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Carnivory maintains cranial dimorphism between males and females: Evidence for niche divergence in extant Musteloidea

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... En particular, las aproximaciones ecomorfológicas se basan en cuantificar la variación de la forma y evaluar en qué medida dicha variación es atribuible al ambiente (factor ecológico) y a sus relaciones de parentesco (factor filogenético) (30). De este modo, se puede estudiar la relación forma-función y vincular las características anatómicas a funciones ecológicas esenciales como la alimentación (9,27,(31)(32)(33)(34). ...
... Para los análisis morfométricos, se seleccionaron landmarks (LMs) y semi-landmarks (SLMs) considerando puntos homólogos observables y repetibles en todos los especímenes. Los puntos escogidos corresponden a regiones asociadas a la mecánica de masticación y, por ende, a su rol potencial en la alimentación (27,(32)(33)(34). Mediante los softwares TpsUtil32 (37) y TpsDig232 (38), se digitalizaron 10 LMs y 50 SLMs en la vista lateral de la mandíbula, 21 LMs y 20 SLMs en la vista ventral del cráneo, 14 LMs y 96 SLMs en la vista dorsal del cráneo y 11 LMs y 65 SLMs en la vista lateral del cráneo (Anexos 4 y 5). ...
... En cuanto a la variación morfológica entre sexos, nuestros resultados indicaron la ausencia de dimorfismo sexual en la forma y el tamaño craneal de Lontra felina, aunque mostraron señales mínimas de diferenciación en la morfología mandibular que dependen únicamente del tamaño. Esta variabilidad casi inexistente puede ser atribuida a diversos factores ambientales e históricos que, según estudios previos, propician la presencia de un menor grado de dimorfismo sexual en las nutrias con respecto a los carnívoros completamente terrestres del clado Musteloidea (34). Law & Mehta (2018) señalaron que el amplio rango dietético y la mayor abundancia de presas en ambientes acuáticos contribuyen a la disminución de la competencia intersexual en Lutrinae, por lo cual es más probable que las especies de nutrias presenten pocas o nulas diferencias entre sexos. ...
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La nutria marina, Lontra felina, es un carnívoro generalista con marcadas diferencias en la dieta a lo largo de su distribución en las costas del Pacífico sudeste, pues las poblaciones del norte (Perú) son principalmente piscívoras, mientras que las del sur (Chile) son predominantemente durófagas (e.g. crustáceos). Diferencias alimenticias existen entre especies vivientes de nutrias (Lutrinae) y han sido asociadas a disparidad en las proporciones del cráneo y la mandíbula. Dado que L. felina no ha sido incluida en análisis cuantitativos, se desconoce si su anatomía corresponde con alguno de los ecomorfotipos establecidos para las nutrias –piscívoro o durófago– y si éste presenta variaciones geográficas o sexuales. El objetivo de este estudio fue establecer el ecomorfotipo de alimentación de Lontra felina en un contexto filogenético e investigar su variación morfológica intraespecífica. Utilizando morfometría geométrica, se cuantificó la morfología craneal y mandibular de especímenes de Lontra felina de Perú y Chile y otras 15 especies de mustélidos. La comparación interespecífica se realizó mediante análisis de componentes principales con mapeo filogenético y las variaciones intraespecíficas se evaluaron usando análisis discriminantes y pruebas de t. Los análisis morfométricos demostraron que la nutria marina presenta un ecomorfotipo alimenticio mixto, con características tanto del piscívoro (cráneos más alargados, planos y estrechos, mandíbulas más alargadas y procesos angulares más grandes) como del durófago (cráneos más cortos, convexos y anchos, áreas molariformes más grandes y mandíbulas más cortas). Los análisis intraespecíficos indicaron la ausencia de dimorfismo sexual, pero revelaron diferencias geográficas en la forma asociadas a la variación latitudinal en la dieta entre Perú y Chile. Así, las poblaciones de Perú exhibieron cráneos más alargados y estrechos, propios del ecomorfotipo piscívoro, y, las de Chile, cráneos más cortos y anchos, típicos del ecomorfotipo durófago. Considerando las diferencias genéticas previamente reportadas, nuestros resultados apoyan la existencia de un proceso de especiación alopátrica en marcha conducido por la dieta y cuyo conocimiento es de relevancia para su conservación.
... Therefore, hyperallometric patterns in both mammals and birds also suggest that sexual selection influences the evolution of SSD 4,8 . However, researchers have also found inconsistent relationships between the degree of SSD and the degree of polygynous mating systems; i.e. the most polygynous or territorial species are not always the most sexually dimorphic and monogamous species are not always the least sexually dimorphic 4,14,19,20 . These inconsistent patterns suggest that additional evolutionary processes may also influence the evolution and maintenance of SSD. ...
... In terrestrial carnivorans, researchers similarly hypothesized that the greatest degree of male-biased SSD should occur in species that are highly territorial and exhibit polygynous mating systems. However, the pattern between social/mating system and the degree of sexual dimorphism is not consistent among terrestrial carnivoran clades 4,14,20,22,27 . Furthermore, Carnivora does not follow Rensch's rule 4 , suggesting that alternative processes may also influence SSD. ...
... Furthermore, Carnivora does not follow Rensch's rule 4 , suggesting that alternative processes may also influence SSD. In the carnivoran clades Musteloidea and Canidae, the evolution and maintenance of sexual dimorphism appears to be influenced by the degree of carnivory 14,22,27 . This pattern is attributed to the hypothesis that competition for terrestrial vertebrate prey is greater than plant material and non-vertebrate prey 23,28,29 . ...
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Although sexual size dimorphism (SSD) is widespread across the animal tree of life, the underlying evolutionary processes that influence this phenomenon remains elusive and difficult to tease apart. In this study, I examined how social system (as a proxy for sexual selection) and diet (as a proxy for natural selection) influenced the evolution of SSD in terrestrial carnivorans (Carnivora; Mammalia). Using phylogenetic comparative methods, I found that are territorial solitary and carnivorous carnivorans exhibited selection towards increased degree of male-biased SSD compared to other carnivorans with alternative social systems and diets. I also found the absence of Rensch’s rule across most carnivoran clades, suggestion a relaxation of the influences of sexual selection on SSD. These results together suggest that sexual selection and niche divergence together are important processes influencing the evolution of male-biased SSD in extant terrestrial carnivorans.
... Specialization toward highly restrictive diet such as hypercarnivory has been thought to cause extreme morphological shifts and significant limitations on subsequent phenotypic evolution leading to low diversity and specialized morphotypes (Van Valkenburgh 1991;Holliday and Steppan 2004;Johnson 2006;Figueirido et al. 2011;Goswami et al. 2014). However, the definition of hypercarnivory may be misleading as evidenced by the multiplicity of definitions used in different studies (Van Valkenburgh 1991;Christiansen and Wroe 2007;Van Valkenburgh 2007;Goswami et al. 2011;Figueirido et al. 2013;Maga and Beck 2017;Law and Mehta 2018;Zrzavý et al. 2018;Tarquini et al. 2020). Hypercarnivorous species are most often characterized by the vertebrate flesh proportion composing their diet. ...
... Our results also indicated a positive correlation between the predator body mass and the relative prey size preference. Indeed, the increase in size within carnivores induces the enhancement of jaw gape and bite force, thus modifying the range of potential prey size Law and Mehta 2018). Moreover, our results coincided with the 21.5 kg average threshold proposed by Carbone et al. (1999) to explain major dietary shifts observed in carnivorous species. ...
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The skeleton is a complex arrangement of anatomical structures that covary to various degrees depending on both intrinsic and extrinsic factors. Among the Feliformia, many species are characterized by predator lifestyles providing a unique opportunity to investigate the impact of highly specialized hypercarnivorous diet on phenotypic integration and shape diversity. To do so, we compared the shape of the skull, mandible, humerus, and femur of species in relation to their feeding strategies (hypercarnivorous vs. generalist species) and prey preference (predators of small vs. large prey) using three-dimensional geometric morphometric techniques. Our results highlight different degrees of morphological integration in the Feliformia depending on the functional implication of the anatomical structure, with an overall higher covariation of structures in hypercarnivorous species. The skull and the forelimb are not integrated in generalist species, whereas they are integrated in hypercarnivores. These results can potentially be explained by the different feeding strategies of these species. Contrary to our expectations, hypercarnivores display a higher disparity for the skull than generalist species. This is probably due to the fact that a specialization toward high-meat diet could be achieved through various phenotypes. Finally, humeri and femora display shape variations depending on relative prey size preference. Large species feeding on large prey tend to have robust long bones due to higher biomechanical constraints.
