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Correlates of horn and antler shape in bovids and cervids

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Abstract

Using analyses that control for phylogeny, we examine whether the shapes of horns and antlers in ungulates are related to style of fighting, environmental factors, mating and social systems, or are simply arbitrary. Many of the predictions that relate horn shape to species' fighting tactics and to their mating and social systems are supported. Bovids with tips facing inwards are likely to wrestle with their horns, be monogamous and solitary, whereas those with tips facing out tend to be polygynous and live in large groups. Smooth horns are used for stabbing, and are found in females of polygynous species living in large groups. In female bovids, twisted horns are found in large species and are used in wrestling whereas, in male bovids, straight horns are found in solitary species. Finally, deer with more than five antler tines tend to be large and to fight by fencing. There was little support for shape of horns and antlers being related to environmental factors, nor were shapes arbitrary as might be expected if they had arisen through female choice. Body size had little effect on these results. In general, monogamous solitary bovids have straight inwardly facing horns whereas polygynous group-living species demonstrate a wide variety of horn shapes.

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... The males of all species bear sexually selected horns growing from the frontal bones which are constructed of a bony core, the os cornu, covered by a horn sheath formed of keratinised epidermis [31]. Bovid horn shape appears to be correlated with male fighting style in intrasexual competition [32]. Although female bovids do not physically compete for mates and are therefore not expected to be subject to sexual selection in the same way as males, female horns are found in roughly half of all extant bovid species [19,33]. ...
... Predator defence, male mimicry, and genetic linkage to males [35] are the most frequently cited explanations for the presence of horns in female bovids. However, predator defence is seldom observed in C. taurinus and is often ineffective [35], although horns may act as a visual deterrent to predators in the open habitat in which this species tends to live [19,32]. Male mimicry in this species is predicted to allow younger males to benefit from remaining in the maternal herd for longer [35], but there are two main problems with this hypothesis. ...
... Horns in females may occur through genetic linkage, and similar expression of linked traits in both sexes is expected in this case. This is possible, but is not universal among bovids because the females of many bovid taxa are hornless [32,33]. The presence of horns in females likely serves some adaptive function [9,13], given the regularity with which they are lost in females of related taxa, and it is possible that female horns are maintained by a combination of factors in C. taurinus. ...
Article
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Sexual selection is thought to be an important driver of adaptation, speciation and extinction. Empirically testing these predictions across macroevolutionary timescales first requires an understanding of the morphology of secondary sexual traits in extant taxa. We used three-dimensional geometric morphometrics to analyse a large sample of the skull of the blue wildebeest, Connochaetes taurinus , in which horns are found in both sexes but only used in intrasexual competition in males. We show that the horns fit several predictions of secondary sexual traits; overall skull shape is significantly correlated with size (R ² = 0.38, p = 0.001), and the sexually selected horns show drastically higher growth rates and variation than any other skull element, supporting previous findings. We also find that despite showing significant sexual dimorphism in shape and size (R ² = 0.21, p = 0.001), allometric growth trajectories of sexes are identical (R ² = 0.01, p = 0.635) and dimorphism is not readily detectable without prior knowledge of sex, and is not possible when shape is corrected for size. Our results show that even with strong sexual selection operating in only one sex, the expression of secondary sexual traits may show characteristic and indistinguishable patterns of growth and variance in both sexes.
... These structures are used for both prey capture (Smilodon saber teeth) and sexual combat (deer antlers, beaked whale tusks), but among extant species, sexually selected weaponry is most prevalent among the artiodactyls in the form of cranial appendages and elongated tusks. Most studies focus on understanding the ecological, social, and phylogenetic underpinnings of horns (Bovidae) and antlers (Cervidae) (Clutton-Brock et al. 1980;Packer 1983;Estes 1991b;Lundrigan 1996;Caro et al. 2003;Bro-Jørgensen 2007;Stankowich and Caro 2009;Goss 2012;), while the factors promoting the evolution, retention, and elaboration of tusks have received little attention (but see Geist 1971;Raia et al. 2015). Here, we investigate the patterns of tusk evolution in artiodactyls while exploring specific ecological factors that might favor their use over cranial weapons (e.g., antlers, horns). ...
... If more than one source provided a mass, we used the average of those values as our mean sex mass. Mating strategies were coded as 0 if monogamous and 1 if polygynous (Caro et al. 2003). Following Stankowich et al. (2014), we used collected habitat use data from IUCN (2017; Version 3.1) and assigned an openness score (0-1) to each habitat type: Temperate Forest (0.2), Tropical Forest (0.1), Savanna (0.7), Temperate Shrubland (0.6), Tropical Shrubland (0.5), Tundra (0.9), Temperate Grassland (0.8), Tropical Grassland (0.8), Wetlands (0.3), Rocky (0.8), Desert (0.95), Marine (0.8), Artificial Grassland (0.8), Urban (0.8), Artificial Marine (0.8), and Caves/Subterranean (0.05). ...
... We edited the IUCN habitat list to only include the primary two to three habitats inhabited per species giving preference to those in which species spend the most time (Stankowich and Caro 2009). Finally, we coded sociality as the typical group size for species: (1) solitary or pairs, (2) 3-10, (3) 11-50, (4) > 50 (Caro et al. 2003). Blinded methods were not used in this study as no behavioral data were recorded and/or analyzed; other experimenters, however, collected different elements of the dataset and compiled them together once completed. ...
Article
Full-text available
Combat weaponry, including elaborate horns and antlers and complex dentition, evolved independently several times among mammals. While it is evident that tusk and tusk-like dentition have emerged primarily among males for intrasexual combat, it is unclear what ecological factors favor the retention or re-evolution of tusks. We investigated patterns of tusk evolution in artiodactyls while exploring specific ecological factors that might favor their use over other cranial weapons (e.g., antlers, horns). We show that among males, small (<15 kg), solitary species tend to retain well-developed canines, and more solitary species live in more closed habitats. These results suggest that tusks are a better weapon option for smaller, slinking artiodactyls in forested environments with low visibility, whereas larger taxa living in more open environment can bear the cost of elaborate headgear and are better served by communicating across distances an honest signal of fighting ability. Small species in dense habitats may also be more likely to be ambushed by predators and have a need to defend themselves; small, slicing daggers may be a better defensive weapon and allow more maneuverability and faster escape than cumbersome headgear in densely vegetated habitats.
... Among land vertebrates, cranial appendages, as well as cranial ornamentation and structures are widely distributed and occur within different vertebrate groups, e.g. [41][42][43][44][45][46][47], where they have many purposes and are often used for sexual display or intra-specific combat [48][49][50][51][52]. Pecora e.g. are known for their morphological variety of cranial appendages that resulted in different types of headgear (antlers, horns, pronghorns, and ossicones [53]), similarly used for sexual display and during intra-specific combat [43,49,[54][55][56][57]. ...
... Among land vertebrates, cranial appendages, as well as cranial ornamentation and structures are widely distributed and occur within different vertebrate groups, e.g. [41][42][43][44][45][46][47], where they have many purposes and are often used for sexual display or intra-specific combat [48][49][50][51][52]. Pecora e.g. are known for their morphological variety of cranial appendages that resulted in different types of headgear (antlers, horns, pronghorns, and ossicones [53]), similarly used for sexual display and during intra-specific combat [43,49,[54][55][56][57]. Among living ruminants, tragulids are a non-pecoran group, lacking cranial appendages. ...
... These frontoparietal bulges are only present in males of D. naui as in Pecora, where cranial appendages are mainly present and stronger developed in males than in females [49,53,54,60,61]. The bulges in Dorcatherium are difficult to interpret, because there is no analogue in tragulids or any other closer related group. ...
Article
Full-text available
Tragulids, chevrotains or mouse deer, were common faunal elements during the Miocene. During that time, Dorcatherium was the most abundant genus, with D . naui being the first described species. Besides their abundance, until recently only very limited cranial material was available for investigation. Here we present a redescription of the first complete skull of D . naui from the middle to late Miocene locality of Eppelsheim, Germany, based on micro-computed tomography. Furthermore, we present a description and comparison of two additional, new skulls of D . naui from the late Miocene hominid locality Hammerschmiede, Germany. Within Dorcatherium , so far, only three other complete skulls are known, all belonging to D . crassum . A comparison between the three skulls of D . naui and the already known skulls of D . crassum shows that these two species differ in morphological features of the skull, such as laterally facing orbitae, separation of supraorbital foramen from supraorbital groove by a bony bridge, well-developed parietal plateau, prominent nuchal tubercle, less-developed nuchal crests, and the presence of an occipital crest. Moreover, two different osteological morphotypes are present in the skulls of D . naui that can be interpreted as a previously unknown sexual dimorphism. Very similar features are observed in D . crassum , which can be likewise related to the same dimorphism. However, males of D . naui differ from males of D . crassum by the presence of frontoparietal bulges, which were probably used for sexual display and during male-male combats in males of D . naui . For the first time, sexual dimorphism in Dorcatherium is described based on skull characteristics, which are, so far, unknown from any other fossil or extant tragulid.
... In a thorough review of the issue, Clutton-Brock (1982) concluded that the principal function of antlers is their use as weapons in intra-specific combat between males for establishing dominance and access to females during the rut. However, other researchers emphasized the role of antlers as display organs that allow competing males to assess their opponents without fighting (e.g., Andersson, 1994;Bubenik, 1990a;Caro et al., 2003;Geist, 1998;Gould, 1974;Kitchener, 1991). Thus, antler morphology seems to be primarily influenced by intraspecific competition for females and possibly also over other resources (Caro et al., 2003;Geist, 1966). ...
... However, other researchers emphasized the role of antlers as display organs that allow competing males to assess their opponents without fighting (e.g., Andersson, 1994;Bubenik, 1990a;Caro et al., 2003;Geist, 1998;Gould, 1974;Kitchener, 1991). Thus, antler morphology seems to be primarily influenced by intraspecific competition for females and possibly also over other resources (Caro et al., 2003;Geist, 1966). ...
... An eco-functional link was suggested for antler size and complexity with different mating systems. Compared with males from smaller breeding groups, those from males of larger groups would require relatively larger (and often more complex) antlers in order to successfully compete for females (Caro et al., 2003;Clutton-Brock et al., 1980;Kitchener, 1985;Roberts, 1996). An additional adaptive explanation for antler size and complexity is female choice. ...
Article
Full-text available
Antlers are the most conspicuous trait of cervids and have been used in the past to establish a classification of their fossil and living representatives. Since the availability of molecular data, morphological characters have generally become less important for phylogenetic reconstructions. In recent years, however, the appreciation of morphological characters has increased, and they are now more frequently used in addition to molecular data to reconstruct the evolutionary history of cervids. A persistent challenge when using antler traits in deer systematics is finding a consensus on the homology of structures. Here, we review early and recent attempts to homologise antler structures and objections to this approach, compare and evaluate recent advances on antler homologies, and critically discuss these different views in order to offer a basis for further scientific exchange on the topic. We further present some developmental aspects of antler branching patterns and discuss their potential for reconstructing cervid systematics. The use of heterogeneous data for reconstructing phylogenies has resulted in partly conflicting hypotheses on the systematic position of certain cervid species, on which we also elaborate here. We address current discussions on the use of different molecular markers in cervid systematics and the question whether antler morphology and molecular data can provide a consistent picture on the evolutionary history of cervids. In this context, special attention is given to the antler morphology and the systematic position of the enigmatic Pere David's deer (Elaphurus davidianus). This article is protected by copyright. All rights reserved.
... Highly complex structures are more likely to be used in nonlethal or ritualized competition, whereas simpler structures can indicate more violent competition (Emlen, 2008). This relationship between shape and fighting style holds true for both bovid horns and cervid antlers (Caro, Graham, Stoner, & Flores, 2003). Further, the size of horns correlates with strength of sexual selection and competition (Bro-Jørgensen, 2007). ...
