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Graph illustrating natural selection driving phenotypic divergence between males (grey lines) and females (black lines). The directions of selection (indicated by arrows) are predicted to track the mean dimensions of the prey. Please note that three possible scenarios exist: (1) Male phenotype remains stable through time (stabilizing selection), with females changing their mean phenotype: (2) the reverse of 1: and (3) males and females both diverge in mean phenotype as depicted in this figure. These scenarios are highly similar to the predictions based on quantitative genetic modeling of ecological character displacement (Slatkin 1984), but can also evolve as a result of adaptation to divergent ecological niches (see text for details).
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Sexual dimorphism in phenotypic traits associated with the use of resources is a widespread phenomenon throughout the animal kingdom. While ecological dimorphisms are often initially generated by sexual selection operating on an animal's size, natural selection is believed to maintain, or even amplify, these dimorphisms in certain ecological settin...
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... dimorphism in phenotypic traits associated with the use of resources is a widespread phenomenon throughout the animal kingdom (Darwin 1871; Selander 1972; Ralls 1976; Slatkin 1984; Shine 1989, 1991; Cullum 1998; Myersterud 2000; Temeles et al. 2000; Lailvaux et al. 2003; Heatwole et al. 2005; Vincent 2006). As discussed in the introduction to this symposium (Lailvaux and Vincent in review), ecological dimorphisms are often generated initially by sexual selection acting on an animal’s body size (either male–male combat or selection for increased fecundity leading to larger body sizes), but are typically amplified or maintained by natural selection (Slatkin 1984; Myersterud 2000). Even so, the potential role(s) of natural selection in this process is not always clear-cut. For example, Slatkin (1984) showed by taking a quantitative genetic modeling approach that sex-based divergence in phenotype and in ecological traits can evolve in a manner similar to competitive character displacement (i.e., one or both sexes diverge in ways that reduce intersexual competition for resources) (Fig. 1). In contrast, empirical work has shown that in species exhibiting size dimorphism in size (SSD; body size in females differs from that in males), intersexual ecological divergence is more likely to evolve purely as the result of adaptation to divergent niches (i.e., the sexes have different adaptive peaks due to differences in body size) (Selander 1972; Ralls 1976; Shine 1991; Myerstud 2000; Vincent 2006). Although previous authors have formulated several sub-hypotheses (e.g., predation risk, differential energetic requirements) under the ‘‘dimorphic niche hypothesis’’ (reviewed by Myersterud 2000; Shine and Wall 2004), here we will largely restrict our attention to the broad implications of the hypothesis. Understanding the evolutionary origins and adaptive significance of ecological dimorphisms is still further complicated by the fact that hormones can play a pivotal role in the evolution of SSD in many animal species (Lerner and Mason 2001; Cox and John-Alder 2005 and references therein; but see Taylor and DeNardo 2005 for a counterexample), which may in turn influence the ecology of the sexes. The feeding apparatus of gape-limited snakes (that do not reduce the size of their prey before ingestion) has proven to be a model system for testing the adaptive nature of ecological dimorphisms for two reasons: (1) head size is believed to rarely be under sexual selection in snakes (e.g., Shine 1991), and (2) because head size limits the maximum size of prey that can be consumed; the sex with the larger head should thus consume larger prey (Shine 1991; Houston and Shine, 1993; Pearson et al. 2002; Shetty and Shine 2002; Shine et al. 2002; Shine and Wall 2004; Vincent et al. 2004a). In contrast, significantly less attention has been paid to the evolution of dietary dimorphisms in other types of squamates such as lizards. This lack of research is likely due to the fact that the feeding apparatus of lizards can be under both sexual and natural selection simultaneously, and these animals extensively chew their prey prior to ingestion (see Herrel et al. 1996, 1999, 2001a, b for an overview; Reilly et al. 2001). Consequently, it is significantly more difficult to generate clear-cut hypotheses to test for the presence of adaptive ecological dimorphisms in lizards compared to snakes. Furthermore, a potentially serious confounding factor in the analysis of adaptive sex-based morphological divergence is the pervasive influence of hormones on animal growth and development. This issue is particularly relevant for both