... Such reduced competition could, theoretically, allow intrasexual territoriality. This intuitively satisfying niche-partitioning hypothesis has, however, experienced mixed success when tested (Selander, 1966;Schoener, 1967;Husar, 1976;Snyder and Wiley, 1976;Powell, 1981Powell, , 1993Dayan and Simberloff, 1994, 1998Holmes and Powell, 1994;King and Powell, 2007;Law and Mehta, 2018;Law, 2019). Resource partitioning by size can only occur when the smaller sex has access to resources not available to the larger (Wilson, 1975;Powell and Zielinski, 1983), a diet requirement that is seldom quantified (for example, Simms, 1979). ...
... Resource partitioning by size can only occur when the smaller sex has access to resources not available to the larger (Wilson, 1975;Powell and Zielinski, 1983), a diet requirement that is seldom quantified (for example, Simms, 1979). Sexual size dimorphism does correlate strongly with a carnivorous diet (Powell, 1979b;Law and Mehta, 2018;Law, 2019). Yet, for those mustelids with large sexual size dimorphism, teeth, jaws and other skull structures related to capturing and killing prey are less dimorphic than are the rest of their bodies, indicating similar use of resources between sexes rather than niche partitioning (Holmes and Powell, 1994). ...
Article
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Animals exploring a new environment develop cognitive maps using diverse sensory input and, thereby, gain information needed to establish home ranges. Experiencing, and learning information about, resources should be advantageous to the resident of a home range while lack of such information should put invaders into the home range at a disadvantage. Conspecifics, especially, should avoid the home ranges of one another to ensure that they do not experience reduced resource availability caused by resource depression or depletion. Yet, encountering conspecific competitors of different sexes may elicit responses that can lead to spacing on a landscape that has different costs and benefits on males and females. We tested the hypothesis that female fishers ( Pekania pennanti ) avoid competition from both males and female conspecifics whereas male fishers avoid competition only from other males. We reintroduced fishers onto our study site in the presence or absence of competitors’ home ranges during late 2009 through 2011. Using satellite transmitters (Argos) and land-based (VHF) telemetry, we monitored fishers and estimated their locations, movements and use of the surrounding landscape during their first 500 days after release. All fishers settled in relatively high-quality habitat but females that encountered the home ranges of conspecifics moved farther, explored larger areas, and settled farther from their release locations than did females that did not encounter a conspecific’s home range. Male fishers exhibited diverse responses upon encountering the home ranges of conspecifics. Thus, female fishers avoid conspecific competition from all fishers, but males tolerate, or impose, competition with females, apparently to increase mating opportunities. These observations are consistent with the movements and strategies of other solitary carnivores.
... One of the characters that can be sexually dimorphic in any species is bite force. Sexual dimorphism in bite force (SDBF) is well studied in lizards (Order Squamata) (Herrel et al., 2010(Herrel et al., , 2007Lappin et al., 2006;Herrel et al., 2002Herrel et al., , 1995, but with relatively less attention in mammals, except in members of Musteloidea superfamily, and lemurs (Law and Mehta, 2018;Campbell and Santana, 2017;Thomas et al., 2015). In Canidae, the effect was assessed in the red fox (Vulpes vulpes) only, in which no significant SDBF was found (Forbes-Harper et al., 2017). ...
... Our results show that canids have almost no bite force differences between males and females, which could be reflecting a lack of feeding niche divergence between sexes in the entire family. Thus, we should expect a consistent food composition overlap and low variation in resource-use between sexes (Law and Mehta, 2018;Campbell and Santana, 2017). However, Forbes-Harper et al. (2017) found feeding differences between sexes and age in red foxes introduced in Australia (juveniles and males feed more on sheep carrion than females which thrive more on rodents and invertebrates) and no differences in bite force. ...
Article
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Bite force is a key trait for understanding aspects of vertebrate ecology and evolution, as it relates directly to different evolutionary pressures, like diet and behaviour. Sexual dimorphism in bite force (SDBF) is an underexplored condition that may shed light on niche divergence between sexes and the effects of sexual selection in species. Here we evaluated differences in modelled bite forces between sexes within Canidae (33 species and two subspecies) and assessed their possible correlations with diet, sociality, hunting strategies, and size dimorphism. We calculated SDBF and bite force quotients through indexes and compared them among different diets, hunting strategies, and sociality groups. Furthermore, we correlated the indexes and size sexual dimorphism using phylogenetic independent contrasts. Only two species showed significant SDBF: the Cape fox (Vulpes chama) and the Ethiopian wolf (Canis simensis). We found no significant differences in bite force dimorphism intensity between sociality levels, dietary levels, or hunting strategies. We found a relationship between bite force sexual dimorphism and size sexual dimorphism, and a correlation between bite force and the bite force quotient dimorphism. However, we found no association between sexual dimorphism in bite force quotient and sexual size dimorphism. Our findings show that Canidae do not have bite force dimorphism, possibly due to the widespread social monogamy in the family, when compared to other Carnivora. This implies possible restrictions that constraint the range of bite strength in adults, especially in females.
... Specialization toward highly restrictive diet such as hypercarnivory has been thought to cause extreme morphological shifts and significant limitations on subsequent phenotypic evolution leading to low diversity and specialized morphotypes (Van Valkenburgh 1991;Holliday and Steppan 2004;Johnson 2006;Figueirido et al. 2011;Goswami et al. 2014). However, the definition of hypercarnivory may be misleading as evidenced by the multiplicity of definitions used in different studies (Van Valkenburgh 1991;Christiansen and Wroe 2007;Van Valkenburgh 2007;Goswami et al. 2011;Figueirido et al. 2013;Maga and Beck 2017;Law and Mehta 2018;Zrzavý et al. 2018;Tarquini et al. 2020). Hypercarnivorous species are most often characterized by the vertebrate flesh proportion composing their diet. ...
... Our results also indicated a positive correlation between the predator body mass and the relative prey size preference. Indeed, the increase in size within carnivores induces the enhancement of jaw gape and bite force, thus modifying the range of potential prey size Law and Mehta 2018). Moreover, our results coincided with the 21.5 kg average threshold proposed by Carbone et al. (1999) to explain major dietary shifts observed in carnivorous species. ...
... Pronounced sexual dimorphism is often correlated with ecological divergence between sexes in sexually polymorphic species (Houston & Shine 1993;Voight 1995;Walker & Rypstra 2002;Law & Mehta 2018;Li & Kokko 2021). Examples of extreme cases of pronounced sexual dimorphism in size and shape are found in the Dynastinae or rhinoceros beetles, a subfamily of the scarab beetle family Scarabaeidae (e.g., Endrödi 1985;Ratcliffe & Cave 2006, 2015Ratcliffe et al. 2013;Milani 2021). ...
... Our study suggests that mature forests should house larger Augosoma beetles than plantations and secondary forests. Sexual size dimorphism is often accompanied by intersexual differences in niche characteristics (e.g., see Teder & Tammaru 2005;Stillwell & Fox 2007;Law & Mehta 2018;Li & Kokko 2021). As shown in our study, A. centaurus exhibits significant sexual size dimorphism where males are considerably larger than females and possess massive head horns (Endrödi 1985). ...
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The sexually dimorphic dynastine centaurus beetle, genus Augosoma (Coleoptera: Scarabeidae), is endemic to tropical Africa where two species are found (A. centaurus and A. hippocrates). These beetles are consumed by rural populations, cause damage in plantations and are targets of insect collectors and traders. We present information on size differences and analysed intersexual niche divergence and seasonality of A. centaurus in seven study sites in three West African countries (Ivory Coast, Togo and Nigeria). We recorded 711 light-attracted and/or opportunistically encountered individuals, as well as another 97 beetles in standardized transect surveys. In the latter, we found the adult sex ratio was equal but was significantly skewed towards females in light-attracted and/or opportunistically encountered individuals. In a sample of 298 adult beetles, males were significantly larger than females, with almost no size overlap between sexes. Beetle activity was highly seasonal with most animals observed in November, active during 19:00–24:00 h. Differences in habitat use were not significant between sexes, with most individuals observed in secondary forest. Males were found higher on vegetation than females and beetles of both sexes were found on Pandanus and Raffia palms. Beetles were larger in sites with more vegetation cover, and there was a significant effect of tree species on body size of both sexes. Study area or country had no effect on any of the studied parameters. Our study confirms that transect surveys without light trapping can be an effective tool for understanding large-sized tropical beetles of similar ecological characteristics.
... However, sexual dimorphism can also provide associated benefits beyond reproduction, such as where sexually selected differences in feeding structures of males and females can lead to intersexual niche partitioning (Herrel et al., 1999;Forbes-Harper et al., 2017;Law & Mehta, 2018), but this can be reduced owing to interspecific competition (Williams, 1980). What drives sexual dimorphism across species may even differ between the sexes (Vanhooydonck et al., 2010). ...