... Recent studies of cranial appendage evolution propose three main explanations for structural and shape diversity: species recognition, habitat controls, and sexual selection. Some researchers argue that cranial appendage diversity is related to the need for animals to expend reproductive effort only on members of the same species (Vrba, 1984), although there is minimal evidence that female mate-choice influences cranial appendage shape (Caro et al., 2003). ...
... Although this explanation would appear to have the most support, it does not consider females that have cranial appendages (Emlen, 2008;Kiltie, 1985). Shape diversity driven solely by female choice rather than selection in sexual competition is expected to produce completely random shapes, but there is little support for complete randomness of cranial appendage shapes, and the correlation between shape and fighting strategy is well supported (Caro et al., 2003). ...
Article
Full-text available
Morphological disparity arises through changes in the ontogeny of structures; however, a major challenge of studying the effect of development on shape is the difficulty of collecting time series of data for large numbers of taxa. A proxy for developmental series proposed here is the age at sexual maturity, a developmental milestone potentially tied to the development of structures with documented use in intrasexual competition, such as cranial appendages in Artiodactyla. This study tested the hypothesis that ruminant cranial appendage shape and size correlate with onset of sexual maturity, predicting that late sexual maturity would correlate with larger, more complicated cranial appendages. Published data for cranial appendage shape and size in extant taxa were tested for correlations with sexual maturity using linear mixed-effect models and phylogenetic generalized least-squares analyses. Ancestral state reconstructions were used to assess correlated variables for developmental shifts indicative of heterochrony. These tests showed that phylogeny and body mass were the most common predictors of cranial appendage shape and sexual maturity was only significant as an interaction with body mass. Nevertheless, using developmental milestones as proxies for ontogeny may still be valuable in targeting future research to better understand the role of development in the evolution of disparate morphology when correlations exist between the milestone and shape.
... In fact, males often fight by pushing the rival with the contact of the horn area of the spiral, using intertwined movements (Kiley-Worthington, 1978). The spiral of eland horn has been proposed to increase grip in wrestling between males (Geist, 1966;Caro et al. 2003). This hypothesis received support from the fact that wrestling was associated with twisted horns in polygynous bovids (Caro et al. 2003;Solounias, 2007). ...
... The spiral of eland horn has been proposed to increase grip in wrestling between males (Geist, 1966;Caro et al. 2003). This hypothesis received support from the fact that wrestling was associated with twisted horns in polygynous bovids (Caro et al. 2003;Solounias, 2007). For this reason, we hypothesized that the spiral zone of the horns may play an important role in understanding the mechanics and functionality-grip of the horn's tissues in spiral-horned antelopes. ...
... So, what is the role of this spiral-shaped structure in the horn bone? Caro et al. (2003) already pointed out that the form of intraspecific fighting and the degree to which individuals are expected to fight influence the shape of horns (wrestling was associated with twisted horns in polygynous bovids). This was already discussed by Geist (1966), who indicated that the presence of spiral antelope horns could allow the locking of males' horns, avoiding more impact but allowing only pushes in fighting. ...
Article
Horns are permanent structures projecting from the head of bovids, consisting of a bony horncore covered with a layer of skin and then a sheath of keratinous material showing variability of growth intensity based on nutrition. From the point of view of the horn's mechanical properties, the keratin sheath has been widely studied, but only a few studies have considered the complete structure of the horn and fewer studies have focused on the bony horncore and its characteristics. The latter showed the important role of the bony core, when cranial appendages are subject to mechanical stress (as happens during fighting). The mechanical properties of bone material, along with its mineral profile, are also important, because they can show effects of different factors, such as nutrition and mineral deficiencies in diet. For this reason, eight horncores of captive common eland male were sampled at four positions along the vertical axis of the horn. The main aim was to study variation in mechanical properties and the mineral content along the vertical axis of the horncores. We further analysed whether the spiral bony ridge present on eland horncores differs in any of the studied properties from adjacent parts of the horncore. In other antelopes, spiral ridges on the horns have been proposed to increase grip during wrestling between males. Cross-sections of the horncores were performed at four positions along the longitudinal axis and, for each position, two bone bars were extracted to be tested in impact and bending. Moreover, in the first sampling position (the closest position to the base) two bars were extracted from the spiralled bony area. The resulting fragments were used to measure ash content, bone density and mineral content. Results showed that horn bone decreased along the vertical axis, in ash (-36%), density (-32%), and in impact work 'U' (marginally significant but large effect: -48%). The concentration of several minerals decreased significantly (Mg, Cr, Mn and Tl by -33%, -25%, -31%, -43%, respectively) between the basal and the uppermost sampling site. The bone tissue of the horncore spiral compared with non-spiral bone of the same position showed a lower ash content (53% vs. 57%), Mg and Mn; in addition to showing approximately half values in work to peak force 'W', bending strength 'BS' and 'U', but not in Young's modulus of elasticity 'E'. In conclusion, similarly to the results in a totally different fighting bony structure, the antlers, the horncore of eland shows advantageous parameters in bone tissue of the base in respect to the tip, with higher values for mechanical properties, density and mineral profile. Moreover, the spiral bone tissue showed lower material mechanical properties. Probably the spiral tissue of the horn may have a role in deflecting potential cross-sectional fractures during wrestling. In addition, it may serve to improve the grip during wrestling, and we propose that it may also prevent risk of rotation of sheath with respect to internal bone not only in this, but also in other straight bovid horns.
... 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]). ...
... Male sexual weapon length (n = 135;, i.e., antlers in cervids and horns in bovids), sperm head, midpiece and total flagellum length (all n = 53), and male muzzle width (n = 88) were represented by linear measurements. Although antlers, and to a lesser degree horns, can exhibit complex geometries (e.g., [44]), our analyses focused on sexual weapon length, measured as the curvilinear distance along the main axis from the base to the most distal tip of the weapon. Importantly, in ungulates, weapon length is commonly used as a proxy measure of weapon size (e.g., [60]), consistently predicts the overall strength of precopulatory sexual selection [43,44], and alternative measures of weapon size and complexity are strongly correlated (e.g., [40]). ...
... Although antlers, and to a lesser degree horns, can exhibit complex geometries (e.g., [44]), our analyses focused on sexual weapon length, measured as the curvilinear distance along the main axis from the base to the most distal tip of the weapon. Importantly, in ungulates, weapon length is commonly used as a proxy measure of weapon size (e.g., [60]), consistently predicts the overall strength of precopulatory sexual selection [43,44], and alternative measures of weapon size and complexity are strongly correlated (e.g., [40]). Moreover, weapon length measurements were available for a wider range of species than weapon complexity scores or weapon mass values. ...
Article
Full-text available
In polyandrous species, males face reproductive competition both before and after mating. Sexual selection thus shapes the evolution of both pre- and postcopulatory traits, creating competing demands on resource allocation to different reproductive episodes. Traits subject to strong selection exhibit accelerated rates of phenotypic divergence, and examining evolutionary rates may inform us about the relative importance and potential fitness consequences of investing in traits under either pre- or postcopulatory sexual selection. Here, we used a comparative approach to assess evolutionary rates of key competitive traits in two artiodactyl families, bovids (family Bovidae) and cervids (family Cervidae), where male–male competition can occur before and after mating. We quantified and compared evolutionary rates of male weaponry (horns and antlers), body size/mass, testes mass, and sperm morphometrics. We found that weapons evolve faster than sperm dimensions. In contrast, testes and body mass evolve at similar rates. These results suggest strong, but differential, selection on both pre- and postcopulatory traits in bovids and cervids. Furthermore, we documented distinct evolutionary rates among different sperm components, with sperm head and midpiece evolving faster than the flagellum. Finally, we demonstrate that, despite considerable differences in weapon development between bovids and cervids, the overall evolutionary patterns between these families were broadly consistent.
... The shape and presence of horns are sexually dimorphic for some species and populations of bovids; shape differences between males and females are most predictable at the smallest and largest body sizes and considerably more variable at middle size ranges (Geist, 1971;Jarman, 1983;Packer, 1983;Rice, 1984). There is a strong correlation between the shape of horns and the animal's fighting style (Caro et al., 2003;Lundrigan, 1996) and between horn size and the intensity of sexual selection within a species (Bro-Jørgensen, 2007). Horn shape is also important because animals assess size of horns visually when deciding to engage in intrasexual competition, and females appear capable of recognizing conspecifics based on horn shape (Bubenik, 1990;Coulon et al., 2007). ...
... Recent investigations of horn shape evolution used horn length or qualitative binary variables (e.g., smooth vs. crenulated horns, straight vs. twisted horns) rather than geometric shape to test for correlations between aspects of shape and ecology, habitat, behavior, and life history (Bro-Jørgensen, 2007;Calamari, 2016;Caro et al., 2003). Other attempts to quantify horn shape have relied on complex classification systems (Mloszewski, 1981), or cross-sectional areas, single curves, or silhouettes (Kitchener, 1985;Lundrigan, 1996). ...
... Within Caprini, Naemorhedus and Capricornis are sister genera (Hassanin et al., 2012). Both bison and aoudads fight by ramming their heads together or locking horns to wrestle, gorals ram but also stab at each other with the points of their horns, and serows fight only by stabbing (Caro et al., 2003). The gorals and serows are found in closed habitats, aoudads in open habitats, and bison in both (Caro et al., 2003). ...
Article
The bony cranial structures of even-toed hoofed mammals are important for understanding ecology and behavior of ruminants. Horns, the cranial appendages of the family Bovidae, are covered in a layer of keratin that is often not preserved in the fossil record; however, this keratin sheath is intimately involved in the processes that influence horn shape evolution. To understand the relationship between these two components of horns, we quantified both core and sheath shape for four extant species using three-dimensional geometric morphometric analyses in separate, core- and sheath-specific morphospaces as well as a combined morphospace. We assessed correlations between the horn and sheath morphospaces using two-block partial least squares regression, a Mantel test of pairwise distances between species, and Procrustes ANOVA. We measured disparity in the combined morphospace as Procrustes distances between mean shapes of cores and sheaths within and between species and as Procrustes variance. We also tested whether core and sheath shapes could be discriminated by taxon with a canonical variate analysis. Results show that horn core and sheath morphospaces are strongly correlated. The differences in shape between a species' core and sheath were statistically significant, but not as great as those between the cores and sheaths of different species when close relatives were not considered, and core and sheath Procrustes variances are not significantly different within species. Cores and sheath shapes were highly identifiable and were assigned to the correct clade 93% of the time in the canonical variate analysis. Based on these tests, horn cores are distinguishable in geometric morphometric analyses, extending the possibility of using geometric morphometrics to study the ecology and evolution of bovid horns to the fossil record.
... The dramatic nature of these contests and the variety of behaviours exhibited across the clade has led many researchers to search for associated traits that allow ruminants to cope with the mechanical demands of combat. Horn and antler shape in male bovids and cervids is correlated with broad trends in social behaviour [25] and with the use of specific behaviours when fighting [26,27]. The material properties of the weapons may also play a role in resisting forces generated by combat, including those of the keratin sheath [28] (but see [29]) and bony horn core [30,31] of bovid horns, and the antler of red deer [32]. ...
... We also conducted a third set of PGLS analyses to test whether vertebral morphology was associated with the presence of horns in females, as female weapons are not present in all ruminant species. We used female horn presence/absence data from the published literature [25] and included it as a categorical variable in conjunction with the body mass. ...
... We found that key aspects of cervical vertebra morphology are associated with differences in combat behaviour in ruminant taxa across the bovid and cervid radiations. The wide range of intraspecific competitive behaviour and a corresponding array of conspicuous cranial appendages in these animals has spurred many researchers to address the relationship between weapon morphology and fighting behaviour [11,12,25,26]. However, hypotheses about the role of the neck in resisting combat forces were limited to isolated species and had not been quantitatively tested [23,29,33,45]. ...