lizards and snakes because steroid hormones such as testosterone have been shown to directly influence the degree of SSD, as well as the shape of the feeding apparatus (Crews et al. 1985; Shine and Crews 1988; Lerner and Mason 2001; Cox et al. 2006), due to its inhibitory effect on male growth in some species (reviewed by Cox and John-Alder 2005). For example, red-sided garter snakes ( Thamnophis sirtalis parietalis ) exhibit significant sexual dimorphism in both body size and relative jaw length (corrected for body size), with females being larger in both aspects. The proximate cause of this dimorphism, however, was shown to be higher levels of circulating testosterone in males than in females, thus causing males to grow more slowly than females (Crews et al. 1985; Lerner and Mason 2001), and not sex-based ecological divergence (but see Krause et al. 2003; Krause and Burghardt in press for recent counterevidence). Further supporting this claim, Shine and Crews (1988) showed that the marked dimorphism in relative jaw length did not always translate into differences between the sexes in maximum size of prey consumed in natural populations. As a result, sex-based divergence in head shape—in the absence of quantitative dietary data—should not be viewed as com- pelling evidence for adaptive ecological divergence between the sexes. In order to tease apart the respective influences of natural and sexual selection on the one hand, and developmental effects on the feeding apparatus of male and female squamates on the other, we formulate two straightforward testable hypotheses for snakes and lizards, respectively. Our overarching goal is to provide a quantitative framework that will enable future researchers to clearly distinguish between adaptive and nonadaptive influences on the feeding apparatus of male and female animals, and to point out areas that still need to be addressed with empirical data. We subsequently test these hypotheses by both reviewing the scientific literature on intersexual dietary divergence in lizards and snakes, and by providing empirical data to fill in critical gaps for some species. Theoretically, there are two nonmutually exclusive mechanisms by which the sexes in gape-limited snakes may exploit different types, shapes, and/or maximum sizes of prey. (1) The sexes can differ in absolute size of head and body because larger-bodied snakes can consume larger maximum sizes of ...
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The relative numbers of trunk (body) and caudal (tail) vertebrae in snakes might be influenced by at least four processes: (1) natural selection for crawling speed, (2) fecundity selection for larger trunk size in females, (3) sexual selection for longer bodies or tails in males and/or (4) developmental constraints (if an increase in the number of...
1. Two often cited hypotheses explaining sexual head size dimorphism in lizards are: sexual selection acting on structures important in intrasexual competition, and reduction of intersexual competition through food niche separation.
2. In this study some implicit assumptions of the latter hypothesis were tested, namely that an increase in gape dist...
Although differential selective pressures on males and females of the same species may result in sex-specific evolutionary trajectories, comparative studies of adaptive radiations have largely neglected within-species variation. In this study, we explore the potential effects of natural selection, sexual selection, or a combination of both, on bite...
In the stag beetle family (Lucanidae), males have diverged from females by sexual selection. The males fight each other for mating opportunities with their enlarged mandibles. It is known that owners of larger fighting apparatuses are favoured to win the male-male fights, but it was unclear whether male stag beetles also need to produce high bite f...
Citations
... However, evidence supporting sexual size dimorphism primarily originating from ecological divergence is limited across different groups (e.g. Slatkin, 1984;Shine, 1989;Fairbairn, 1997;Mysterud, 2000;Vincent & Herrel, 2007). Furthermore, underlying ecological factors may also influence sexual size dimorphism in tiger beetles. ...
Background
Sexual dimorphism, driven by sexual selection, leads to varied morphological distinctions in male and female insects, providing insights into selection pressures across species. However, research on the morphometric variability within specific taxa of tiger beetles (Coleoptera: Cicindelidae), particularly arboreal and semi-arboreal species, remains very limited.
Methods
We investigate sexual dimorphism in six semi-arboreal Therates tiger beetle taxa from the Philippines, focusing on morphological traits. We employed morphometric measurements and multivariate analyses to reveal patterns of sexual dimorphism between sexes within the taxa.