Article
Morphological differences between the sexes are a common feature in many groups of animals and can have important ecological implications for courtship, mating, access to prey and, in some cases, intersex niche partitioning. In this study, we evaluated the role of sexual dimorphism in the performance of the two structures that mediate the ability to access prey, the pinchers or chelae and the venomous stinger, in two species of scorpions with contrasting morphologies: Chactas sp., which has marked sexual dimorphism in the chelae, and Centruroides sp., which does not have such marked dimorphism in the chelae. We evaluated aspects such as chela pinch force, toxicity to prey (LD50) and the volume of venom in males and females of each species. We found significant differences between males and females of Chactas sp. in the chela pinch force, volume of venom and LD50. In contrast, for Centruroides sp., no differences between males and females were found in any of these traits. We discuss several potential selective regimes that could account for the pattern observed.
... As such, the investigation of variation between the sexes in body form can shed light both on their respective reproductive and social functions (Kaliontzopoulou et al. 2007) and their ecological roles (Butler and Losos 2002;Kaliontzopoulou et al. 2010). Several studies have documented patterns of sexual dimorphism in body proportions (e.g., Butler and Losos 2002;Butler et al. 2007;Kaliontzopoulou et al. 2015), the shape of various structures (e.g., Gidaszewski et al. 2009;Kaliontzopoulou et al. 2012;Berns and Adams 2013;Kelly et al. 2013;Sanger et al. 2013;Laporte et al. 2017;Law and Mehta 2018), ornamentation (e.g., Geist and Bayer 1988;Emlen 2008;Watson and Simmons 2010), or coloration (e.g., Endler 1983) to provide insights regarding the selective mechanisms that generate such patterns in nature. From a methodological perspective, sexual dimorphism in complex traits is typically quantified using multivariate approaches that fully integrate information on different aspects of the phenotype (Wyman et al. 2013). ...
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Allometric trends in the degree of sexual dimorphism with body size have long fascinated evolutionary biologists. Many male‐biased clades display more prominent sexual dimorphism in larger taxa (Rensch's rule), with most examples documenting this pattern for body size dimorphism. While sexual dimorphism in traits other than body size is equally functionally relevant, characterizing allometric patterns of sexual dimorphism in such traits is hampered by lack of an analytical framework that can accommodate multivariate phenotypes. In this paper we derive a multivariate equivalency for investigating trends in sexual dimorphism – relative to overall body size – across taxa and provide a generalized test to determine whether such allometric patterns correspond with Rensch's rule. For univariate linear traits like body size, our approach yields equivalent results to those from standard procedures, but our test is also capable of detecting trends in multivariate datasets like shape. Computer simulations reveal the method displays appropriate statistical properties, and an empirical example in Mediterranean lizards provides the first demonstration of Rensch's rule in a multivariate phenotype (head shape). Our generalized procedure substantially extends the analytical toolkit for investigating macroevolutionary patterns of sexual dimorphism and seeking a better understanding of the processes that underlie them. This article is protected by copyright. All rights reserved
... However, sexual dimorphism can also provide associated benefits beyond reproduction, such as where sexually selected differences in feeding structures of males and females can lead to intersexual niche partitioning (Herrel et al., 1999;Forbes-Harper et al., 2017;Law & Mehta, 2018), but this can be reduced owing to interspecific competition (Williams, 1980). What drives sexual dimorphism across species may even differ between the sexes (Vanhooydonck et al., 2010). ...
Article
Morphological differences between the sexes are a common feature in many groups of animals and can have important ecological implications for courtship, mating, access to prey and, in some cases, intersex niche partitioning. In this study, we evaluated the role of sexual dimorphism in the performance of the two structures that mediate the ability to access prey, the pinchers or chelae and the venomous stinger, in two species of scorpions with contrasting morphologies: Chactas sp., which has marked sexual dimorphism in the chelae, and Centruroides sp., which does not have such marked dimorphism in the chelae. We evaluated aspects such as chela pinch force, toxicity to prey (LD 50) and the volume of venom in males and females of each species. We found significant differences between males and females of Chactas sp. in the chela pinch force, volume of venom and LD 50. In contrast, for Centruroides sp., no differences between males and females were found in any of these traits. We discuss several potential selective regimes that could account for the pattern observed. ADDITIONAL KEYWORDS: pinch force-LD 50-scorpions-sexual dimorphism-venom.
... They hypothesized that gene flow was the main source of the greater variability in mainland populations. Otherwise, recently Law & Mehta (2018) highlighted niche divergence as an important mechanism that maintains the evolution of sexual dimorphism in musteloids, displaying in cranial size and bite force dimorphism rather than in cranial shape. Korablev et al. (2013) interpreted differences in the degree of SSD in four Mustelidae species in accordance with the niche variation hypothesis. ...
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A brief review of the phylogeny and nomenclature of the weasels, genus Mustela Linnaeus, 1758 in the broad sense, indicates continuing confusion over the appropriate name for the well-supported American clade included within it. A case is made that the American mink (Neovison vison) and three allied species (Mustela frenata, M. felipei, and M. africana) should now be recognized in the genus Neogale Gray, 1865. The ages and morphological disparities of both Neogale and Mustela sensu stricto indicate that both are in need of comprehensive revisions.
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Background The lesser grison ( Galictis cuja ) is one of the least known carnivores in the Neotropical region. Its wide geographical occurrence and range of habitats could lead to morphological variations along its distribution. So, this study aimed to investigate the variation in skull shape and size of this species, by testing the existence of ecotypes adapted to their respective environments (Uruguayan savanna and Atlantic Forest), as well as its relationship with selected abiotic variables. Methods The skulls of 52 museum specimens were photographed in the ventral, dorsal, and lateral views, and were analyzed using geometric morphometric techniques. Results We found sexual size dimorphism, with males being larger than females. The shape variation between sexes, as well as between ecoregions, is mostly explained by the effect of allometry. The specimens from Uruguayan savanna are larger than the ones from the Atlantic Forest. Size variation was also significantly correlated to latitude, temperature and precipitation patterns. No correlation between skull shape with geographical distance was detected. Discussion Morphometric measurements and diet data of lesser grison in regions from higher latitudes than our sampling show a tendency to heavier individuals, and the consumption of bigger prey compared to Uruguayan savanna. The results indicated the smaller specimens associated to low variability in annual temperature, congruent to Atlantic Forest region. An explanation for observed variation may be related to the “resource rule” but, due the minimal natural history information regards this species, we can just speculate about this.
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European and American minks (Mustela lutreola and Neovison vison, respectively) are very similar in their ecology, behavior, and morphology. However, the American mink is a generalist predator and seems to adapt better to anthropized environments, allowing it to outcompete the European mink in areas where it has been introduced, threatening the survival of the native species. To assess whether morphological differences may be contributing to the success of the American mink relative to the European mink, we analyzed shape variation in the cranium of both species using 3D geometric morphometrics. A set of 38 landmarks and 107 semilandmarks was used to study shape variation between and within species, and to assess how differences in size factored into that variation. Sexual dimorphism in both size and shape was also studied. Significant differences between species were found in cranial shape, but not in size. Relative to American mink, European mink have a shorter facial region with a rounder forehead and wider orbits, a longer neurocranium with less developed crests and processes, and an antero‐medially placed tympanic bullae with an anteriorly expanded cranial border. Within species, size‐related sexual dimorphism is highly significant, but sexual dimorphism in shape is only significant in American mink, not in European mink. Additionally, two trends common to both species were discovered, one related to allometric changes and another to sexual size dimorphism. Shape changes related to increasing size can be subdivided into two, probably related, groups: increased muscle force and growth. The first group somewhat parallels the differences between both mink species, while the second group of traits includes an anterodorsal expansion of the face, and the neurocranium shifting from a globous shape in small individuals to a dorsoventrally flattened ellipse in the largest ones. Finally, the sexual dimorphism trend, while also accounting for differences in muscle force, seems to be related to the observed dietary differences between males and females. Overall, differences between species and sexes, and shape changes with increasing size, seem to mainly relate to differences in masticatory‐muscle volume and therefore muscle force and bite force, which, in turn, relate to a wider range of potential prey (bigger prey, tougher shells). Thus, muscle force (and dietary range) would be larger in American mink than in European mink, in males than in females, and in larger individuals than in smaller ones. Cranial shape and size variation in European (Mlu) and American minks (Nvi) were studied. Both species differed in shape but not size, while sexes within each differed in size but in shape only for Nvi. Allometric shape changes were related to growth and sexual dimorphism. Overall, differences among species, sexes, and sizes related to masticatory‐muscle volume, which suggests that muscle force (and dietary range) would be larger in Nvi than in Mlu, in males than in females, and in larger mink.