Article
Cranial weapons of all shapes and sizes are common throughout the animal kingdom and are frequently accompanied by the evolution of additional traits that enhance the use of those weapons. Bovids (cattle, sheep, goats, antelope) and cervids (deer) within the mammal clade Ruminantia are particularly well known for their distinct and varied cranial appendages in the form of horns and antlers, which are used as weapons in intraspecific combat between males for access to mates. Combat in these species takes many forms, including head-on collisions (ramming); stabbing an opponent's head or body with horn tips (stabbing); rearing and clashing downwards with horns (fencing); or interlocking antlers or horns while vigorously pushing and twisting (wrestling). Some aspects of weapon and skull morphology have been linked to combat behaviours in bovid and cervid species, but the contribution of postcranial structures that support these weapons, such as the neck, has not been explored. To investigate the role of the neck in intraspecific combat, we quantified biomechanically relevant linear variables of the cervical vertebrae (C1-C7) from males and females of 55 ruminant species. We then used phylogenetic generalized least-squares regression to assess differences among species that display primarily ramming, stabbing, fencing and wrestling combat styles. In males, we found that wrestlers have longer vertebral centra and longer neural spines than rammers, stabbers or fencers, while rammers have shorter and wider centra and taller neural spine lever arms. These results suggest a supportive role for the cervical vertebrae in resisting forces generated by male-male combat in ruminant mammals and indicate that evolutionary forces influencing cranial weapons also play a role in shaping the supporting anatomical structures.
... In bovids (cattle, sheep, goats and antelopes) males typically have horns, but in many species females also possess such weapons, albeit usually smaller and differently shaped (Caro et al., 2003;Bro-Jørgensen, 2008;Stankowitch and Caro, 2009). In male bovids, horn evolution is thought to be driven by male-male competition for females, whereas female horns may serve as defence against predators or in female-female competition over territories (Roberts, 1996;Stankowitch and Caro, 2009). ...
... Female sexually selected weaponry thus seems al-most non-existent. Rather, when females possess weapons, they are naturally selected, used in competition over resources other than mates, in predator defence, or are non-functional (Caro et al., 2003;Bro-Jørgensen, 2007;Emlen, 2008). In other words, female weapons may often belong to the realm of social selection (West-Eberhard, 1983;Lyon and Montgomerie, 2012;Tobias et al., 2012), but not to that of sexual selection as defined in Box 1. ...
Article
In sex role reversed species, predominantly females evolve sexually selected traits, such as ornaments and/or weapons. Female ornaments are common and their function well documented in many species, whether sex role reversed or not. However, sexually selected female weapons seem totally absent except for small wing spurs in three jacana species, present in both males and females. This poor female weaponry is in sharp contrast to the situation in species with conventional sex roles: males com-monly have evolved sexually selected weapons as well as ornaments. At the same time, females in many taxa have naturally se-lected weapons, used in competition over resources or in predator defence. Why are sexually selected weapons then so rare, al-most absent, in females? Here I briefly review weaponry in females and the function of these weapons, conclude that the near ab-sence of sexually selected weapons begs an explanation, and suggest that costs of sexually selected weapons may exceed costs of ornaments. Females are more constrained when evolving sexually selected traits compared to males, at least compared to those males that do not provide direct benefits, as trait costs reduce a female's fecundity. I suggest that this constraining trade-off be-tween trait and fecundity restricts females to evolve ornaments but rarely weapons. The same may apply to paternally investing males. Whether sexually selected weapons actually are more costly than sexually selected ornaments remains to be investigated.
... This process of antlerogenesis is controlled intrinsically by fluctuating hormone levels, which in turn are dependent on extrinsic photoperiodicity, resulting in an annual antler cycle with distinct seasonality at higher latitudes (Goss, 1983). Antlers are usually restricted to male individuals (except in the case of reindeer, Rangifer tarandus; e.g., Geist, 1998) and play a crucial role in socio-reproductive behavior (e.g., Brown, 1980;Clutton-Brock, 1989;Caro et al., 2003). The size and/or morphological complexity of antlers are species specific, increasing with successive generations of the appendages until the animal reaches its prime (Geist, 1998); there is no simple linear correlation between body size and relative antler size (Lemaître et al., 2014). ...
... Their role in establishing a social ranking of individuals may be more important than their role in sexual behavior or intraspecific fighting (Brown, 1980). There is little to support a correlation of antler morphology and environmental factors; it is more likely that antler morphology is mainly driven by female preference (Caro et al., 2003). A detailed review of antlerogenesis is in the Supporting Information. ...
Article
Antlers are unique appendages. They are shed and rebuilt at intervals, and are synapomorphic for all living Cervidae (except for the Chinese water deer, Hydropotes inermis, in which they have presumably been lost). The antlerogenic process is controlled by a complex interaction of fluctuating levels of several hormones, most importantly testosterone. The oldest antler remains are recorded from the early Miocene; these have been interpreted as non-deciduous appendages because of supposed permanent skin coverage and the lack of a burr (a well-developed osseous protuberance around the base of the antler, which is always present in extant cervids). The aim of this study is to test the hypothesis that antler shedding was possible in these early Miocene cervids. Antlers of all extant and eight Miocene cervid genera, including burr-less antler fragments of the earliest cervids Procervulus, Ligeromeryx, and Lagomeryx were studied. An extensive comparative morphological analysis of external features of the antler, and of the abscission area and the base of the antler in particular, was undertaken. The results indicate that a regular, porous, and rugose abscission surface at the proximal end of the antler indicates antler shedding in both living and fossil cervids. The antler shedding mechanism must therefore have already been present in all early/mid Miocene cervid genera included in this study. On this basis, it is suggested that the presence of a burr is not prerequisite in order to shed antlers, that the presence of perpetual antlers has not yet been verified, and that the process of shedding and regeneration developed with the first appearance of these organs. This insight is particularly important for the systematic classification of early Miocene species as Cervidae, because the absence of the antler shedding and rebuilding mechanism would exclude them from the taxon Cervidae and from any relationship with extant cervids. J. Morphol., 2016.
... In the few groups where this has been done (e.g. bovids; Caro et al., 2003) such variation has been overlooked as a result of linking fighting style of a species to a single behaviour (and discarding displays). However, since the behaviours adopted during a contest may change throughout the contest, ascribing fighting style to a single behaviour may underestimate the influence of the weapon on contest success. ...
... By evolving displays, weapons would then tend to increase in size and complexity, which could decrease piercing performance and change the function of the weapon altogether (Emlen, 2008). This pathway is believed to have occurred in cervids, where short, pointed antlers have increased in size and complexity in recent species (Barrette, 1977;Caro et al., 2003;Davis, Brakora & Lee, 2011). Our results show that the adoption of noncontact displays before fights is a common strategy among animals (Table 3). ...
Article
In many species that fight over resources, individuals use specialized structures to gain a mechanical advantage over their rivals during contests (i.e. weapons). Although weapons are widespread across animals, how they affect the probability of winning contests is still debated. According to theory, understanding weapon function during contests is essential to: (i) understanding its importance in determining the winner, and (ii) identifying what weapon traits (e.g. weapon length versus shape versus performance) are most relevant for contest success. However, quantitative evaluations of how weapon function affects the extent to which weapon traits influence contest success are still lacking. Here, we first develop an individual‐based model to evaluate how increasing the influence of the weapon in determining the winner translates to differences between winners and losers. Then, we use a meta‐analysis to identify: (i) whether different weapon measures influence contest outcome differently; (ii) how animals use their weapons during fights – i.e. weapon function; and (iii) if weapon function correlates to how weapons influence contest outcome. Our model showed that, as weapons increased the chance of determining the winner, the mean difference between winners and losers also increased. Therefore, in our meta‐analysis we used the mean trait difference between winners and losers as a proxy for the extent to which weapons influence contest success. The literature search identified 49 suitable studies, containing information for 52 species, totalling 107 effect sizes. Four main patterns emerged. First, most of the literature focuses on linear measures of weapons, while performance measures are concentrated on Crustacea and Squamata; other types of measures were rare. Second, differences between winners and losers in linear measurements were greater than differences in performance measurements when all species were combined (and when we used only a subset). Third, species that bear weapons almost always perform visual/tactile displays before engaging in physical contact. And fourth, while the way individuals display their weapons did not influence the importance of weapon size on contest outcomes, fighting style predicted when differences between winners and losers would be higher. Species that used their weapons to push or lift (even when combined with other functions) showed greater differences between winners and losers when compared to species that used their weapons to impact, pierce, pull or squeeze. Overall, our results show that we have an incomplete understanding of animal weapons built mostly on weapon size and a few select taxa. Thus, we should start focusing on measuring weapons according to how they are used during contests and in a wider diversity of species. One way forward is to conduct studies that integrate weapon morphology to weapon function to ensure we are measuring the most ecologically relevant variables.
... While the mechanisms of weapon divergence are poorly understood (McCullough et al. 2016), 2 hypotheses have received some support. The first posits that selection should shape weapons to be most effective for a given style of fighting (e.g., ramming, wrestling, or tossing; Lundrigan 1996; Caro et al. 2003;McCullough et al. 2014). The second posits that the costs and benefits of agonistic signals, fighting styles, or weapons may vary across environments. ...
... Many studies tend to focus on a single competitive trait, but it is important to note that success in male competition is influenced by multiple traits (i.e., signal/weapon characteristics, competitor recognition, and aggressive behavior) that may evolve in concert and shape the process of population differentiation and speciation. Male competitive traits can be functionally linked to other lifehistory traits such as immunity or antioxidant defenses (Zera and Harshman 2001), genetically linked to other seemingly unrelated traits through pleiotropy or linkage disequilibrium, or correlated with other behavioral traits (Lundigran 1996;Caro et al. 2003). For example, coordinated changes in male coloration and stress sensitivity in the melanocortin system (Ducrest et al. 2008) could facilitate speciation if alternative trait combinations are adaptive (Dijkstra et al. 2017). ...
Article
Support for the role of sexual selection in speciation has grown over the last 30 years. Work in this area, however, has largely focused on a single dominant question: when and how do divergent male sexual signals and corresponding female preferences lead to reproductive isolation? The field has not given adequate attention to the role that male competition, Darwin’s second mechanism of sexual selection, might also play in speciation. In this review, we summarize recent work that shows precopulatory male competition can initiate speciation in sympatry, drive divergence of competitive phenotypes in allopatry, and strengthen reproductive barriers between competitive types during secondary contact. The manner by which male competition contributes to divergence in allopatry is a poorly understood yet compelling area of research; similar to female choice, male competition may be more likely to lead to speciation when working in concert with divergent ecology, and allopatry sets the stage for divergence among environments with reduced gene flow. To encourage future research in this area, we place potential mechanisms for speciation by male competition into existing speciation frameworks and propose a theoretical and empirical research agenda to reveal how male competition contributes to the accumulation of reproductive isolation. Our current understanding of when and how divergence in competitive phenotypes leads to reproductive isolation is limited, and theoretical work may be particularly well-suited to reveal when divergence by male competition is fastest and most likely.
... Therefore, if the weapon evolved in a given ancestor, and was followed by prolific speciation, variation in complexity of the ISW would be a consequence of diversification of fighting and display strategies among species and subsequent weapon adjustment (e.g. Eberhard, 1979;Caro et al., 2003;Schutze et al., 2007;Emlen, 2008). ...
... Hughes et al., 2014) signals. Perhaps counterintuitively, some formidable ISWs, like deer antlers or shrimp chelae, have increased in size and complexity but have decreased the absolute damage inflicted in combat and/or the likelihood of engaging in combat, probably due to the evolution of stereotyped displays and pushing scrimmages to assess resource-holding potential and strength respectively (Lincoln, 1994;Caro et al., 2003;Hughes et al., 2014;Emlen, 2014a). Two other well-studied examples of contests that if not resolved by ritualized displays escalate to pushing duels and jousting are drosophilid flies (Grimaldi & Fenster, 1989) and scarab beetles (Emlen, 2008). ...