Results
Our results indicate significant sexual dimorphism in elytra width, with females consistently displaying broader elytra, potentially enhancing fecundity. Notable sexual size dimorphism was observed in Therates fulvipennis bidentatus and T. coracinus coracinus , suggesting heightened sexual selection pressures on male body size. Ecological factors, mating behavior, and female mate choice might contribute to the observed morphological variation. These findings emphasize the need for further studies to comprehend mating dynamics, mate choice, and ecological influences on morphological variations in semi-arboreal and arboreal tiger beetles.
... The likely origination of the FG raptorial specialist subgroup at the beginning of the Kasimovian represents an enhancement in jaw robusticity and anterior biting efficiency (Figs. 2 and 7 and Supplementary Fig. 6a) that enabled these new raptorial specialists to better grasp, pierce and hold prey. Bite force has been found to scale positively with prey hardness and size in extant terrestrial reptiles 77,78 , indicating that these new predators were likely feeding on larger prey and subject to greater stresses on their jaws during prey capture/consumption (Fig. 3). The proliferation of FG feeding functionality among eothyridids, ophiacodontids, and sphenacodontians during the Kasimovian coincides with the first diversification of herbivorous tetrapods (Fig. 7) such as diadectids, captorhinids, and edaphosaurids 76,79,80 . ...
Terrestrial ecosystems evolved substantially through the Palaeozoic, especially the Permian, gaining much new complexity, especially among predators. Key among these predators were non-mammalian synapsids. Predator ecomorphology reflect interactions with prey and competitors, which are key controls on carnivore diversity and ecology. Therefore, carnivorous synapsids may offer insight on wider ecological evolution as the first complex, tetrapod-dominated, terrestrial ecosystems formed through the late Palaeozoic. Using morphometric and phylogenetic comparative methods, we chart carnivorous synapsid trophic morphology from the latest Carboniferous to the earliest Triassic (307-251.2 Ma). We find a major morphofunctional shift in synapsid carnivory between the early and middle Permian, via the addition of new feeding modes increasingly specialised for greater biting power or speed that captures the growing antagonism and dynamism of terrestrial tetrapod predator-prey interactions. The further evolution of new hypo- and hypercarnivorous synapsids highlight the nascent intrinsic pressures and complexification of terrestrial ecosystems across the mid-late Permian.
... Previous work has suggested that factors affecting the direction and magnitude of SSD in turtles may include fecundity (Cox et al., 2003). Other works, noted that sexual dimorphism is influenced by the characteristics of habitats as variables differ between localities such as food availability and partitioning and differences in hormone levels (Cox and John Alder, 2005 ;Shine, 1989, Vincent andHerrel, 2007). ...
The Mediterranean pond turtle Mauremys leprosa is widely distributed in Morocco. Significant variations could be observed in this species due to the fact that it occupies a vast and environmentally diverse area. Thus, the analysis of population parameters is crucial in elucidating the differences between populations. Differences between individuals may be linked to environmental factors, including many habitat characteristics. In the present study, variation in population structure, body condition, morphology and sexual dimorphism, were examined for the Sahara Desert blue-eyed turtle M. l. saharica in southern Morocco from seven distinct localities distributed in four different drainages in southern Morocco. A total of 224 turtles were captured, sexed, weighed and then measured for their carapace and tail dimensions before being released. Among the studied populations, four were dominated by adult individuals (Taakilt: 96%, Oued Guir: 87%, Oued Ziz: 75%, Sidi ElMehdaoui: 70%), two comprised 100% of adults (Oued Noun and Tarmigt) and one population consisted of 50% adults (Lower Draa). The sex ratio was significantly different between populations, being balanced in two populations (Tarmigt and Sidi El Mehdaoui), male-biased in three populations (Oued Zizi, Oued Noun, Lower Draa) and female-biased in the two remaining populations (Oued Guir and Taakilt). Body Condition Index (BCI, g/cm3) was significantly different among populations but not between sexes, or for the Sex × Locality interaction. The Sexual Size Dimorphism (SSD), females being larger than males, varies among population with the greatest degree observed in Oued Guir population (SSD = 0.41) and the lowest in Sidi El Mehdaoui population (SSD = -0.14). Length-Weight Relation (LWR) shows that M. l. saharica has isometric growth for both male and female of Oued Guir and Tarmigt, for female of Oued Ziz, Taakilt, Oued Noun and lower Draa and hypo-allometric for male of Oued Ziz, Taakilt, Oued Noun, Lower Draa and for both male and female of Sidi El Mehdaoui. Morphological variation of carapace is shown in this species associated with population variation among basins. Increased understanding of population characteristics and their fitness status must be considered in conservation and management action plans for the species and its habitat.