Article
The carnivoran cranium undergoes tremendous growth in size and development of shape to process prey as adults and, importantly, these ontogenetic processes can also differ between the sexes. How these ontogenetic changes in morphology actually relate to the underlying jaw musculature and overall bite performance has rarely been investigated. In this study, I examined sex‐specific ontogenetic changes in cranial morphology, jaw adductor muscles, and theoretical bite force between subadults and adults in the fisher (Pekania pennanti ) and American marten (Martes americana ). I found evidence that cranial size alone does not completely explain ontogenetic increases in bite forces as found in other mammalian species. Instead, cranial shape development also drives ontogenetic increases in relative bite force by broadening the zygomatic arches and enlargement of the sagittal crest, both of which enable relatively larger jaw adductor muscles to attach. In contrast, examination of sexual dimorphism within each age‐class revealed that cranial shape dimorphism did not translate to dimorphism in either size‐corrected bite forces or size‐corrected physiological cross‐sectional area of the jaw adductor muscles. These results reveal that morphological size and shape variation can have different influences on bite performance depending on the level of intraspecific variation that is examined (i.e. ontogenetic versus sexual dimorphism).
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Cyclical parthenogenesis is a widespread reproductive strategy in which organisms go through one or multiple rounds of clonal reproduction before sexual reproduction. In populations of the planktonic cladoceran Daphnia magna sexual reproduction is typically less common than parthenogenesis and therefore hardly studied. We studied the sexual process and its relation to sexual selection in Daphnia rockpool populations, where sex is common throughout the summer, by observing natural mating in these shallow habitats. While microsatellite markers revealed no evidence for disassortative mating and thus, inbreeding avoidance, body length and infection status revealed assortative mating, suggesting sexual selection to act. In cases where two males mated with a single female, larger male remained longer, possibly giving them an advantage in sperm competition. Indirect evidence points at the brood pouch as the likely site of fertilization and thus, sperm competition. Sperm length was as variable within ejaculates as it was among males from different populations. Our data give firm evidence that sexual selection is present in this species and that it likely manifests itself by a combination of female choice and male - male competition.
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An elongate body with reduced or absent limbs has evolved independently in many ectothermic vertebrate lineages. While much effort has been spent examining the morphological pathways to elongation in these clades, quantitative investigations into the evolution of elongation in endothermic clades are lacking. We quantified body shape in 61 musteloid mammals (red panda, skunks, raccoons, and weasels) using the head‐body elongation ratio. We also examined the morphological changes that may underlie the evolution towards more extreme body plans. We found that a mustelid clade comprised of the subfamilies Helictidinae, Guloninae, Ictonychinae, Mustelinae, and Lutrinae exhibited an evolutionary transition towards more elongate bodies. Furthermore, we discovered that elongation of the body is associated with the evolution of other key traits such as a reduction in body size and a reduction in forelimb length but not hindlimb length. This relationship between body elongation and forelimb length has not previously been quantitatively established for mammals but is consistent with trends exhibited by ectothermic vertebrates and suggests a common pattern of trait covariance associated with body shape evolution. This study provides the framework for documenting body shapes across a wider range of mammalian clades to better understand the morphological changes influencing shape disparity across all vertebrates. This article is protected by copyright. All rights reserved
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Mustela sibirica Pallas, 1773, commonly known as the Siberian weasel, is a widely distributed Palearctic musteline with natural populations ranging from west of the Ural Mountains of Siberia to the Far East and south to Taiwan and the Himalayas. A key characteristic that distinguishes M. sibirica from most sympatric musteline species is the occurrence of a black mask on its face that surrounds the eyes, a white muzzle and chin, and the presence of a nearly completely monotone yellowish-brown coat. Although M. sibirica is hunted to make “kolinsky stable-hair” paintbrushes, populations remain stable and the species is currently listed as “Least Concern” by the International Union for Conservation and Nature and Natural Resources.
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Size and shape are often considered important variables that lead to variation in performance. In studies of feeding, size‐corrected metrics of the skull are often used as proxies of biting performance; however, few studies have examined the relationship between cranial shape in it's entirety and estimated bite force across species and how dietary ecologies may affect these variables differently. Here, we used geometric morphometric and phylogenetic comparative approaches to examine relationships between cranial morphology and estimated bite force in the carnivoran clade Musteloidea. We found a strong relationship between cranial size and estimated bite force but did not find a significant relationship between cranial shape and size‐corrected estimated bite force. Many‐to‐one mapping of form to function may explain this pattern because a variety of evolutionary shape changes rather than a single shape change may have contributed to an increase in relative biting ability. We also found that dietary ecologies influenced cranial shape evolution but did not influence cranial size nor size‐corrected bite force evolution. While musteloids with different diets exhibit variation in cranial shapes, they have similar estimated bite forces suggesting that other feeding performance metrics and potentially non‐feeding traits are also important contributors to cranial evolution. We postulate that axial and appendicular adaptations and the interesting feeding behaviors reported for species within this group also facilitate different dietary ecologies between species. Future work integrating cranial, axial, and appendicular form and function with behavioral observations will reveal further insights in the evolution of dietary ecologies and other ecological variables. This article is protected by copyright. All rights reserved.
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Among adults of closely related species, a trend in craniofacial evolutionary allometry (CREA) for larger taxa to be long-faced and smaller ones to have paedomorphic aspects, such as proportionally smaller snouts and larger braincases, has been demonstrated in some mammals and two bird lineages. Nevertheless, whether this may represent a ‘rule’ with few exceptions is still an open question. In this context, Felidae is a particularly interesting family to study because, although its members are short-faced, previous research did suggest relative facial elongation in larger living representatives. Using geometric morphometrics, based on two sets of anatomical landmarks, and traditional morphometrics, for comparing relative lengths of the palate and basicranium, we performed a series of standard and comparative allometric regressions in the Felidae and its two subfamilies. All analyses consistently supported the CREA pattern, with only one minor exception in the geometric morphometric analysis of Pantherinae: the genus Neofelis. With its unusually long canines, Neofelis species seem to have a relatively narrow cranium and long face, despite being smaller than other big cats. In spite of this, overall, our findings strengthen the possibility that the CREA pattern might indeed be a ‘rule’ among mammals, raising questions on the processes behind it and suggesting future directions for its study.
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Adaptive radiation is hypothesized to be a primary mechanism that drives the remarkable species diversity and morphological disparity across the Tree of Life. Tests for adaptive radiation in extant taxa are traditionally estimated from calibrated molecular phylogenies with little input from extinct taxa. With 85 putative species in 33 genera and over 400 described extinct species, the carnivoran superfamily Musteloidea is a prime candidate to investigate patterns of adaptive radiation using both extant- and fossil-based macroevolutionary methods. The species diversity and equally impressive ecological and phenotypic diversity found across Musteloidea is often attributed to 2 adaptive radiations coinciding with 2 major climate events, the Eocene-Oligocene transition and the Mid-Miocene Climate Transition. Here, we compiled a novel time-scaled phylogeny for 88% of extant musteloids and used it as a framework for testing the predictions of adaptive radiation hypotheses with respect to rates of lineage diversification and phenotypic evolution. Contrary to expectations, we found no evidence for rapid bursts of lineage diversification at the origin of Musteloidea, and further analyses of lineage diversification rates using molecular and fossil-based methods did not find associations between rates of lineage diversification and the Eocene-Oligocene transition or Mid-Miocene Climate Transition as previously hypothesized. Rather, we found support for decoupled diversification dynamics driven by increased clade carrying capacity in the branches leading to a subclade of elongate mustelids. Supporting decoupled diversification dynamics between the subclade of elongate mustelids and the ancestral musteloid regime is our finding of increased rates of body length evolution, but not body mass evolution, within the decoupled mustelid subclade. The lack of correspondence in rates of body mass and length evolution suggest that phenotypic evolutionary rates under a single morphological metric, even one as influential as mass, may not capture the evolution of diversity in clades that exhibit elongate body shapes. The discordance in evolutionary rates between body length and body mass along with evidence of decoupled diversification dynamics suggests that body elongation might be an innovation for the exploitation of novel Mid-Miocene resources, resulting in the radiation of some musteloids.
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Extant aquatic mammals are a key component of aquatic ecosystems. Their morphology, ecological role and behaviour are, to a large extent, shaped by their feeding ecology. Nevertheless, the nature of this crucial aspect of their biology is often oversimplified and, consequently, misinterpreted. Here, we introduce a new framework that categorizes the feeding cycle of predatory aquatic mammals into four distinct functional stages (prey capture, manipulation and processing, water removal and swallowing), and details the feeding behaviours that can be employed at each stage. Based on this comprehensive scheme, we propose that the feeding strategies of living aquatic mammals form an evolutionary sequence that recalls the land-to-water transition of their ancestors. Our newconception helps to explain and predict the origin of particular feeding styles, such as baleen-assisted filter feeding in whales and raptorial ‘pierce’ feeding in pinnipeds, and informs the structure of present and past ecosystems.