Article
We propose a practical concept that distinguishes the particular kind of weaponry that has evolved to be used in combat between individuals of the same species and sex, which we term intrasexually selected weapons (ISWs). We present a treatise of ISWs in nature, aiming to understand their distinction and evolution from other secondary sex traits, including from ‘sexually selected weapons’, and from sexually dimorphic and monomorphic weaponry. We focus on the subset of secondary sex traits that are the result of same‐sex combat, defined here as ISWs, provide not previously reported evolutionary patterns, and offer hypotheses to answer questions such as: why have only some species evolved weapons to fight for the opposite sex or breeding resources? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that are only present or enlarged in adults of one of the sexes, and are used as weapons during intrasexual fights. Because of the absence of behavioural data and, in many cases, lack of sexually discriminated series from juveniles to adults, we exclude the fossil record from this review. We merge morphological, ontogenetic, and behavioural information, and for the first time thoroughly review the tree of life to identify separate evolution of ISWs. We found that ISWs are only found in bilateral animals, appearing independently in nematodes, various groups of arthropods, and vertebrates. Our review sets a reference point to explore other taxa that we identify with potential ISWs for which behavioural or morphological studies are warranted. We establish that most ISWs come in pairs, are located in or near the head, are endo‐ or exoskeletal modifications, are overdeveloped structures compared with those found in females, are modified feeding structures and/or locomotor appendages, are most common in terrestrial taxa, are frequently used to guard females, territories, or both, and are also used in signalling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only a few species possess better‐developed weapons than males, that the cases of independent evolution of ISWs are not evenly distributed across the phylogeny, and that animals possessing the most developed ISWs have non‐hunting habits (e.g. herbivores) or are faunivores that prey on very small prey relative to their body size (e.g. insectivores). Bringing together perspectives from studies on a variety of taxa, we conceptualize that there are five ways in which a sexually dimorphic trait, apart from the primary sex traits, can be fixed: sexual selection, fecundity selection, parental role division, differential niche occupation between the sexes, and interference competition. We discuss these trends and the factors involved in the evolution of intrasexually selected weaponry in nature.
... There, females of most medium-to-large species have straight, sharp horns that provide an effective weapon for predator defence. For males with antlers, sexual selection determined the general characteristics of their antlers (for example, shape, number of tines) 62 , as is also the case for bovids 34,62 . But the prolonged retention of antlers after the breeding season in elk, and possibly other medium-to-large species such as red deer that are also often preferred by predators 53,54 , appears to have instead been influenced by selection for a predatory deterrent. ...
... There, females of most medium-to-large species have straight, sharp horns that provide an effective weapon for predator defence. For males with antlers, sexual selection determined the general characteristics of their antlers (for example, shape, number of tines) 62 , as is also the case for bovids 34,62 . But the prolonged retention of antlers after the breeding season in elk, and possibly other medium-to-large species such as red deer that are also often preferred by predators 53,54 , appears to have instead been influenced by selection for a predatory deterrent. ...
Article
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Sexually selected weapons evolved to maximize the individual reproductive success of males in many polygynous breeding species. Many weapons are also retained outside of reproductive periods for secondary reasons, but the importance of these secondary functions is poorly understood. Here we leveraged a unique opportunity from the predator-prey system in northern Yellowstone National Park, WY, USA to evaluate whether predation by a widespread, coursing predator (wolves) has influenced a specific weapon trait (antler retention time) in their primary cervid prey (elk). Male elk face a trade-off: individuals casting antlers early begin regrowth before other males, resulting in relatively larger antlers the following year, and thus greater reproductive success, as indicated by research with red deer. We show, however, that male elk that cast their antlers early are preferentially hunted and killed by wolves, despite early casters being in better nutritional condition than antlered individuals. Our results run counter to classic expectations of coursing predators preferring poorer-conditioned individuals, and in so doing, reveal an important secondary function for an exaggerated sexually selected weapon-predatory deterrence. We suggest this secondary function played a key evolutionary role in elk; uniquely among North American cervids, they retain their antlers long after they fulfil their primary role in reproduction.
... Even in the face of these multiple independent transitions, clavicles, digits, and patella display the same trait in males and females across species. So far, the intersection of sexual dimorphism and bone losses and gains has only been observed in the mammalian bovids (Family Bovidae) (Caro et al., 2003;Packer, 1983;Stankowich & Caro, 2009). Female and male expression of horns are not perfectly correlated in bovid evolution. ...
... Interestingly, similar to our study, there are no known species where females have horns but males do not. Bovid horns are sexually dimorphic in shape, and the most comprehensive analyses conclude they function primarily in males for intrasexual competition and for defense in females (Caro et al., 2003;Stankowich & Caro, 2009). The baculum and baubellum thus represent a highly unusual case of widespread independently evolving sexually dimorphic bones, with the baubellum demonstrating more evolutionary and developmental lability compared to the baculum. ...
Article
Full-text available
Understanding the evolutionary forces that influence sexual dimorphism is a fundamental goal in biology. Here, we focus on one particularly extreme example of sexual dimorphism. Many mammal species possess a bone in their penis called a baculum. The female equivalent of this bone is called the baubellum and occurs in the clitoris, which is developmentally homologous to the male penis. To understand the potential linkage between these two structures, we scored baculum/baubellum presence/absence across 163 species and analyzed their distribution in a phylogenetic framework. The majority of species (N = 134) shared the same state in males and females (both baculum and baubellum present or absent). However, the baubellum has experienced significantly more transitions, and more recent transitions, so that the remaining 29 species have a baculum but not a well-developed baubellum. Even in species where both bones are present, the baubellum shows more ontogenetic variability and harbors more morphological variation than the baculum. Our study demonstrates that the baculum and baubellum are generally correlated across mammals, but that the baubellum is more evolutionarily and developmentally labile than the baculum. The accumulation of more evolutionary transitions, especially losses in the baubellum, as well as noisier developmental patterns, suggests that the baubellum may be nonfunctional, and lost over time.
... Sexually selected weapons used in male-male competition include some of the most ornate and diverse traits in the animal kingdom (Andersson 1994;Emlen 2008;McCullough et al. 2016). Despite their shared function as tools of mate competition, weapons differ dramatically in shape, size, and position on the body between closely related taxa (Rosenberg 2002;Caro et al. 2003;Emlen et al. 2005a;Bro-Jorgensen 2007;Schutze et al. 2007;Emlen 2008;McCullough et al. 2015). Their extreme size and rapid diversification imply a history of strong and likely divergent selection (e.g., West-Eberhard 1983;Kingsolver et al. 2001;McCullough et al. 2016), but the * Co-first authors. ...
... Large-scale (e.g., family-level) comparative studies suggest that changes in the monopolizability of females (e.g., harem size) and fighting style (Kitchener 1991;Lundrigan 1996;Caro et al. 2003;Bro-Jørgensen 2007), and changes in the types of costs incurred from weapon expression (Emlen 2001;Emlen et al. 2005b), can drive evolutionary changes in weapon form; and biomechanical modeling suggests that changes in fighting style can drive changes in weapon form as well Klinkhamer et al. 2019). ...
Article
Full-text available
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.
... The most apparent difference between deer antlers and bovid horns is that antlers are branched, which seems to have evolved for interlocking, enabling the typical "pushing fights" during the rut [1,2,46,47]. In contrast, horns allow for a wider array of fighting modes. ...
... In contrast, horns allow for a wider array of fighting modes. Some species deliver violent bumps with the basal part of their horns [48], while the spiral horns of others could allow interlocking and pushing fights between males [47,49,50]. ...
... Regardless of the selection pressure, the environment in which the trait is used can constrain its form. For example, cranial appendages are shaped by their environment, such that species living in open grasslands often wield the largest structures, but species living in the dense forest often wield reduced or no cranial structures [3,4]. Cumbersome structures like antlers decrease manoeuvrability in densely vegetated environments and species that live in these environments are more likely to evolve small tusks, as opposed to large antlers [5]. ...
... Small changes in eyestalk form could result in significant reductions in flight agility and manoeuvrability [7]. Thus, the environment strongly influences the size, shape and presence of exaggerated morphological structures [3,4,[8][9][10][11] . ...
Article
Full-text available
Humans are inherently fascinated by exaggerated morphological structures such as elk antlers and peacock trains. Because these traits are costly to develop and wield, the environment in which they are used can select for specific sizes or shapes to minimize such costs. In aquatic environments, selection to reduce drag can constrain the form of exaggerated structures; this is presumably why exaggerated morphologies are less common in aquatic environments compared to terrestrial ones. Interestingly, some crayfish species possess claws with an exaggerated gape between their pinching fingers, but the function of this claw gape is unknown. Here, I describe and test the function of the exaggerated claw gape of the New River crayfish, Cambarus chasmodactylus. Specifically, I test the hypothesis that the claw gape aids in movement against flowing currents. I found that both claw size and gape size were sexually dimorphic in this species and that males have disproportionately larger gapes compared to females. By experimentally covering their claw gape and testing crayfish locomotor performance, I found that individuals with their gape blocked were 30% slower than crayfish with a natural gape. My results highlight a unique adaptation that compensates for wielding an exaggerated structure in aquatic environments.
... Given that horns are weapons, and thus functional structures that need to perform well in combat, the most intuitive and compelling explanation for horn diversity is that it reflects adaptations to different fighting styles (Geist, 1966;Kitchener, 1991;Lundrigan, 1996;Caro et al., 2003;McCullough, Tobalske & Emlen, 2014). However, it is unclear how different fighting styles may drive corresponding changes in horn design, because the mechanical properties of beetle horns are still poorly understood. ...
Article
Full-text available
Sexual selection has equipped male rhinoceros beetles with large horns on their head and prothorax to aid in battle over access to females. Horns are used to pry and dislodge opponents from resource sites that attract females, so an optimal horn should be able both to withstand the high stresses imposed during fights, and to resist deflection in response to these loads. We examined the cross-sectional morphology of horns using micro-computed tomography scanning to determine how horn structure changes with horn length to withstand the different fighting loads. Specifically, we measured the second moment of area of horns within and among rhinoceros beetle species to assess whether changes in cross-sectional morphology accompany changes in body size in order to maintain high strength and stiffness during fights. We find that the second moment of area of horns increases with body size both intra-specifically and inter-specifically, and that these relationships closely fit those predicted if horns have been selected to be strong and stiff fighting structures. Our results therefore support the hypothesis that rhinoceros beetle horns are structurally adapted for combat.
... The tensile crack strength in different directions is consistent with the average orientation degree h in the corresponding direction. 17 Arrow, Caro et al. investigated the shapes of beetle horns, 18,19 and in 2014, McCullough 5 demonstrated the constant relationship between the cross-sectional shapes of the horn and the mechanical characteristics of the horns by comparing three species of rhinoceros beetles. ...
Article
Full-text available
To learn about the fiber-lamination design methods of light-weighted cylindrical composites, this study investigates the relationships between the exoskeleton of Oryctes Rhinoceros horns and their structural functions: 1) The odd and even numbered exterior layers of the fibers of the horn are oriented at 20° and 150°, respectively, and the change of the fiber angles outside-in suits the mechanical needs of horns; 2) O. Rhinoceros horn has a flat top cross-section with a mellow root, which increases the bending performance of the root and can make the top thrust easily into trees. 3) The maximum average orientation degree of the back is 0.93, which is beneficial for the tensile properties in the y-direction. By comparison, the fibers in the front are oriented primarily in the x-direction, which is suitable for resisting horizontal forces.
... Although sexual selection is the most pervasive explanation for the evolution and persistence of ornaments, there are several cases of ornaments that are used or at least recruited for functions not explicitly related to mate choice. For instance, horns in females of several ungulate species may serve as antipredator devices (Caro et al. 2003). Facial ornaments may have served for species recognition in di-nosaurs (Padian andHorner 2011, 2014) as they do in Neotropical primates (Santana et al. 2012). ...
