lek breeding mating systems than in species adopting resource-defence territoriality, a finding that can be explained by more emphasis on direct benefits than indirect benefits in resource-defence systems, where male mating success may also be less skewed due to difficulty in monopolizing mates. The results also indicate that gestural repertoire size was positively correlated with the number of closely related species occurring in sympatry. This is consistent with display complexity being selected to facilitate species recognition during courtship and thereby avoid interspecific hybridization. At the same time, repertoire size was negatively associated with male body mass, possibly due to the energetic and mechanical constraints imposed on movements in very large species. By contrast, we found no evidence that the habitat drives selection for complex gestural courtship displays.
In polygynous ungulates, males are often larger than females and bear more elaborate/larger weapons. Quantifying sexual dimorphism in different traits could provide insights into species-specific evolutionary pathways of sexual selection. Concerning the combination of secondary sexual traits, we found that Himalayan tahr (Hemitragus jemlahicus) is unique among the ~20 species in the tribe Caprini, as its body mass di-morphism is ~2-fold greater than the dimorphism in horn size, whereas horn shape appears to be near-monomorphic. Whilst horns show the same growth rate in both sexes, body mass increases faster in males. Considering age variation, dominant, golden-ruffed males are also heavier than brown-ruffed, lower-ranking males. Unlike most bovids, male-male competition in tahr does not seem to have influenced weapon development , suggesting a lower importance of horns in male-male competition compared to body mass, as their unritualized combat style also suggests. Our study highlights alternative evolutionary pathways occurring in the Caprinae, where intraspecific signals involve different traits, from weapons to pelage features. Accordingly, male tahr use their ruff colour as an 'honest' signal of rank.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
A minimum of 51 tamarao (Bubalus mindorensis) occurred in a 20-km 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.
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.
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.
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.
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.
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.
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.
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
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.
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))
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.
Sexual dimorphism in mammals is not entirely satisfactorily explained by the models that are advanced to account for it among birds. This may be because species‐specific styles of being dimorphic, and of attaining mature dimorphic state, are not clearly recognized. Mature dimorphism is a syndrome involving body size, appearance and weaponry; each facet and the whole syndrome may have functions in both fighting and signalling. The mature dimorphic stage has to be reached by growth and change from juvenile and sub‐adult states.
The occurrence of the separate facets of the dimorphic syndrome are reviewed in species of Bovidae, Cervidae and Macropodidae, large, diverse families of eutherian (the first two) and metatherian mammals, which have broadly similar ecological adaptations. In each family the smallest species tend to be homomorphic, with small, inconspicuous weapons. Greatest dimorphism in size is found in medium‐sized bovids and cervids, and the larger macropodids (in which no species exceeds 100 kg male weight); the range of species showing greatest dimorphism in size also shows the most exaggerated weapons. Mature dimorphism is reached by different patterns of growth, which may be determinate and similar in the sexes (leading to homomorphism), determinate but differing between the sexes, or indeterminate and differing, both of which lead to heteromorphism.
The syndromes of dimorphism and patterns of growth are associated and a classification of styles of dimorphism is presented. The adaptiveness of the styles is suggested in terms of what is known of the socio‐ecology, in particular the male reproductive strategies, of the species. The various styles of heteromorphy appear to be associated with males' way of achieving polygyny: such as by non‐resource‐based territoriality, by dominance‐determined access to oestrous females, or by wandering and formation of a consortship with pro‐oestrous females. The relevance of the species' ecology of use of resources to these styles of dimorphism and mate‐acquisition is briefly discussed.
Abstract. At the National Bison Range (western Montana, U.S.A.) mate choice by female pronghorn, Antilocapra americana, is clearly observable. Females move independently, and copulate once per oestrus. Males cannot force copulation, only temporarily block female movement, and do not monopolize resources critical to females. Females practice three distinct strategies of mate choice. 'Sampling' females visit several harem-holding males, remain with each male a short time, and switch between males at an increased rate as oestrus approaches. Switches often appear to be energetically expensive. The actual mating visit is brief, about 1·5 days. Sampling females always leave males that fail to defend an adequate zone of tranquillity around the harem, but also leave males when there is no apparent cause. The majority (71%) of exits from males are of the latter type. Most sampling females return to and mate with a male that they visited within the week before oestrus. 'Inciting' females behave as samplers unti