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Weaning represents a major ontogenetic dietary shift in southern sea otters (Enhydra lutris nereis), as juveniles must transition from depending on mother’s milk to independently processing hard-shelled invertebrates. When the skulls of juveniles have reached sufficient maturity to transition to a durophagous diet remains to be investigated. Here, we conducted a comprehensive analysis of skull development and growth and sexual dimorphism using geometric morphometric approaches in 204 southern sea otter skulls. We found that southern sea otters of both sexes exhibit dramatic changes in cranial and mandibular shape and size over ontogeny. Although the majority of these changes occur in the pup stage, full development and growth of the skull does not occur until well after weaning. We hypothesize that the slower maturation of the crania of newly weaned juveniles serves as a handicap by constraining jaw adductor muscle size, biting ability and feeding on hard-shelled prey. In our analysis of sexual dimorphism, we found significant sexual shape and size dimorphism in adult craniomandibular morphology that arose through differences in developmental and growth rates and duration. We postulate that males are selected to attain mature crania faster to presumably reach adult biting ability sooner, gaining a competitive advantage in obtaining food and in male–male agonistic interactions.
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Rensch's rule states that sexual size dimorphism (SSD) increases with body size in taxa where males are larger, and decreases when females are larger. The dominant explanation for the trend is currently that competitive advantage for males is greater in larger individuals, whereas female size is constrained by the energetics of rearing offspring. This rule holds for a variety of vertebrate taxa, and opposing trends are rare. We examine the allometry of SSD within the Musteloidea and demonstrate a hypo-allometry contrary to Rensch's rule, with lower SSD associated with larger body size. We provide evidence that feeding ecology is involved. Where diet promotes group-living, the optimal strategy for the males of larger species is often not to attempt to defend access to multiple females, obviating any competitive advantage of relatively greater size. We conclude that the effect of feeding ecology on mating systems may be a hitherto neglected factor explaining variation in SSD.
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The niche divergence hypothesis suggests that if a species exhibits intersexual differences in diet, selection should favor divergence in the feeding apparatus between the sexes. Recent work revealed that male and female southern sea otters (Enhydra lutris nereis) utilize different dietary resources in response to increased population density; females exhibit more specialized diets as a function of smaller home ranges, whereas males exhibit larger home ranges, potentially allowing them to expand their dietary breadths by feeding on prey items that are not found in female home ranges. These dietary differences suggest the potential for sexual dimorphism of the feeding apparatus (i.e., the skull). Here, we tested the hypothesis that male and female southern sea otters exhibit differences in craniomandibular traits directly related to biting ability. Univariate and multivariate analyses of 12 craniomandibular traits showed that size is the primary axis of skull variation, whereas only a handful of craniomandibular traits demonstrated significant shape differences between the sexes. Relative postorbital constriction breadth, masseter in-lever length, and cranial height differed significantly between the sexes. These 3 traits can increase the surface area of jaw muscle attachment sites and thus are directly linked to the mechanics of biting ability. Collectively, these morphological differences indicate that niche divergence may be an important mechanism maintaining sexual dimorphism in southern sea otters.
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Sexual dimorphism attributed to niche divergence is often linked to differentiation between the sexes in both dietary resources and characters related to feeding and resource procurement. Although recent studies have indicated that southern sea otters (Enhydra lutris nereis) exhibit differences in dietary preferences as well as sexual dimorphism in skull size and shape, whether these intersexual differences translate to differentiation in feeding performances between the sexes remains to be investigated. To test the hypothesis that scaling patterns of bite force, a metric of feeding performance, differ between the sexes, we calculated theoretical bite forces for 55 naturally deceased male and female southern sea otters spanning the size ranges encountered over ontogeny. We then used standardized major axis regressions to simultaneously determine the scaling patterns of theoretical bite forces and skull components across ontogeny and assess whether these scaling patterns differed between the sexes. We found that positive allometric increases in theoretical bite force resulted from positive allometric increases in physiological cross-sectional area for the major jaw adductor muscle and mechanical advantage. Closer examination revealed that allometric increases in temporalis muscle mass and relative allometric decreases in out-lever lengths are driving these patterns. In our analysis of sexual di- morphism, we found that scaling patterns of theoretical bite force and morphological traits do not differ between the sexes. How- ever, adult sea otters differed in their absolute bite forces, revealing that adult males exhibited greater bite forces as a result of their larger sizes. We found intersexual differences in biting ability that provide some support for the niche divergence hypothesis. Continued work in this field may link intersexual differences in feeding functional morphology with foraging ecology to show how niche divergence has the potential to reinforce sexual dimorphism in southern sea otters.
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A major goal of evolutionary studies is to better understand how complex morphologies are related to the different functions and behaviours in which they are involved. For example, during locomotion and hunting behaviour, the head and the eyes have to stay at an appropriate level in order to reliably judge distance as well as to provide postural information. The morphology and orientation of the orbits and cranial base will have an impact on eye orientation. Consequently, variation in orbital and cranial base morphology is expected to be correlated with aspects of an animal's lifestyle. In this study, we investigate whether the shape of the skull evolves in response to the functional demands imposed by ecology and behaviour using geometric morphometric methods. We test if locomotor habitats, diet, and activity pattern influence the shape of the skull in musteloid carnivorans using (M)ANOVAs and phylogenetic (M)ANOVAs, and explore the functional correlates of morphological features in relation to locomotor habitats, diet, and activity pattern. Our results show that phylogeny, locomotion and, diet strongly influence the shape of the skull, whereas the activity pattern seems to have a weakest influence. We also show that the locomotor environment is highly integrated with foraging and feeding, which can lead to similar selective pressures and drive the evolution of skull shape in the same direction. Finally, we show similar responses to functional demands in musteloids, a super family of close related species, as are typically observed across all mammals suggesting the pervasiveness of these functional demands.
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The societies of group-living carnivores that neither hunt nor interact cooperatively may arise due to ecological drivers and/or constraints. In this study we evaluate whether group-living may be intrinsically associated with fossoriality; a link that is well supported in other taxa, but hitherto under-evaluated in the Carnivora. We make two over-arching predictions: (i) that fossoriality will be associated with carnivoran sociality; and (ii) that this association will be most evident in those species making extended use of subterranean dens. From a meta-analysis of key behavioral, ecological, ontological, and trophic traits, we demonstrate that three quarters of carnivore species exhibit some reliance on underground dens. Congruence between life-history traits and metrics of fossoriality evidenced that: (1) there are phylogenetic, and morphological constraints on wholly fossorial life-histories; (2) fossoriality correlated positively with the extent of offspring altriciality, linked to the use of natal dens; (3) burrow use increased with latitude; and (4) insectivorous carnivores were more fossorial than predatory carnivores. Corroborating work in the Rodentia, fossorial traits associated strongly with carnivoran group-living tendencies, where species utilizing subterranean natal dens are 2.5 times more likely to form groups than those that do not. Furthermore, using comparative analyses, we evidence support for an evolutionary relationship between diet, fossoriality, and sociality. We propose that fossorial dens act as a safe haven, promoting fitness benefits, territorial inheritance and cooperative breeding. We conclude that, among smaller (<15 kg) den-using carnivores, and especially for omnivorous/insectivorous species for which food resource dispersion is favorable, continued cohabitation at natal dens can promote cohabitation among adults; that is, philopatric benefits leading to (not necessarily cooperative) spatial groups.
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Among closely related species, larger mammals tend to have a longer face and proportionally smaller braincase. This putative ‘rule’ in mammalian macroevolution has been proposed for the first time in 2013 based on 3D geometric morphometrics of antelopes, fruit bats, tree squirrels and mongooses. To firmly demonstrate that this trend holds as a ‘rule’ requires expanding the analysis in more lineages and other mammalian orders: if supported in most groups, it may indeed become a new evolutionary ‘rule’ besides famous ones such as Bergmann’s and Allen’s. In this study, using statistical shape analysis and both standard and comparative methods on a sample of kangaroos, wallabies and other macropodine marsupials, we show that the ‘big size-long face’ pattern is indeed found also outside the placentals. This provides support to the hypothesis of an important role of size-related shape changes (i.e., allometry) in the origin of the exceptional disparity of mammals, that, only in terms of size, span more orders of magnitude than any other animal: from 3 to 4 g of a tiny bat to more than 100 tons in blue whales.
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Five native terrestrial mustelids are found in Great Britain. Only three of these occur in Ireland. Farmed American mink have recently established feral populations on both islands. We studied inter- and intraspecific size relationships, sexual size dimorphism, and morphological variation among these mustelids. We viewed each size as a separate morphospecies, skull length as a measure of body size, and the upper canine tooth as the organ used to kill prey. Geographic variation was low in both islands, so we considered the mustelid population of each island a single unit. Community-wide character displacement (evidence by equal size ratios) was found among British mustelids for canine diameter. For skull length it was seen only when the largely vermivorous badger was excluded. When we added feral mink the regular pattern disappeared, but when we substituted the mink for the polecat, which is now restricted to parts of Wales and adjacent England, community-wide character displacement was manifest. For Irish mustelids size ratios were not equal, but the pattern for canines was more regular than for skull lengths. Adding the local feral mink did not result in a regular pattern, but addition of the mink and exclusion of the badger yielded equal ratios for skull length but not for canines. These patterns plus published empirical data support a hypothesis of prey size partitioning. The significant differences in size between some of the British and Irish populations of the same morphospecies suggest the possibility of ecological release among Irish mustelids, whose populations originally derived from British ones. In particular, canine sexual size dimorphism is greater for Irish pine martens, stoats, and mink, as would be expected if there were fewer competitors. For the marten and the stoat, Irish females have evolved to be strikingly smaller than their British counterparts, in each case approximating the size of the male of a missing species (polecat for the marten, weasel for the stoat). For skull length there is no consistent patterns. Finally, morphological variation is greater in Ireland for five or six morphospecies, as predicted by the niche-variation hypothesis.