Article
Full-text available
Luxuriant, bushy antlers, bizarre crests, and huge, twisting horns and tusks are conventionally understood as products of sexual selection. This view stems from both direct observation and from the empirical finding that the size of these structures grows faster than body size (i.e., ornament size shows positive allometry).We contend that the familiar evolutionary increase in the complexity of ornaments over time in many animal clades is decoupled from ornament size evolution. Increased body size comes with extended growth. Since growth scales to the quarter power of body size, we predicted that ornament complexity should scale according to the quarter power law as well, irrespective of the role of sexual selection in the evolution and function of the ornament. To test this hypothesis, we selected three clades (ammonites, deer, and ceratopsian dinosaurs) whose species bore ornaments that differ in terms of the importance of sexual selection to their evolution.We found that the exponent of the regression of ornament complexity to body size is the same for the three groups and is statistically indistinguishable from0.25.We suggest that the evolution of ornament complexity is a by-product of Cope’s rule. We argue that although sexual selection may control size in most ornaments, it does not influence their shape.
... Although sexual selection is the most pervasive explanation for the evolution and persistence of ornaments, there are several cases of ornaments that are used or at least recruited for functions not explicitly related to mate choice. For instance, horns in females of several ungulate species may serve as antipredator devices (Caro et al. 2003). Facial ornaments may have served for species recognition in di-nosaurs (Padian andHorner 2011, 2014) as they do in Neotropical primates (Santana et al. 2012). ...
Research
Full-text available
ABSTRACT - Luxuriant, bushy antlers, bizarre crests, and huge, twisting horns and tusks are conventionally understood as products of sexual selection. This view stems from both direct observation and from the empirical finding that the size of these structures grows faster than body size (i.e., ornament size shows positive allometry).We contend that the familiar evolutionary increase in the complexity of ornaments over time in many animal clades is decoupled from ornament size evolution. Increased body size comes with extended growth. Since growth scales to the quarter power of body size, we predicted that ornament complexity should scale according to the quarter power law as well, irrespective of the role of sexual selection in the evolution and function of the ornament. To test this hypothesis, we selected three clades (ammonites, deer, and ceratopsian dinosaurs) whose species bore ornaments that differ in terms of the importance of sexual selection to their evolution.We found that the exponent of the regression of ornament complexity to body size is the same for the three groups and is statistically indistinguishable from0.25.We suggest that the evolution of ornament complexity is a by-product of Cope’s rule. We argue that although sexual selection may control size in most ornaments, it does not influence their shape.
... Although sexual selection is the most pervasive explanation for the evolution and persistence of ornaments, there are several cases of ornaments that are used or at least recruited for functions not explicitly related to mate choice. For instance, horns in females of several ungulate species may serve as antipredator devices (Caro et al. 2003). Facial ornaments may have served for species recognition in di-nosaurs (Padian andHorner 2011, 2014) as they do in Neotropical primates (Santana et al. 2012). ...
Article
Full-text available
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... Although sexual selection is the most pervasive explanation for the evolution and persistence of ornaments, there are several cases of ornaments that are used or at least recruited for functions not explicitly related to mate choice. For instance, horns in females of several ungulate species may serve as antipredator devices (Caro et al. 2003). Facial ornaments may have served for species recognition in di-nosaurs (Padian and Horner 2011, 2014) as they do in Neotropical primates (Santana et al. 2012). ...
Research
Luxuriant, bushy antlers, bizarre crests, and huge, twisting horns and tusks are conventionally understood as products of sexual selection. This view stems from both direct observation and from the empirical finding that the size of these structures grows faster than body size (i.e., ornament size shows positive allometry).We contend that the familiar evolutionary increase in the complexity of ornaments over time in many animal clades is decoupled from ornament size evolution. Increased body size comes with extended growth. Since growth scales to the quarter power of body size, we predicted that ornament complexity should scale according to the quarter power law as well, irrespective of the role of sexual selection in the evolution and function of the ornament. To test this hypothesis, we selected three clades (ammonites, deer, and ceratopsian dinosaurs) whose species bore ornaments that differ in terms of the importance of sexual selection to their evolution.We found that the exponent of the regression of ornament complexity to body size is the same for the three groups and is statistically indistinguishable from0.25.We suggest that the evolution of ornament complexity is a by-product of Cope’s rule. We argue that although sexual selection may control size in most ornaments, it does not influence their shape.
... While antlers undoubtedly qualify as osseous structures, the horns of bovids include both osseous and non-osseous structures. Horn occurrence and shape has been included in various comparative studies (Caro et al. 2003;Stankowich & Caro 2009). It should be noted that certain aspects of horn (and antler) shape correlate with body size (e.g.Lemaître et al. 2014). ...
Article
In the attempt to derive phylogenetic relationships from the most comprehensive of character matrices, not only molecular, biochemical and osteological, but also data from less frequently used domains such as behaviour, life history, physiology and soft tissue anatomy are sought. Here, soft tissue traits that have been used in ruminant phylogenetic analyses, and that are potentially available for such analyses, are reviewed. The use of certain measures, such as the presence of an ileocaecal gland or certain skin glands, or the number of colic coils, appears unfounded. Using the presence of the gall bladder as a family trait disregards notable exceptions in the Bovidae. The largest set of potential, easily available soft tissue data appears to be related to the digestive tract, but has so far not been utilized. Generally, the paucity of recent anatomical studies is striking, which means that many available anatomical reports or drawings stem from the beginning of the 20th century. Currently no dataset exists that describes soft tissue anatomy for various organ systems in the same specimens (as is standard for osteological traits taken from whole skeletons). Suggestions are presented how data evaluation and scoring can be performed avoiding circular reasoning, and a plea is made for using data on a species, not a family level. The importance of assessing data independence and correcting for body mass-related effects is emphasized. For most organ systems, new, coordinated, systematic dissections are necessary before a reliable inclusion of soft tissue traits will become possible.
... Comp. by:Saravanakumar Date:24/10/05 Time:12:36:05 Stage:First Proof File Path:// spsind002s/cup_prod1/PRODENV/000000~1/00DA26~1/S00000~2/00D226~1/ 000000~3/000007331.3D Proof by: QC by: of secondary sexual characters (Jarman 1983, Caro et al. 2003. Large herbivores vary greatly along the gradient of size dimorphism, with at one end, females that are larger than males (e.g. ...
... Although sexual selection is the most pervasive explanation for the evolution and persistence of ornaments, there are several cases of ornaments that are used or at least recruited for functions not explicitly related to mate choice. For instance, horns in females of several ungulate species may serve as antipredator devices (Caro et al. 2003). Facial ornaments may have served for species recognition in dinosaurs (Padian and Horner 2011, 2014) as they do in Neotropical primates (Santana et al. 2012). ...
Article
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Ceratopsidae represents a group of quadrupedal herbivorous dinosaurs that inhabited western North America and eastern Asia during the Late Cretaceous. Although horns and frills of the cranium are highly variable across species, the lower jaw historically has been considered to be relatively conservative in morphology. Here, the lower jaws from 58 specimens representing 21 ceratopsoid taxa were sampled, using geometric morphometrics and 2D finite element analysis (FEA) to explore differences in morphology and mechanical performance across Ceratopsoidea (the clade including Ceratopsidae, Turanoceratops and Zuniceratops). Principal component analyses and non-parametric permuted manovas highlight Triceratopsini as a morphologically distinct clade within the sample. A relatively robust and elongate dentary, a larger and more elongated coronoid process, and a small and dorso-ventrally compressed angular characterize this clade, as well as the absolutely larger size. By contrast, non-triceratopsin chasmosaurines, Centrosaurini and Pachyrhinosaurini have similar morphologies to each other. Zuniceratops and Avaceratops are distinct from other taxa. No differences in size between Pachyrhinosaurini and Centrosaurini are recovered using non-parametric permuted anovas. Structural performance, as evaluated using a 2D FEA, is similar across all groups as measured by overall stress, with the exception of Triceratopsini. Shape, size and stress are phylogenetically constrained. A longer dentary as well as a long coronoid process result in a lower jaw that is reconstructed as relatively much more stressed in triceratopsins.
... Defending a territory is expected be more energetically costly than defending a harem, and both to be more costly than tending (Owen-Smith 1977). Large males in harem species display large and complex weapons that generally lead to dissuade the small males to engage in a fight, whereas males in territorial species have short and sharp weapons often used in risky fights (Caro et al. 2003). Moreover, territorial males have to patrol and mark the territory they defend using glandular secretions, which is likely to be costly. ...
Article
In most mammals, both sexes display different survival patterns, often involving faster senescence in males. Being under intense sexual competition to secure mating opportunities, males of polygynous species allocate resources to costly behaviors and conspicuous sexual traits, which might explain these observed differences in longevity and senescence patterns. However, comparative studies performed to date have led to conflicting results. We aimed to resolve this problem by first reviewing case studies of the relationship between the strength of sexual selection and age-specific survival metrics. Then, we performed a comprehensive comparative analysis to test whether such relationships occur among species of captive ruminants. We found that the strength of sexual selection negatively influenced the onset of actuarial senescence in males, with males senescing earlier in polygynous than in monogamous species, which led to reduced male longevity in polygynous species. Moreover, males of territorial species senesced earlier but slower, and have a shorter longevity than males of species displaying other mating tactics. We detected little influence of the strength of sexual selection on the rate of actuarial senescence. Our findings demonstrate that the onset of actuarial senescence, rather than its rate, is a side effect of physiological mechanisms linked to sexual selection, and potentially accounts for observed differences in longevity. This article is protected by copyright. All rights reserved.
... Species that possess horns with ridges use this structural feature to provide grip during fights, effectively holding the opponents together (Geist 1966a, Walther 1974, Lundrigan 1996. Moreover, it has been argued that the shape of the structure has developed in line with the style of combat; for example, bovids with inward facing tips to their horns fight by wrestling, smooth horns are used for stabbing at opponents and antlers with more than five points are adapted for a fencing style of combat (Caro et al. 2003). Longer horns or antlers are closely associated with wrestling type contests such as the twisting, pushing contests of deer (Lundrigan 1996). ...
... moschops' morph specimens representing females. This is by analogy with extant mammals: robust cranial structures such as facial bosses are frequently utilized in inter-male agonistic behavior (Estes, 1991;Caro et al., 2003), as has also been proposed for dinocephalian therapsids (Barghusen, 1975). Additionally or alternatively, these bosses may have functioned as display structures, which in sexually dimorphic mammals also tend to be exaggerated in males (Andersson, 1994). ...
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Cryptodont dicynodonts are some of the most abundant therapsid taxa in the upper Permian fossil record. Despite extensive taxonomic study of these families, the species diversity of geikiid cryptodonts remains problematic, particularly for the set of edentulous taxa in the genera Pelanomodon, Propelanomodon, and Geikia. Here, all known specimens of tuskless geikiid dicynodonts from the upper Permian Karoo Basin of South Africa are reevaluated. The genus Pelanomodon is redescribed based on a series of skulls representing a range of sizes. All tuskless geikiids from the Karoo Basin are considered to represent a single species (Pelanomodon moschops) that exhibited cranial dimorphism as adults. The nominal species Pe. moschops and Pe. rubidgei differ only in degree of facial boss development and are most parsimoniously interpreted as sexual dimorphs. Propelanomodon is considered to represent the juvenile morphotype of Pelanomodon moschops, as indicated by the geographic and stratigraphic overlap of the two taxa, the lack of any small skulls of the Pelanomodon morphotype in well-sampled localities, and the presence of specimens of intermediate size and morphology. Propelanomodon specimens are distinctive among juvenile dicynodonts in having a significantly narrower intertemporal region than adults. Analysis of intertemporal width relative to total skull size in Pelanomodon and the abundant cryptodont taxa Aulacephalodon, Oudenodon, and Tropidostoma suggests that Pelanomodon had a unique growth trajectory. Based on available stratigraphic data, Pelanomodon was restricted to the uppermost Permian Daptocephalus Assemblage Zone of the South African Beaufort Group and was a victim of the end-Permian mass extinction.