... In H. pastazae, females also have a larger head size compared to males (Garcia-Cobos et al., 2021). While sexual dimorphism in predatory behaviour or diet has not been reported in these species, sexual size dimorphism has been linked to trophic divergence between sexes of other snake species (Vincent & Herrel, 2007). ...
... For H. pastazae, females also have a larger head size compared to males (Garcia-Cobos et al., 2021). Sexual dimorphism on head size in aquatic snakes often leads to differential prey use, in which females obtain larger prey items compared to males (Mushinsky et al., 1982;Shine, 1986;Camilleri & Shine, 1990;Houston & Shine, 1993;Avila et al., 2006;Luiselli et al., 2007;Vincent & Herrel, 2007). This difference in prey size between sexes may lead to variation in the prey species as well as in the frequency of prey capture and feeding. ...
Understanding the roles of ecological and sexual selection in the variation of sensory systems may elucidate aspects of the natural history of organisms. Little is known about the evolution of mechanoreception in snakes and how the function and structure of mechanoreceptors vary between species or sexes. Here, we describe the internal and external morphology of cephalic mechanoreceptor sensilla and quantify inter- and intraspecific variation in four sensilla traits of two freshwater snake species that differ in their habitat and diet preferences, Helicops pastazae and Helicops angulatus, by combining scanning electron microscopy (SEM), histological techniques and image analyses. SEM showed sensilla as prominent evaginations of the epidermis surrounded by concentric rings, with H. pastazae having larger and more heterogeneous sensilla. In both species, histology showed a reduction in the outer epidermal layer above the sensilla with a grouping of dermally derived central cells below it. Higher values of sensilla traits were found in H. pastazae, except for the chin-shields. We also found that males of both species had significantly higher values of sensilla traits on all of the scales examined. We hypothesize that the variation in both qualitative and quantitative traits in scale sensilla might be a consequence of differences in foraging and/or reproductive strategies between species and sexes.
... Squamates have played, and continue to play, an important role in research on SSD (e.g. Carothers 1984;Shine 1991Shine , 1994Stamps 1993;Cox et al. 2003Cox et al. , 2007Stuart-Fox & Moussalli 2007;Vincent & Herrel 2007;Meiri et al. 2014). Squamates constitute excellent model organisms on which to test macroevolutionary patterns because they are widespread, rich, and abundant, and exhibit a wide SSD range, from marked male bias to strong female bias (Cox et al. 2003(Cox et al. , 2007Scharf & Meiri 2013;Tarr et al. 2019). ...
Rensch's rule relates to a pattern whereby sexual size dimorphism (SSD) is more female-biased in small-sized species and more male-biased in large-sized ones. We collected literature and museum data on the body size of males and females belonging to 4,032 lizard species, as well as data on their reproductive modes and clutch sizes. We used phylogenetic comparative analyses, and general linear mixed models, to test Rensch's rule and examined how reproductive mode and clutch size affect sexual size dimorphism. Sexual size dimorphism was independent of clutch size in lizard species with variable clutch sizes and in oviparous lizards. Large litters were associated with female-biased sexual dimorphism in viviparous and in scincomorph lizards. Inference regarding Rensch's rule depended on the analytical method used to identify it. The widely used, but less conservative, reduced major axis regression usually support Rensch's rule while ordinary least squares regressions mostly show isometric relationships. The rule tended to apply more to oviparous than viviparous lizards. We infer that Rensch's rule is, at best, a weak pattern in lizards. This is especially true in viviparous lineages, where females reproduce infrequently, therefore evolve large sizes to maximise fecundity, resulting in female-biased dimorphism.