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Intraspecific variation in behavior and diet can have important consequences for population and ecosystem dynamics. Here, we examine how differences in reproductive investment and spatial ecology influence individual diet specialization in male and female southern sea otters (Enhydra lutris nereis). We hypothesize that greater reproductive constraints and smaller home ranges of females lead to more pronounced intraspecific competition and increased specialization. We integrate stable carbon (δ(13)C) and nitrogen (δ(15)N) isotope analysis of sea otter vibrissae with long-term observational studies of five subpopulations in California. We define individual diet specialization as low ratios of within-individual variation (WIC) to total population niche width (TNW). We compare isotopic and observational based metrics of WIC/TNW for males and females to data on population densities, and movement patterns using both general linear and linear mixed-effects models. Consistent with our hypothesis, increasing population density is associated with increased individual diet specialization by females but not by males. Additionally, we find the amount of coastline in a sea otter's home range positively related with individual dietary variability, with increased range span resulting in weaker specialization for both males and females. We attribute our results to sex-based differences in movement, with females needing to specialize in their small ranges to maximize energy gain, and posit that the paradigm of individual prey specialization in sea otters with increased intraspecific competition may be a pattern driven largely by females. Our work highlights a potentially broader role of sex in the mechanistic pressures promoting and maintaining diet specialization.
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I examined influence of body size and mating systems on sexual-size dimorphism by summarizing characteristics and testing for associations among the most dimorphic mammalian taxa-Macropodidae, Primates, Mustelidae, Pinnipedia, Elephantidae, Ruminantia. The most dimorphic taxa were seals in Otariidae. On average, males were three times larger than females, and all otariids displayed extensive dimorphism. Except for the Strepsirhini, most taxa had dimorphism ratios (mass of males:mass of females) between 1.2-1.8. Extent of dimorphism increased with body size but the effect was slight (power function between masses of males and females, 1.04-1.05) for most taxa. Phocid seals and macropodid marsupials had power functions of ca. 1.2. Mating systems were associated with size dimorphism in simian primates and ruminants. Monogamous simian primates were less dimorphic than simians that had polygynous mating systems. Ruminants with tending and harem mating systems were more dimorphic than those with territorial polygynous and monogamous mating systems. Polygyny and how it was conducted were associated with the extent of sexual size dimorphism.
Chapter
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This chapter explores the pattern of sexual size dimorphism (SSD) in mammals and the processes that underlie its evolution. Most mammalian orders have male-biased SSD, although some orders are not sexuallydimorphic for body size or show significantly female-biased SSD. In general, SSD increases with body size across mammals (Rensch's rule). Malebiased dimorphism relates to sexual selection on males through male-male competition for females, since sexual selection as indicated by mating systems is positively correlated with male-biased SSD. Selection pressure on female mass, identified in that age at weaning, is higher in polygynous species. However, the reproductive rate is lower for large females, indicating that fecundity selection selects small females. Although these patterns hold across mammals as a whole, the data presented in the chapter also reveal considerable variation across orders.
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Our understanding of macroevolutionary patterns of adaptive evolution has greatly increased with the advent of large-scale phylogenetic comparative methods. Widely used Ornstein–Uhlenbeck (OU) models can describe an adaptive process of divergence and selection. However, inference of the dynamics of adaptive landscapes from comparative data is complicated by interpretational difficulties, lack of identifiability among parameter values and the common requirement that adaptive hypotheses must be assigned a priori. Here, we develop a reversible-jump Bayesian method of fitting multi-optima OU models to phylogenetic comparative data that estimates the placement and magnitude of adaptive shifts directly from the data. We show how biologically informed hypotheses can be tested against this inferred posterior of shift locations using Bayes Factors to establish whether our a priori models adequately describe the dynamics of adaptive peak shifts. Furthermore, we show how the inclusion of informative priors can be used to restrict models to biologically realistic parameter space and test particular biological interpretations of evolutionary models. We argue that Bayesian model fitting of OU models to comparative data provides a framework for integrating of multiple sources of biological data—such as microevolutionary estimates of selection parameters and paleontological timeseries—allowing inference of adaptive landscape dynamics with explicit, process-based biological interpretations.
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Facial length is one of the best known examples of heterochrony. Changes in the timing of facial growth have been invoked as a mechanism for the origin of our short human face from our long-faced extinct relatives. Such heterochronic changes arguably permit great evolutionary flexibility, allowing the mammalian face to be remodelled simply by modifying postnatal growth. Here we present new data that show that this mechanism is significantly constrained by adult size. Small mammals are more brachycephalic (short faced) than large ones, despite the putative independence between adult size and facial length. This pattern holds across four phenotypic lineages: antelopes, fruit bats, tree squirrels and mongooses. Despite the apparent flexibility of facial heterochrony, growth of the face is linked to absolute size and introduces what seems to be a loose but clade-wide mammalian constraint on head shape.
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Many ecological and evolutionary studies seek to explain patterns of shape variation and its covariation with other variables. Geometric morphometrics is often used for this purpose, where a set of shape variables are obtained from landmark coordinates following a Procrustes superimposition.We introduce geomorph: a software package for performing geometric morphometric shape analysis in the r statistical computing environment.Geomorph provides routines for all stages of landmark-based geometric morphometric analyses in two and three-dimensions. It is an open source package to read, manipulate, and digitize landmark data, generate shape variables via Procrustes analysis for points, curves and surfaces, perform statistical analyses of shape variation and covariation, and to provide graphical depictions of shapes and patterns of shape variation. An important contribution of geomorph is the ability to perform Procrustes superimposition on landmark points, as well as semilandmarks from curves and surfaces.A wide range of statistical methods germane to testing ecological and evolutionary hypotheses of shape variation are provided. These include standard multivariate methods such as principal components analysis, and approaches for multivariate regression and group comparison. Methods for more specialized analyses, such as for assessing shape allometry, comparing shape trajectories, examining morphological integration, and for assessing phylogenetic signal, are also included.Several functions are provided to graphically visualize results, including routines for examining variation in shape space, visualizing allometric trajectories, comparing specific shapes to one another and for plotting phylogenetic changes in morphospace.Finally, geomorph participates to make available advanced geometric morphometric analyses through the r statistical computing platform.
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Superimposition methods for comparing configurations of landmarks in two or more specimens are reviewed. These methods show differences in shape among specimens as residuals after rotation, translation, and scaling them so that they align as well as possible. A new method is presented that generalizes Siegel and Benson's (1982) resistant-fit theta-rho analysis so that more than two objects can be compared at the same time. Both least-squares and resistant-fit approaches are generalized to allow for affine transformations (uniform shape change). The methods are compared, using artificial data and data on 18 landmarks on the wings of 127 species of North American mosquitoes. Graphical techniques are also presented to help summarize the patterns of differences in shape among the objects being compared.
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The mammalian skull has proven to be remarkably plastic during ontogeny and phylogeny in response to the demands of mastication. I examine whether the bending strength of the skull in some mammals correlates with the maximal loads imposed through the masticatory apparatus. The approach is analytical, using the methods of beam theory. Cranial strength is estimated from the second moment of area and other geometrical measurements made from 20–30 transverse CT scans through the skulls of 20 opossums (Didelphis virginiana), and through single skulls of five felid and five canid genera of different sizes. Maximal biting forces were first estimated from areas on the dried skulls bounding the spaces filled in life by the jaw-adducting muscles. These estimates were then adjusted with reference to forces recorded in vivo or, for other specimens, to estimates based on dissections of the jaw muscles. Stress distribution in the face, and peak stresses, were calculated for each animal. Stress levels are low (5–35 MPa) compared with peak stresses in limb bones (40–100 MPa), which correlates with the lower in vivo strains in cranial bones reported in the literature. Stress estimates are in a range that is plausible, which supports the validity of the procedure. Patterns of stress distribution along the face are comparable within each group of animals. Peak stress is independent of size for the carnivorans, but decreases with increasing skull length in D. virginiana. High bending strength of the skull is a consequence of cranial form in mammals; having to enclose the brain, for example, increases the bending strength of the skull. Furthermore, factors such as stiffness or shear and torsional strength may be more important than bending strength. However, bending stress levels appear to be closely regulated, as in other bones that have been studied. The threshold for optimising bending strength and weight is simply at a different level.