... First and foremost, we need more information on how weapons are used in fights. Ungulates are by far the most well-studied taxa in this regard; comparative analyses have found correlations between weapon morphology and species-specific fighting styles, suggesting that differences in how or where males fight favor corresponding changes in weapon form [50][51][52][53][54]. For example, bovid species with short, smooth horns tend to be stabbers; species with robust, curved horns tend to be rammers; and species with long, reaching horns tend to wrestle and fence [51,53]. ...
Article
The elaboration and diversification of sexually selected weapons remain poorly understood. Here, we argue that progress in this topic has been hindered by a strong bias in sexual selection research, and a tendency for weapons to be conflated with ornaments used in mate choice. Here, we outline how male-male competition and female choice are distinct mechanisms of sexual selection, and why weapons and ornaments are fundamentally different types of traits. We call for research on the factors contributing to weapon divergence, the potential for male-male competition to drive speciation, and the specific use of weapons in the context of direct fights versus displays. Given that weapons are first and foremost fighting structures, biomechanical approaches are an especially promising direction for understanding weapon design.
... shaping the evolution of bony ornamentation, namely studies of bovid horns and cervid antlers. Caro et al. (2003) found that antler and horn shape is related to a species' mating and social system as well as their fighting technique. They also found that horn and antler shape were not related to body size or environmental factors, nor were they arbitrary. ...
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The discovery in central Montana of the first ever multi-individual, monotaxic stegosaur site in North America prompted a revaluation of previous Stegosaurus material. Of particular interest was the plate morphology and function, as the plates of these specimens come in two morphs. This study examined a majority of known Stegosaurus material in collections across the USA and in Switzerland using a combination of quantitative techniques, histological analysis, and CT scanning. Several major insights into the biology of North American stegosaurs were made. First, in agreement with previous research, the taxa “Hesperosaurus mjosi” was found to be similar enough to specimens of Stegosaurus such that it should be considered a species within this genus. Second, dimorphism in the plates of S. mjosi was confirmed, with one morph being wide and oval and the other being tall and narrow. The wide morph plates reach sizes 45% larger in surface area than do the tall morph plates. This variation was determined to be sexual dimorphism by the elimination of alternate hypotheses. These included individual, ontogenetic, and interspecific variation as well as the possibility that one individual possessed both morphs of plates. There appear to be no intermediate morphs. Both morphs are known from sexually mature, young adults and fully-grown, old adults. Taphonomy of the Montana site suggests that these two morphs comprised a social group and the only dimorphic feature is plate shape. Isolated specimens possess only one morph of plate or the other and plates of both morphs can be identified along the entire plate series from neck to tail. This is the strongest evidence for sexual dimorphism in dinosaurs yet presented and the first to rigorously test all alternate hypotheses for the observed dimorphism. Third, a simple model was developed to estimate the relative intensities of predation risk to sexual selection in a species based on the degree of dimorphism observed in their horny structures. By applying this to S. mjosi and available data on extant bovids, it was found that modern analogs for S. mjosi include the mountain goat (Oreamnos americanus), American bison (Bison bison), African or Cape buffalo (Syncerus caffer), and the genus of antelope Gazella. The comparison to extant bovids was further explored. A tail spike found at the central Montana quarry was preserved with fossilized keratin extending off of the tip. By developing a proxy for heat flow into or out of horny structures based on the surface areas of bony core and keratin sheath, it was found that Stegosaurus likely thermoregulated to the same extent as would be expected for an extant, tropical bovid of the same body size. Fourth, an examination of some of the variation within Stegosaurus suggests that the genus likely exhibited high species richness and varying degrees of sexual dimorphism between species within the genus. Overall these results provide evidence for multiple functions of plates including inter- and intraspecific display, predator deterrence, and thermoregulation. The relative importance of these different functions probably varied between species of Stegosaurus. Future paleontological and stratigraphic work on Morrison Formation stegosaurs, in combination with comparative approaches to extant species, will likely provide new insights into the details of the paleobiology of Stegosaurus.
... The great phenotypic diversity of weapons in living species is known to be influenced by multiple factors including mating system, intensity of sexual selection, fighting style, body size and mechanical constraints [4,[7][8][9][10][11][12]. However, rigorous studies attempting to define common ecological or anatomical correlates of weaponry within a broad phylogenetic framework are lacking and no research to our knowledge has attempted to study weaponry broadly in extinct vertebrates. ...
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Weaponry, for the purpose of intraspecific combat or predator defence, is one of the most widespread animal adaptations, yet the selective pressures and constraints governing its phenotypic diversity and skeletal regionalization are not well understood. Here, we investigate the evolution of tail weaponry in amniotes, a rare form of weaponry that nonetheless evolved independently among a broad spectrum of life including mammals, turtles and dinosaurs. Using phylogenetic comparative methods, we test for links between morphology, ecology and behaviour in extant amniotes known to use the tail as a weapon, and in extinct taxa bearing osseous tail armaments. We find robust ecological and morphological correlates of both tail lashing behaviour and bony tail weaponry, including large body size, body armour and herbivory, suggesting these life-history parameters factor into the evolution of antipredator behaviours and tail armaments. We suggest that the evolution of tail weaponry is rare because large, armoured herbivores are uncommon in extant terrestrial faunas, as they have been throughout evolutionary history.
... Second, mandibular serrations could increase the likelihood of being fed by a parent by influencing sibling competition for parental attention. Larvae of some parasitoid wasps use sharpened mandibles as weapons in siblicide (Mayhew & van Alphen, 1999), and sharpened structures are used as weapons in other animal taxa (Caro, Graham, Stoner, & Flores, 2003;Frazzetta, 1988). N. orbicollis larvae have a size advantage when reared in smaller broods (Benowitz & Moore, 2016) and could therefore benefit from improved performance in intrafamilial combat. ...
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Background In species with parental care, there is striking variation in offspring dependence at birth, ranging from feeding independence to complete dependency on parents for nutrition. Frequently, highly dependent offspring further evolve reductions or alterations of morphological traits that would otherwise promote self‐sufficiency. Here, we examine evidence for morphological evolution associated with dependence in burying beetles (Nicrophorus spp.), in which dependence upon parents appears to have several independent origins. In many species, precocial first instar larvae can survive without parenting, but several altricial species die at this stage on their own. We focused specifically on the mandibles, which are expected to be related to feeding ability and therefore independence from parents. Results We find no evidence that the size of the mandible is related to dependence on parents. However, we do find a developmental and phylogenetic correlation between independence and the presence of serrations on the inner edge of the mandible. Mandibles of independent species bear serrations at hatching, whereas dependent species hatch with smooth mandibles, only developing serrations in the second instar when these larvae gain the ability to survive on their own. Phylogenetic evidence suggests that serrations coincide with independence repeatedly. We note a single exception to this trend, a beetle with a serrated mandible that cannot survive without parents. However, this exception occurs in a species that has recently evolved the loss of independence. Conclusions We argue that the absence of mandible serrations occurs due to alternative selection pressures incurred in larvae dependent upon parents to survive. We suggest that this may have led to a variable function for mandibles, perhaps related to increased competitive ability among siblings or increased efficiency in receiving nutrition from parents. Furthermore, we propose that the phylogenetic pattern we see is consistent with the long‐held evolutionary hypothesis that evolutionary change in behavior and physiology precede morphological change.
... Many factors affecting the evolution of horn size and shape in male and female bovids have been identified (Bro-Jørgensen, 2007;Caro, Graham, Stoner, & Flores, 2003;Geist, 1966;Lundrigan, 1996;Packer, 1983;Stankowich & Caro, 2009). In several instances, these factors covary with body size. ...
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Allometric relationships describe the proportional covariation between morphological, physiological, or life‐history traits and the size of the organisms. Evolutionary allometries estimated among species are expected to result from species differences in ontogenetic allometry, but it remains uncertain whether ontogenetic allometric parameters and particularly the ontogenetic slope can evolve. In bovids, the nonlinear evolutionary allometry between horn length and body mass in males suggests systematic changes in ontogenetic allometry with increasing species body mass. To test this hypothesis, we estimated ontogenetic allometry between horn length and body mass in males and females of 19 bovid species ranging from ca. 5 to 700 kg. Ontogenetic allometry changed systematically with species body mass from steep ontogenetic allometries over a short period of horn growth in small species to shallow allometry with the growth period of horns matching the period of body mass increase in the largest species. Intermediate species displayed steep allometry over long period of horn growth. Females tended to display shallower ontogenetic allometry with longer horn growth compared to males, but these differences were weak and highly variable. These findings show that ontogenetic allometric slope evolved across species possibly as a response to size‐related changes in the selection pressures acting on horn length and body mass.
... The link between fighting behavior and weapon morphology is thought to be a primary driver of the morphological diversification in sexually selected traits (Mccullough et al., 2016). In this way, one mechanism that decapod claws may have diversified is through their differential use in differential fighting contexts or fighting style (i.e., the divergent-context hypothesis (Lundrigan, 1996;Caro et al., 2003;Emlen, 2008)). The behavioral observations that I conducted on C. chasmodactylus provide preliminary evidence to support this hypothesis. ...
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Despite the diversity in the size, performance, and function of crustacean claws (chelae), our understanding of the functional morphology and evolution of these claws are lacking. Since crayfish claws are multi-function organs that face selection from fighting, mating, foraging, and predation, these pressures all interact to shape the claws morphology and performance. I studied a unique claw morphology, the exaggerated claw gape of the New River crayfish, Cambarus chasmodactylusJames, 1966, by investigating how this claw morphology relates to claw strength and fighting style. Claw length increased in both males and females as body size increased, although this trend was more pronounced in males, and maximal claw strength increased as claw length increased in both males and females. I describe the fighting behavior of the species in relation to previously studied species and speculate on how changes in claw morphology have led to changes in species-specific fighting style. Observations of fighting behavior in C. chasmodactylus revealed a previously undescribed claw grasping behavior, in which competitors grasp onto the manus of their opponent’s claw and laterally twist to flip their opponent. When assessing the relative role of claw size and claw strength on contest success in male individuals, individuals with larger, but not necessarily stronger claws were most likely to attain dominance. These results highlight the functional morphology of a unique claw morphology and provides initial evidence for how claw form relates to fighting style within decapod crustaceans.
... theory of sexual selection provides an explanation for the evolution of extravagant differences between males and females in several phenotypical traits. Weaponry in male ungulates is one of the best-studied secondary sexual characters (Emlen, 2008), whose shape and size are largely driven by selection operating through male-male competition over females (Andersson, 1994;Caro, Graham, Stoner, & Flores, 2003;Geist, 1966). Positive correlations between weapon size and male breeding success have been reported in several species such as red deer Cervus elaphus (Kruuk et al., 2002), bighorn sheep Ovis canadensis (Coltman, Festa-Bianchet, Jorgenson, & Strobeck, 2002), Soay sheep Ovis aries (Robinson, Pilkington, Clutton-Brock, Pemberton, & Kruuk, 2006), and Alpine ibex Capra ibex (Willisch, Biebach, Marreros, Ryser-Degiorgis, & Neuhaus, 2015). ...