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... It is possible that these two either represent different species or that the size difference originates from sexual dimorphism or ontogeny, because variation in the size of the skull between adult males and adult females or juveniles is common among lacertids (e.g. Klemmer, 1957;Darevsky, 1967), and it is usually male-biased (Vincent and Herrel, 2007, and references therein). The smaller morphotype M3 might represent the adult female or juvenile, and the larger morphotype M4 the adult male version of a single species. ...
Several extinct species are known from the family Lacertidae, but due to poor preservation, many of them are based on single bones. Here, we compare phylogenetic signals of disarticulated premaxillae, maxillae and dentaries of lacertids from four French Oligocene localities (Coderet, La Colombière, Roqueprune 2, Mas de Got B). We identified five morphotypes among the premaxillae, six among the maxillae, and ten among the dentaries. These morphotypes were scored as individual taxa per locality into three separate character matrices with the same 246 characters, one matrix for each jaw element. Subsequently, the phylogenetic position of the morphotypes was tested using maximum parsimony. The consensus trees with the dentaries and the maxillae found a large polytomy including all taxa except the outgroup taxon Gekko gecko. The consensus tree with the premaxillae showed a considerably more resolved topology but found all morphotype taxa outside Lacertidae. In a second step, we compared the constitution of our three datasets and the morphotype taxa. Our results suggest that a combination of convergent characters and missing data led to the outgroup position of the premaxilla morphotype taxa. The poor resolution of the maxillae strict consensus is likely a consequence of their fragmentary preservation. For the dentaries, a high amount of missing data due to the high number of morphotype taxa most likely caused the poor tree resolution. Indeed, tests with fewer morphotypes found tree resolutions comparable to the premaxilla data. When linking the morphotypes, five possible lacertid “species” were found. Comparison with already known French Oligocene lacertid species points to a slightly higher species richness of Lacertidae at that time than known before. Reliable species classification based on phylogeny only seems possible when combining the jaw elements or in association with other cranial and postcranial material, putting some doubt on species identifications based on single bones.
... The reason for this finding in the case of P. siculus could be related to the ability of this generalist species to rapidly shift its diet towards a more omnivorous one to counteract the scarcity/instability of trophic resources (Pérez-Mellado and Corti, 1993;Taverne et al., 2019). Due to these often controversial and inconsistent results, sexual selection has been claimed to be the main driver of sexual dimorphism divergence in Podarcis lizards (Vincent and Herrel, 2007). ...
Islands are compelling natural laboratories for studying evolutionary processes. Nevertheless, the existence of general rules underlying morphological evolution on islands remains an unresolved issue. In this study, we investigated the insular phenotypic variability of the Italian wall lizard (Podarcis siculus) on a large geographical scale, in order to assess the putative existence of an island effect on three morphological head traits: shape, size and degree of sexual dimorphism. A geometric morphometric analysis was performed on 30 island and 24 mainland populations, involving a total of 992 specimens, and we analysed differences in both mean trait values and variances (disparity). We found increased shape disparity in insular lizards with respect to mainland ones. On the other hand, both size disparity and mean head dimensions of males decreased on islands, leading to a reduction in sexual dimorphism. Our results provide evidence for a multidirectional morphological diversification on islands concerning head shape of both sexes, while directional and canalizing selection likely occurred for head size, but only in males. Our findings improve our knowledge on the effect of insularity in Podarcis siculus, and highlight the need for an exstensive sampling scheme and a multi-trait methodological approach.
... The reason for this finding in the case of P. siculus could be related to the ability of this generalist species to rapidly shift its diet towards a more omnivorous one to counteract the scarcity/instability of trophic resources (Pérez-Mellado and Corti, 1993;Taverne et al., 2019). Due to these often controversial and inconsistent results, sexual selection has been claimed to be the main driver of sexual dimorphism divergence in Podarcis lizards (Vincent and Herrel, 2007). ...