Book
The editors of this book have used their combined 90 years of experience working on the behaviour and ecology of wild carnivores to draw together a unique network of the world’s experts on musteloid biology and conservation. The musteloids are the most speciose and diverse super-family among carnivores, ranging from little known, exotic, and highly-endangered species to the popular and familiar, and include a large number of introduced invasives. They feature terrestrial, fossorial, arboreal, and aquatic members, ranging from tenacious predators to frugivorous omnivores, span weights from a 100g weasel to 30kg giant otters, and express a range of social behaviours from the highly gregarious to the fiercely solitary. Their diversity and extensive biogeography inform a wide spectrum of ecological theory and conservation practice. Beginning with a brief account of 93 musteloid species, there follow eight comprehensive review chapters covering topics most relevant to musteloid biology and conservation: evolution, form and function, population dynamics, communication, social organisation, exploitation and conflict with people, study tools and techniques, and disease. Twenty detailed case studies then delve into the very best species investigations worldwide, written by leading figures in the field, and providing a range of geographic and taxonomic coverage. The final chapter synthesises what has been discussed in the book, and reflects on the different and diverse conservation needs of musteloids and the wealth of conservation lessons they offer.
Chapter
Excluded from the pursuit predator niche by better-adapted early felids and canids, the musteloids exploited other hunting strategies as grasslands proliferated in the Oligocene. Unconstrained by specialised running limbs, lineages evolved to excavate prey (badgers) and enter burrows (polecats). Others took to tree-climbing (martens, procynoids) and even swimming (otters). While some species specialised in rodent hunting (weasels) others became more generalist omnivores. In-turn the dispersion of these food types dictated socio-spatial geometries, allowing insectivorous, piscivorous and frugivorous species to congregate with varying degrees of social cohesion, often unified within subterranean burrows – a basis to group-living distinct from the pack-hunting felids and canids. Induced ovulation and delayed implantation feature in the mating systems of several species, evolved to ensure breeding success amongst low-density, solitary ancestors. Group-living musteloids exhibit degrees of reproductive suppression, allo-parental care and other cooperative behaviours, thus this contrarian superfamily provides unique insights into the basis of carnivore societies.
Article
Convergence is widely regarded as compelling evidence for adaptation, often being portrayed as evidence that phenotypic outcomes are predictable from ecology, overriding contingencies of history. However, repeated outcomes may be very rare unless adaptive landscapes are simple, structured by strong ecological and functional constraints. One such constraint may be a limitation on body size because performance often scales with size, allowing species to adapt to challenging functions by modifying only size. When size is constrained, species might adapt by changing shape; convergent shapes may therefore be common when size is limiting and functions are challenging. We examine the roles of size and diet as determinants of jaw shape in Sciuridae. As expected, size and diet have significant interdependent effects on jaw shape and ecomorphological convergence is rare, typically involving demanding diets and limiting sizes. More surprising is morphological without ecological convergence, which is equally common between and within dietary classes. Those cases, like rare ecomorphological convergence, may be consequences of evolving on an adaptive landscape shaped by many-to-many relationships between ecology and function, many-to-one relationships between form and performance, and one-to-many relationships between functionally versatile morphologies and ecology. On complex adaptive landscapes, ecological selection can yield different outcomes. This article is protected by copyright. All rights reserved.
Article
Ecomorphology studies focus on understanding how anatomical and behavioral diversity result in differences in performance, ecology, and fitness. In mammals, the determinate growth of the skeleton entails that bite performance should change throughout ontogeny until the feeding apparatus attains its adult size and morphology. Then, interspecific differences in adult phenotypes are expected to drive food resource partitioning and patterns of lineage diversification. However, Formal tests of these predictions are lacking for the majority of mammal groups, and thus our understanding of mammalian ecomorphology remains incomplete. By focusing on a fundamental measure of feeding performance, bite force, and capitalizing on the extraordinary morphological and dietary diversity of bats, we discuss how the intersection of ontogenetic and macroevolutionary changes in feeding performance may impact ecological diversity in these mammals. We integrate data on cranial morphology and bite force gathered through longitudinal studies of captive animals and comparative studies of free-ranging individuals. We demonstrate that ontogenetic trajectories and evolutionary changes in bite force are highly dependent on changes in body and head size, and that bats exhibit dramatic, allometric increases in bite force during ontogeny. Interspecific variation in bite force is highly dependent on differences in cranial morphology and function, highlighting selection for ecological specialization. While more research is needed to determine how ontogenetic changes in size and bite force specifically impact food resource use and fitness in bats, interspecific diversity in cranial morphology and bite performance seem to closely match functional differences in diet. Altogether, these results suggest direct ecomorphological relationships at ontogenetic and macroevolutionary scales in bats.
Article
Lifetime reproductive success of males is often dependent upon the ability to physically compete for mates. However, species variation in social structure leads to differences in the relative importance of intraspecific aggression. Here we present a large comparative data set on sexual dimorphism in skeletal shape in Carnivora to test the hypotheses that carnivorans exhibit sexual dimorphism in skeletal anatomy that is reflective of greater specialization for physical aggression in males relative to females and that this dimorphism is associated with the intensity of sexual selection. We tested these hypotheses using a set of functional indices predicted to improve aggressive performance. Our results indicate that skeletal shape dimorphism is widespread within our sample. Functional traits thought to enhance aggressive performance are more pronounced in males. Phylogenetic model selection suggests that the evolution of this dimorphism is driven by sexual selection, with the best-fitting model indicating greater dimorphism in polygynous versus non-polygynous species. Skeletal shape dimorphism is correlated with body size dimorphism, a common indicator of the intensity of male-male competition, but not with mean body size. These results represent the first evidence of sexual dimorphism in the primary locomotor system of a large sample of mammals. This article is protected by copyright. All rights reserved.
Article
Morphometrics, a new branch of statistics, combines tools from geometry, computer graphics and biometrics in techniques for the multivariate analysis of biological shape variation. Although medical image analysts typically prefer to represent scenes by way of curving outlines or surfaces, the most recent developments in this associated statistical methodology have emphasized the domain of landmark data: size and shape of configurations of discrete, named points in two or three dimensions. This paper introduces a combination of Procrustes analysis and thin-plate splines, the two most powerful tools of landmark-based morphometrics, for multivariate analysis of curving outlines in samples of biomedical images. The thin-plate spline is used to assign point-to-point correspondences, called semi-landmarks, between curves of similar but variable shape, while the standard algorithm for Procrustes shape averages and shape coordinates is altered to accord with the ways in which semi-landmarks formally differ from more traditional landmark loci. Subsequent multivariate statistics and visualization proceed mainly as in the landmark-based methods. The combination provides a range of complementary filters, from high pass to low pass, for effects on outline shape in grouped studies. The low-pass version is based on the spectrum of the spline, the high pass, on a familiar special case of Procrustes analysis. This hybrid method is demonstrated in a comparison of the shape of the corpus callosum from mid-sagittal sections of MRI of 25 human brains, 12 normal and 13 with schizophrenia.
Article
Fifteen variables, selected primarily to reflect functionally significant aspects of cranial morphology, were measured on one skull each of 62 species of modern carnivores, including viverrids, canids, mustelids and felids. To allow comparisons between species of different sizes without the potentially confounding effects of allometric shape changes, the measurements were transformed to dimmensionless variables, based on the residuals from allometric equations. Fourteen out of 15 of the transformed variables distinguish one or more of the four family groups and the rotated first two axes of a principal components analysis distinguish all four families from each other. The following functional hypotheses are proposed: mustelids and felids have the most powerful bites and canids the weakest among the four family groups studied; mustelids and, to a lesser degree, felids have more powerful neck musculature than do canids and viverrids; and visual abilities are best developed among felids and least developed among mustelids. The first two functional hypotheses suggest possible differences in killing behaviour, which are supported by a preliminary survey of the literature on such behaviour. Allometric analysis of the 15 cranial measures shows that the neurocranial components scale with negative allometry, while most of the other measures scale approximately isometrically.