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Weaponry in ungulates may be costly to grow and maintain, and different selective pressures in males and females may lead to sex-biased natural survival. Sexual differences in the relationship between weapon growth and survival may increase under anthropogenic selection through culling, for example because of trophy hunting. Selection on weaponry growth under different scenarios has been largely investigated in males of highly dimorphic ungulates, for which survival costs (either natural or hunting-related) are thought to be greatest. Little is known, however, about the survival costs of weaponry in males and females of weakly dimorphic species. We collected information on horn length and age at death/shooting of 407 chamois Rupicapra rupicapra in a protected population and in two hunted populations with different hunting regimes, to explore sexual differences in the selection on early horn growth under contrasting selective pressures. We also investigated the variation of horn growth and body mass in yearling males (n=688) and females (n=539) culled in one of the hunted populations over 14 years. The relationship between horn growth and survival showed remarkable sexual differences under different evolutionary scenarios. Within the protected population, under natural selection, we found no significant trade-off in either males or females. Under anthropogenic pressure, selection on early horn growth of culled individuals showed diametrically opposed sex-biased patterns, depending on the culling regime and hunters’ preferences. Despite the selective bias between males and females in one of the hunted populations, we did not detect significant sex-specific differences in the long-term pattern of early growth. The relationship between early horn growth and natural survival in either sex might suggest stabilizing selection in horn size in chamois. Selection through culling can be strongly sex-biased also in weakly dimorphic species, depending on hunters’ preferences and hunting regulations, and long-term data are needed to reveal potential undesirable evolutionary consequences.
... Several hypotheses have been proposed to explain the diversity of weapon shapes among species [8]. One of the most well-supported hypotheses proposes that weapon diversification results from differences in how and/or where the weapons are used, called the divergent-context hypothesis [3,9,10]. For example, bovids that wrestle with their opponents are more likely to have horns with long central arches [9]. ...
Article
Many sexually selected traits function as weapons, and these weapons can be incredibly diverse. However, the factors underlying weapon diversity among species remain poorly understood, and a fundamental hypothesis to explain this diversity remains untested. Although weapons can serve multiple functions, an undeniably important function is their role in fights. Thus, a crucial hypothesis is that weapon diversification is driven by the evolution of weapon modifications that provide an advantage in combat (e.g. causing more damage). Here, we test this fighting-advantage hypothesis using data from 17 species of coreid bugs. We utilize the fact that male–male combat in coreids often results in detectable damage, allowing us to link different weapon morphologies to different levels of damage among species. We find that certain weapon morphologies inflict much more damage than others, strongly supporting the fighting-advantage hypothesis. Moreover, very different weapon morphologies can inflict similarly severe amounts of damage, leading to a weapon performance landscape with multiple performance peaks. This multi-peak pattern could potentially drive different lineages towards divergent weapon forms, further increasing weapon diversity among species. Overall, our results may help explain how sexually selected weapons have evolved into the diversity of forms seen today.
Article
Male stag beetles battle for females with their impressive, oversized mandibles. We describe their fighting behavior, which is essential to understand the evolution and morphology of their weaponry. Our behavioral analysis reveals several anatomical structures that are important for fighting, and our morphological investigations show how these may be adapted for their functions. Stag beetle fights are much more variable than other armed beetles’ battles. They spend considerable time and effort in dislodging their opponent, that clings to the substrate with its tarsal claws. These tarsal claws are also indispensable to maintain balance in the most spectacular battles, when they lift a rival high in the air. The male claws are highly curved and have an increased height for this purpose. The prothoracic muscles are hypertrophied to support the lifting movement. The largest beetle wins in 85 % of the fights and the smaller the difference in mandible length is between the rivals, the longer the battles can last. The long mandibles enable males to reach the opponent’s legs in order to dislodge it. For this purpose, they bite with all parts of their mandibles, even though the distal part is more vulnerable for failure and transfers less bite force. Blindfolded experiments prove that visual information is not a requisite for a successful battle.
Chapter
Animals are exposed to many threats and dangers in nature, with predation being among the most important factors. In response, prey have evolved an array of morphological defenses to protect themselves against predators, ranging from offensive weaponry to defensive armor and body size. In this article, I outline the morphological diversity in defensive traits and provide some examples that illustrate the role of these morphological defenses in a predator-prey context. As I discuss the different types, it is important to bear in mind that it is exceedingly difficult to test the functional significance of defensive morphologies. Although an antipredator function is often taken for granted, defensive morphologies might be subjected to other selective pressures (e.g., sexual selection) or might possess other functionalities besides protection (e.g., thermoregulation). Hence, whenever possible I attempted to discriminate between traits that primarily function as defensive tools and those that more likely function for another purpose but clearly have a (secondary) antipredator benefit.
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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.
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Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.
Article
The horned, ceratopsid dinosaurs can be easily split into two major groups based on their cranial structures, but now a new discovery shows that at least one genus 'switched sides' and convergently evolved the form of the other clade. Copyright © 2015 Elsevier Ltd. All rights reserved.
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Unlike most of the other members of the wild cattle family, the aurochs is extinct. That means that not all data concerning this animal and its life history can be described, and that some aspects will be examined here in a different way than in other wild cattle species. Linnaeus (1758) described domestic cattle under the name of Bos taurus. He mentioned that at the time its wild ancestor was briefly described by the Roman commander-in-chief Julius Caesar under the name of urus (derived from the Germanic word ‘ur’). In 1827, Bojanus made the first osteological research on an aurochs skeleton and gave to this species the name Bos primigenius. Because he thought it came ‘before the Flood’, he added the word antediluvialis. Though the name given by Bojanus is widely used so far, more correctly the name given by Linnaeus should be employed, because he described the species first. The Spanish word for this animal, ‘uro’, comes directly from the Latin word urus. In both the English and the French languages the word ‘aurochs’ comes from the German word ‘auerochs’. For centuries, this latter word was mistakenly used for the wisent (Bison bonasus). Around the twelfth and thirteenth centuries, during which the original aurochs became extinct in Germany, the word ‘ur’ gradually changed into ‘auer’ and ‘auerochs’ (Szalay 1915). When people were no longer aware of the original aurochs its name passed to a related animal, similarly impressive-the wisent. Such a process has been observed in other species too.
Article
Bovidae display a distinctive cranial architecture, characterised by multifarious frontal sinuses varying in size and shape. These features are usually considered to be of phylogenetic importance and their functional importance still remains elusive. Pneumatisation is the process in which cranial sinuses result from the resorption and deposition of the bone in response to biomechanical stress. In this study, we use high resolution computed tomography (CT-scanning) to identify these internal cranial structures on 12 bovid cranial specimens from Upper Miocene Greek localities. The present study focuses on two genera, Tragoportax and Miotragocerus, so as to explore the variance inside this group in terms of frontal sinuses. The non-destructive nature of this method allows 3D digital representation of cranial internal features of fossilised material in high resolution, providing also volumetric data of the sinuses. We demonstrate the unique internal morphology that these species possess. This morphology is comprised by wide frontal sinuses conforming closely to the shape of the frontal bone and that extend up to the base of their horn cores. The volume of the sinuses was strongly correlated to the frontal bone size and cranial volume.
Article
Antlers are unique characteristic males of family Cervidae. It fully regenerates each year in relation to their sexual cycles. It is primarily used as weapons in intra specific fighting or possibly as organs displaying strength. These are formed by the calcification, beneath the velvet of the fast growing pre-osseous tissue known as velvet antlers. Continuous dividing velvet antlers replace dead antlers every year through shedding. Antlers are composed primarily of minerals, protein, carbohydrates and fatty acids. Calcium and phosphorus, primarily present as hydroxyapatite [Ca5(PO4)3OH] (Chapman 1975). In India there are nine species of deer viz. Kashmir stag (Cervus elaphus), mouse deer (Tragulus meminna), musk deer (Moschus moschiferus), swamp deer (Rucervus duvaucelii), thamin (Rucervus eldii), barking deer (Muntiacus muntjak), chital (Axis axis), hog deer (Axis porcinus) and sambar (Rusa unicolor). Mouse deer and musk deer possess only canines and antler is absent in both the deer species. Previously, the export of shed antlers and peacock feathers was allowed in India. Although, antlers are not very extensive in illegal wildlife trade but artifacts of antlers and they are used for traditional medicines for treatment of various diseases in different countries. The problem of illegal trade of antlers is mainly due to the mis-declaration of its quantity. Mostly antlers of chital, sambar and swamp deer are widely traded. Visualizing the current status of some of the deer species, trade of antler in India has been recently banned. Therefore, to provide legal protection to deer and other related species accordingly brow-antlered deer (Rucervus elidii), hog deer (Axis procinus), and swamp deer (R. duvaucelii) were included in Schedule I, whereas barking deer (Muntiacus muntjak), chital (Axis axis), and sambar (R. unicolor) were in schedule III of Wildlife (Protection) Act, 1972. To strengthen the legal measures and to protect deer species, identification of antlers in complete, pieces or pulverized form is very important. Very few scientific studies deal with identification of antlers belonging to different species. Thus, present study is aimed to identify species from antlers using species specific signature using elemental analysis. Morphological features of antlers in artiodactyls often used as a diagnostic tool to identify species. Although, morphometry is very good technique to identify antlers but as malformation in antlers is common also antlers are seized either in complete form or in small pieces or powder form, therefore, identification through morphometry becomes difficult for deformed antlers or pieces. In such cases elemental analysis is helpful. Elemental composition of antlers of different species helps in inter-species distinction. This study will help managers, scientists and enforcement agencies to identify species from parts and products of antler.
Chapter
Names Genus: Bubalus C. H. Smith, 1827 Species: Tamaraw Bubalus mindorensis Heude, 1888 Names in other languages: French: Tamarau, Tamarao; German: Mindoro-Büffel; Spanish: Tamarao, Búfalo de Mindoro; Italian: Tamaraw Other common names: Mindoro dwarf buffalo. Taxonomy Bubalus mindorensis Heude (1888), type specimen Mindoro Island, Philippines. Despite early debates on its proper taxonomic classification (Everett 1878; Meyer 1878; Bartlett 1878), the tamaraw is classified as a distinct species within the genus Bubalus based on morphological characteristics (Groves 1969; Custodio et al. 1996; Braun et al. 2002; Wilson & Reeder 2005) and genetic analyses (Namikawa et al. 1995; Tanaka et al. 1996, 2000). Steere (1888a) also independently described the tamaraw, placing it in the genus Anoa, but was superseded by Heude (1888). Heude’s type specimen was lost and then rediscovered (Braun et al. 2002) in Xujiahui, China, in the Shanghai Natural History Museum, which acquired the museum Heude founded (Zikawei Museum of Natural History). Subspecies and distribution The tamaraw is endemic to the island of Mindoro, Philippines (Plate 9; Heaney et al. 1987; Custodio et al. 1996). Thought to be previously widely distributed in Mindoro (Everett 1878; Steere 1888b, 1891), by 1949 there appears to have been fewer than 1000 animals left on the island, with an estimated 244 in Occidental Mindoro (Manuel 1957) in at least three areas (Harrisson 1969a). By 2007 the three areas where tamaraw are now thought to remain are in Mounts Iglit-Baco National Park (MIBNP), the Mount Calavite Wildlife Sanctuary and the Aruyan-Malati area (R. M. Boyles, unpublished; J. de Leon, unpublished).
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Reproductive competition generates episodes of both pre- and post-copulatory sexual selection. Theoretical models of sperm competition predict that as the fitness gains from expenditure on the weapons of male combat increase, males should increase their expenditure on weapons and decrease their expenditure on traits that contribute to competitive fertilization success. Although traits subject to sexual selection are known to have accelerated evolutionary rates of phenotypic divergence, it is not known whether the competing demands of investment into pre- and post-copulatory traits affect their relative rates of evolutionary divergence. We use a comparative approach to estimate the rates of divergence in pre- and post-copulatory traits among onthophagine dung beetles. Weapons evolved faster than body size while testes mass and sperm length evolved more slowly than body size, suggesting that pre-copulatory competition is the stronger episode of sexual selection acting on these beetles. Although horns evolved faster than testes, evolutionary increases in horn length were not associated with evolutionary reductions in testes mass. Our data for onthophagines support the notion that in taxa where males are unable to monopolise paternity, expenditure on both weapons and testes should both be favored. This article is protected by copyright. All rights reserved.