Islands are compelling natural laboratories for studying evolutionary processes. Nevertheless, the existence of general rules underlying morphological evolution on islands remains an unresolved issue. In this study, we investigated the insular phenotypic variability of the Italian wall lizard (Podarcis siculus) on a large geographical scale, in order to assess the putative existence of an island effect on three morphological head traits: shape, size and degree of sexual dimorphism. A geometric morphometric analysis was performed on 30 island and 24 mainland populations, involving a total of 992 specimens, and we analysed differences in both mean trait values and variances (disparity). We found increased shape disparity in insular lizards with respect to mainland ones. On the other hand, both size disparity and mean head dimensions of males decreased on islands, leading to a reduction in sexual dimorphism. Our results provide evidence for a multidirectional morphological diversification on islands concerning head shape of both sexes, while directional and canalizing selection likely occurred for head size, but only in males. Our findings improve our knowledge on the effect of insularity in Podarcis siculus, and highlight the need for an exstensive sampling scheme and a multi-trait methodological approach. ADDITIONAL KEYWORDS: disparity-evolution on islands-geometric morphometrics-head shape-sexual dimorphism-size.
... Because most ectotherms are ecologically independent from the time of hatching, a population typically contains free-ranging individuals of a wide range of body sizes; mean sizes of males and females can diverge dramatically (e.g., Pough 1980, Pettorelli et al. 2011. Differences in body sizes may engender divergence in trophic roles, with larger individuals focusing on larger prey types and sometimes, foraging in different ways in different habitats (e.g., Vincent et al. 2005, Vincent andHerrel 2007). Additionally, the low maintenance metabolic requirements of ectotherms facilitate niche specialization of subgroups within the population; for example, one sex may feed only at one time of year, on prey that are only available at that time, whereas the other sex feeds on prey available at other times (e.g., Goiran et al. 2013). ...
Within a population of apex predators, differences among individuals can influence both their ecological impact and their vulnerability to threatening processes. Our field studies on a large monitor lizard (Varanus panoptes) in the Australian wet–dry tropics show that diets shift seasonally and depend upon a lizard’s sex and body size. Individuals that had previously been recorded to consume frogs were most at risk following biological invasion by toxic cane toads (Rhinella marina), as were individuals with broad diets during the wet season. As a result, mortality of those individual predators likely reduced predation pressure on other taxa (invertebrates and reptiles) that were frequently consumed by the same lizards that ate frogs, but with less benefit for taxa (e.g., rodents) that were consumed by non‐anuran‐eating individuals within the predator population. In particular, individuals killed by cane toads often had consumed agamid lizards, a group whose abundance has been reported to increase due to toad‐induced mortality of V. panoptes. To understand the vulnerability of apex predators, or the ecological consequences of their extirpation, we need to incorporate the role of variation among individuals in critical ecological traits.
... functional properties of food items and food niche breadth) are drivers of variation in bite force. Sexual dimorphism in head dimensions may allow for food partitioning between males and females (Schoener, 1967;Herrel et al., 1999a, b) but is generally considered to be maintained through male-male competition in lizards (Vincent & Herrel, 2007;Vanhooydonck et al., 2010). Dimorphism in head dimensions was previously demonstrated to be decoupled from dimorphism in diet in this study system (Taverne et al., 2019), hence suggesting that niche partitioning for limited resources probably does not explain the observed head dimorphism. ...
Bite force is a key performance trait in lizards because biting is involved in many ecologically relevant tasks, including foraging, fighting and mating. Several factors have been suggested to impact bite force in lizards, such as head morphology (proximate factors), or diet, intraspecific competition and habitat characteristics (ultimate factors). However, these have been generally investigated separately and mostly at the interspecific level. Here we tested which factors drive variation in bite force at the population level and to what extent. Our study includes 20 populations of two closely related lacertid species, Podarcis melisellensis and Podarcis sicula, which inhabit islands in the Adriatic. We found that lizards with more forceful bites have relatively wider and taller heads, and consume more hard prey and plant material. Island isolation correlates with bite force, probably by driving resource availability. Bite force is only poorly explained by proxies of intraspecific competition. The linear distance from a large island and the proportion of difficult-to-reduce food items consumed are the ultimate factors that explain most of the variation in bite force. Our findings suggest that the way in which morphological variation affects bite force is species-specific, probably reflecting the different selective pressures operating on the two species.