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Mammals flex, extend, and rotate their spines as they perform behaviors critical for survival, such as foraging, consuming prey, locomoting, and interacting with conspecifics or predators. The atlas‐axis complex is a mammalian innovation that allows precise head movements during these behaviors. While morphological variation in other vertebral regions has been linked to ecological differences in mammals, less is known about morphological specialization in the cervical vertebrae, which are developmentally constrained in number but highly variable in size and shape. Here, we present the first phylogenetic comparative study of the atlas‐axis complex across mammals. We used spherical harmonics to quantify 3D shape variation of the atlas and axis across a diverse sample of species, and performed phylogenetic analyses to investigate if vertebral shape is associated with body size, locomotion, and diet. We found that differences in atlas and axis shape are partly explained by phylogeny, and that mammalian subclades differ in morphological disparity. Atlas and axis shape diversity is associated with differences in body size and locomotion; large terrestrial mammals have craniocaudally elongated vertebrae, while smaller mammals and aquatic mammals have more compressed vertebrae. These results provide a foundation for investigating functional hypotheses underlying the evolution of neck morphologies across mammals. This article is protected by copyright. All rights reserved
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The misnamed 'Irish Elk' is a late Pleistocene giant deer that ranged southward to North Africa and eastward to China. Since its first scientific description in 1697, it has played a major role in debates about the history of life. Cuvier used it to prove the fact of extinction and set the basis for a geologic time scale. Later, Megaloceros became the rallying point for anti-Darwinians; they invoked orthogenesis to deny natural selection and attributed extinction to an inadaptive trend towards immense antlers. The antlers posed a severe difficulty for the modern synthesis: they were generally explained as allometric correlates of advantageous increases in body size that offset the problems of admittedly disproportionate antlers. Virtually every textbook in evolution cites Megaloceros as a case of allometry contra orthogenesis: nonetheless, no one has ever generated any quantitative data about it. I measured 79 skulls and antlers of Megaloceros to resolve two questions bearing upon the allometric hypothesis: 1) Does Megaloceros, at its maximal body size among cervine deer, lie on the extrapolated line for positive allometry of antlers among smaller cervines? 2) Does the intraspecific variation among adult stags of Megaloceros yield relatively large antlers in stags of large body size? The answer to both questions is undoubtedly yes (Figs. 1-2, 7-10). 1) the static, interspecific allometry of adult cervines is strongly positive; Megaloceros has the predicted antler size for its body size (moose have relatively small antlers for their body size). 2) The exponent of intraspecific allometry is about 2.5. If selection acted to preserve deer of large body size, relatively large antlers would follow as a consequence of correlation. Yet the fact of positive allometry need not provoke the usual interpretation: large bodies might be a consequence of advantageous antlers, or both antlers and bodies might be selected in concert. The assumption of disproportionate antlers is based on the a priori notion that antlers must function as weapons in battle: 90 pound antlers, mounted with tines pointing backward on a 5 pound skull cannot be regarded as well-designed for such a purpose. But deer often use their antlers to establish dominance and win access to females by display and ritualized combat. Moreover, display is especially important in large deer and deer with palmated antlers. The antler morphology of Megaloceros is ideally suited for display: smaller deer must rotate their heads to show the palm. The torque produced by rotation in Megaloceros would have posed severe mechanical problems. But, alone among deer with palmated antlers, Megaloceros displayed its full palm when simply looking straight ahead. The immense antlers of Megaloceros were advantageous in themselves. Its extinction may be traced to late glacial changes in climate.
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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.
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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
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Reviews evidence for 5 functional explanations of the evolution of antlers in male cervids. 1) There is extensive evidence that antlers are used in fights between competing males. Fights are regular during the breeding season and can be damaging. Antlers have proved to be effective weapons of defence and offense, and there is no systematic evidence to support the suggestion that antler-less males (hummels) are more successful in competition for females than antlered stags. 2) Though male deer sometimes use their antlers in defence against predators, the absence of antlers in females of most species suggests that this is not their principal function. 3) Nor does it seem likely that antlers evolved as heat-regulating mechanisms. 4) There is no conclusive evidence that males assess each other by their relative antler size, most measures of antler size and shape are not closely correlated with dominance or fighting ability. 5) Nor is there firm evidence that females selectively mate with large-antlered males. Antlers thus evolved as weapons and are retained by selection because of their function in intra-specific combat. -from Author
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Animal colour patterns are adaptive for three reasons: camouflage, communication and physico-physiological functions. This study proposes a conceptual framework for predicting the main adaptive function of carnivore colour patterns based on three factors: visibility, shape and location on the body, as well as, their behavioural ecological correlates. Using a comparative phylogenetic approach, the colour patterns present on the body, the tail and the eyes of 200 species of mammalian carnivores were analysed. Their evolutionary history was reconstructed using MacClade and Maddison's concentrated-changes test was used to test the association between species' colour patterns and their behavioural ecology on a composite phylogeny for all the Carnivora. The results for dark spots, vertical stripes, horizontal stripes, ringed tails, black tail tips, white tail tips, dark eye contour and dark eye patches, are presented. The comparative analyses indicate that spotted, vertically striped and horizontally striped coats evolved for camouflage. Tail markings seem to have evolved for intra- and/or inter-specific communication, while dark markings near and around the eyes are associated with variables consistent with a physico-physiological function. These findings suggest that both the physical environment and animal behaviour are important selective factors driving the evolution of animal colour patterns and that both need to be taken into consideration in future studies of animal coloration.
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Although native to Corsica and Sardinia, mouflon sheep (Ovis orientalis musimon) have been widely introduced to other areas, including the United States. This study examines the courtship and agonistic behavior of 2 captive mixed-sex herds in west-central Texas, U.S.A. These herds were observed in large enclosures on game ranches during the autumn of 1980. Inter- or intra-sexual interactions consisted of one or more repeated behavioral postures (i.e. low stretch, twist, foreleg kick, sniff rear, urination, flehmen, mount, horn threat, chest push, neck fighting, head butt, clash, horn rubbing, chase). Ninety-five male-male, 63 female-female and 155 male-female interactions were observed. Low intensity agonistic displays (with total number of occurrences in the interactions given in paren-theses) such as foreleg kicks (154) and twists (141) were most frequently seen in male-male interactions. Although head butting (96) was commonly observed in male-male encounters, clashes (52) were also prominent. In female-female agonistic interactions, an unexpectedly high frequency of head butts (99) were regularly exhibited. Agonistic and courtship behavior patterns such as twist (131) and flehmen (106) were prominent in the behavior of males toward females. Although males would occasionally head butt a female (22), they seemed to inhibit clashing in their interactions with the hornless females. In most cases, female-female interactions were initiated by head butts (46) and male-male encounters began with either a head butt (23) or twist (18). Male-female interactions showed more variation in the behaviors which initiated the encounter, but usually began with a twist (39), flehmen (29) or low stretch (20) by the male. The observations of mouflon in this study conformed well with previous reports of sheep behavior.
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Horns of bovids are remarkably diverse. This diversity may reflect functional differences associated with the use of horns as intraspecific weapons. I use measurements from museum specimens and behavioral data from the literature to examine the relationship between morphology of horns of males and fighting behavior in 21 species of bovids, representing 11 of the 12 bovid tribes. A high correlation between morphology of horns and fighting behavior was found. In particular, a short horn reach and undeveloped catching arch is associated with stabbing behavior; a long horn reach, with wrestling and fencing behavior; a well-developed catching arch, with wrestling behavior; and robust, recurved horns, with ramming behavior. A phylogeny of bovid tribes suggests that these features of morphology of horns and fighting behavior are rapidly evolving and frequently convergent.
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Five types of species can be identified in large mammals. The evolution of three types, Ice Age giants, island dwarfs, and hybrids, can be explained, but not that of tropical food specialists and continental paedomorphs. Ice Age giants, which arose while colonizing latitudes (altitudes) with increasingly seasonal climates and productivity pulses, are characterized by ornate social organs, large bodies, and ecological plasticity. Colonizing landscapes with decreasing seasonality appears to conserve (or re-evolve) primitiveness, producing paedomorphs. Island dwarfs appear to be shaped by efficiency selection in the absence of predators. The explanation of mammalian Ice Age evolution hinges on the sensitivity of mammals to environmental factors, in particular nutrition. Extremes in food abundance generate extremes in phenotypes and selection regimes. Abundance is linked to colonization and selection for new social and ecological adaptations; scarcity is typical of settled areas and maintenance regimes. These select for efficiency in the procurement, processing, and use of food. Rapid speciation is predicted during colonization, followed by a gradual, continuous fine tuning of the ecology of the new form. Neither the punctuated nor the gradualistic model of speciation adequately explains evolution in large mammals. Early predictions of the "dispersal hypothesis" of mammalian evolution have now been tested for caprids. Results from cytogenetic, electrophorectic, and immunodiffusion studies support the dispersal hypotheses.
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A review of the behaviour of bovids and cervids when fighting suggests that it is only the dimensions of the base of horns and antlers which are important mechanically in their design. A fourth power function of the diameter of the base of a horn (the second moment of area (I)) increases linearly with body weight so that there is the same maximum stress in the horn during fighting. This is consistent with the efficient use of materials and is found to be the case for different types of fighting. Sheep and goats fight most forcefully and have a higher ratio of I to body weight than antelopes and deer, which tend to use less forceful wrestling.
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The evolutionary bases for horns, antlers, and tusks in male ungulate mammals have been extensively investigated, but the reasons for the presence or absence of hornlike organs in female ungulates have not been thoroughly reviewed. Here I examine (1) the taxonomic distribution of horns and hornlike organs in females and evidence on the evolutionary history of the trait; (2) behavioural evidence on the uses of horns and hornlike organs by females and socio-ecological correlations with female hornedness or hornlessness; and (3) evidence on the role of genes and hormones in controlling the expression of female horns. In cervids, antlered females may be rare because of a constraint to regrow antlers yearly, if at all, small social group sizes, and a direct linkage between androgen production and the initiation of antler growth. For non-cervid ungulate females, there appear to be no overriding or unifying aptive factors governing the evolutionary development of horns or hornlike organs, although the trait may be aptive in some cases.
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We used comparative data to test functional hypotheses for 17 antipredator behaviour patterns in artiodactyls. We examined the literature for hypotheses about auditory and visual signals, defensive behaviour and group-related antipredator behaviour in this taxon and derived a series of predictions for each hypothesis. Next, we documented occurrences of these behaviour patterns and morphological, ecological and behavioural variables for 200 species and coded them in binary format. We then pitted presence of an antipredator behaviour against presence of an independent variable for cervids, bovids and all artiodactyls together using nonparametric tests. Finally, we reanalysed the data using Maddison's (1990, Evolution, 44, 539–557) concentrated-changes tests and a consensus molecular and taxonomic phylogeny. We found evidence that snorting is both a warning signal to conspecifics and a pursuit-deterrent signal, lack of evidence that whistling alerts conspecifics and indications that foot stamping is a visual signal to warn group members. Evidence suggested that tail flagging was a signal to both conspecifics and predators, that bounding, leaping and stotting were used both as a signal and to clear obstacles and that prancing functioned similarly to foot stamping. Analyses of tail flicking, zigzagging and tacking were equivocal. We confirmed that inspection occurs in large groups, freezing enhances crypticity, and species seeking refuge in cliffs tend to be small. Entering water and attacks on predators had few correlates. Finally, group living, a putative antipredator adaptation, was associated with large body size and species living in open habitats, confirming Jarman's (1974, Behaviour, 48, 215–267) classic hypothesis. Bunching and group attack apparently deter predators. Despite limitations, comparative and systematic analyses can bolster adaptive hypotheses and raise new functional explanations for antipredator behaviour patterns in general.
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... STEPHEN JAY GOULD Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 ... no way correspond (as they c