Chapter
This chapter describes the types of dimorphic traits found in marine mammals and explains some of the reasons why these traits might have evolved and what can be inferred about the lives of males and females in a particular species from the pattern of sexual dimorphism. "Sexual dimorphism" means that the two sexes of a species differ in external appearance or other features. Males and females may differ in size, color, shape, the development of appendages, and also in scent or sound production. Darwin considered that most sexual dimorphism was due to sexual selection, in which evolutionary forces acted separately on the sexes. The emerging view is that the degree of sexual dimorphism in a species is the result of the difference between the sum of all the selective pressures affecting the male and the sum of those affecting the female. Sexual size dimorphism is "reversed" among the 13 species of baleen whales with females attaining asymptotic lengths that are generally 5% longer than males. The relative size of the sexes varies widely among the more than 70 species of toothed whales. Males are larger than females in many species, with the most pronounced dimorphism in sperm whales, killer whales, bottlenose whales, narwhals, belugas, and pilot whales. Differences between the sexes may occur in the size and shape of the head, teeth, thoracic girth, flukes, flippers, dorsal fin, caudal peduncle, postanal hump, and length of the beak. The 36 species of pinnipeds show the greatest range in sexual size dimorphism of any higher vertebrate group. Adult males are up to 10 times as heavy as adult females in some species, whereas females are slightly larger than males in others. The variation in sexual dimorphism among marine mammal taxa is striking. The sexes are visually indistinguishable in some species, whereas in others the differences between the sexes are so extreme that males and females live essentially separate lives except when they meet to mate. New techniques, such as scoring molecular genetic markers from tissue samples, are providing insight into social structure and variance in male reproductive success.
Chapter
The greatest strength of the new geometric morphometrics is the system of interrelated multivariate and graphical procedures it offers for a variety of analytic questions involving landmark data. A typical analysis will begin with the conversion of landmark data into a multivariate statistical representation of shape, will continue with a series of broadly familiar multivariate matrix manipulations, and will conclude by inspection of a considerable variety of diagrams that represent the findings in both the space of shape coordinates per se and the space of the two-or three-dimensional image of the organism. The choices under the first heading, the passage to a multivariate representation of shape, include two-point shape coordinates, partial warp scores, and Procrustes residuals. Each of these except the partial warp scores is unsuitable for some subset of the reasonable matrix manipulations; for instance, shape coordinates do not supply sensible principal components analyses, and Procrustes residuals cannot lead to sound canonical variate analyses without modification. The modes of diagramming data include thin-plate splines, partial warp splines and scatters, Procrustes residual scatters, and resistant-fit scatters, among others. Most analyses benefit greatly from exploiting more than one of these.
Article
Convergence in morphology can result from evolutionary adaptations in species living in environments with similar selective pressures. Here, we investigate whether the shape of the forelimb long bones has converged in environments imposing similar functional constraints, using musteloid carnivores as a model. The limbs of quadrupeds are subjected to many factors that may influence their shape. They need to support body mass without collapsing or breaking, yet at the same time resist the stresses and strains induced by locomotion. This likely imposes strong constraints on their morphology. Our geometric morphometric analyses show that locomotion, body mass and phylogeny all influence the shape of the forelimb. Furthermore, we find a remarkable convergence between: (i) aquatic and semi-fossorial species, both displaying a robust forelimb, with a shape that improves stability and load transfer in response to the physical resistance imposed by the locomotor environment; and (ii) aquatic and arboreal/semi-arboreal species, with both groups displaying a broad capitulum. This augments the degree of pronation/supination, an important feature for climbing as well as grasping and manipulation ability, behaviors common to aquatic and arboreal species. In summary, our results highlight how musteloids with different locomotor ecologies show differences in the anatomy of their forelimb bones. Yet, functional demands for limb movement through dense media also result in convergence in forelimb long-bone shape between diverse groups, for example, otters and badgers. © 2015 Anatomical Society.
Article
In mustelids males are always the larger sex. In 24 samples from 15 species there is a significant inverse correlation between the extent of sexual dimorphism (measured as the ratio of male to female weight) and male weight. Two explanations for the selective advantages of dimorphism are discussed. The first hypothesis proposes that it reduces intersexual competition for food by enabling each sex to exploit different prey. Available data on food habits are insufficient to test the theory directly. Other objections, however, lead to the conclusion that avoidance of competition is not the primary advantage of dimorphism. The second hypothesis takes into account the polygynous breeding systems of mustelids and the fact that females alone raise their litters. It proposes that small females are favoured because they need less energy for daily maintenance and are probably more efficient in hunting small prey. Because of this they can channel more energy into reproduction than larger females. Large males are favoured by sexual selection and the ability to exploit a wider range of prey. Data are presented for weasels Mustela nivalis showing that towards the end of lactation an average female requires daily about 20% less energy than a hypothetical male-sized female. The optimum sizes of each sex result from different selective pressures, and probably vary independently. Relationships between reproductive strategy, diet and dimorphism are discussed. /// У куньих самцы всегда более крупные. В 24 пробах на 15 видах установлена обратная корреляция между степенью полового диморфизма (определенного как отношение веса самцов и самок) и весом самцов. Обсуждаются два объяснения селективного преисущества диморфизма. Первая гипотеза предполагает, что диморфизм снижает межполовую конкуренцию за пишу, т.к. представители разного пола могут использовать разные категории жертв. Имеющиеся данные по питанию недостатичны для праямой проверки теории. Но другие возражения приводят к заключению, что устранение конкуренции - не прямое следствие диморфизма. Вторая гипотеза принимает во внимание полигиннуо систему размножения мустелид и тот факт, что самки, одни, воспитывают потомство. Предполагается, сто мелкие самки тратят меньше энергии на поддержание обменных процессов и более эффективно охотятся на мелкую добычу. В результате этого они могут затрачивать больше энертии на процессы размножения, чем крупные самцы. Отбор крупных самцов позволяет использовать более широкий крут жертв. Данные, собранные на Mustela nivalis показали, что в конце лактации суточные энергетические потребности самок в среднем на 20% меньше, чем у гипотетических самок размером с самца. Оптимальные размеры представителей обоих полов - результат различно направленного отбора и по-видимому варьируют независимо друг от друга. Обсуждается соотношения между стратегией размножения, диетой и степенью диморфизма.
Article
It is proposed (1) that the evolution of pinniped polygyny can profitably be examined as an integral part of a complex adaptive suite including physiology, morphology, ecology, and distribution which has involved in relation to an amphibious mode of life, and (2) that the selective factors evolved in this behavioral evolution can be identified by analysis of the processes by which the breeding structure is annually re-established and maintained in the rookeries. Terrestrial parturition and offshore marine feeding appear to have been key determinants of the major adaptive features of pinnipeds, and also to have interacted with characteristics common to most mammals in such a way as to favor both polygyny and sexual dimorphism. A model is presented that starts with terrestrial parturition and offshore marine feeding, which together are unique to pinnipeds, derives from them a series of functions typical of polygynous pinnipeds, relates these to characteristics common to most mammals, and indicates the major feedback loops which have given selective impetus to the evolution of the polygynous breeding system. Key roles are assigned to gregariousness, and male exclusion. The roles of large size, fat deposits, sexual dimorphism, female reproductive patterns, and ontogenetic development are evaluated.
Article
Virtually nothing in nature is uniform. Observed at the right scale, most entities are clustered rather than evenly distributed, spatially and temporally, and this applies across domains from the distribution of matter in the universe, to habitats across the Earth's surface, and to energy in the landscape. Patchiness means organisms cannot carve out even territories. Instead, their shape and size depends on the dispersion of materials needed for survival and reproduction. This fundamental feature of life is intrinsically understood in ecology, for example, in the ideal free distribution and optimal foraging theory, and is represented in the anatomy as well as behaviour of organisms via the structures and strategies for moving, finding and capturing these patchy resources. But perhaps most striking of all is the role of patchiness in facilitating the formation of social groups – of societies. The resource dispersion hypothesis (RDH) suggests that where resources are dispersed and rich enough, multiple individuals can collapse into groups that share the same space at little cost to each other. Cooperation may be absent, but sociality is favoured nevertheless. Thirty years after the origin of the hypothesis, we review the accumulating models, critiques, evidence and experiments, concluding that RDH is a pervasive feature of animal spacing patterns across a wide range of species, taxonomic groups and ecosystems. In the spirit of the original objective of the Huxley Reviews to ‘suggest and inspire research that will improve our knowledge in the future’, we also take the opportunity to consider wider implications of the RDH. If we live and evolved on a patchwork planet, then we should expect broader effects. Indeed, we suggest that the RDH has played an important role in the evolution of cooperation, biodiversity, behaviour and, not least, in the social organization of humans in our evolutionary past and today.
Article
This vignette documents the use of the caper package for R (R Development Core Team, 2011) in carrying out a range of comparative analysis methods for phylogenetic data. The caper package, and the code in this vignette, requires the ape package (Paradis et˜al., 2004) along with the packages mvtnorm and MASS.
Article
Geometric Morphometrics for Biologists is an introductory textbook for a course on geometric morphometrics, written for graduate students and upper division undergraduates, covering both theory of shape analysis and methods of multivariate analysis. It is designed for students with minimal math background; taking them from the process of data collection through basic and more advanced statistical analyses. Many examples are given, beginning with simple although realistic case-studies, through examples of complex analyses requiring several different kinds of methods. The book also includes URLâs for free software and step-by-step instructions for using the software.
Article
Long-term continuous observations of hunting lions Panthera leo in the Kruger National Park were used to assess the variables affecting hunting success of male and female lions. Generalized linear models revealed that seven variables had significant independent influences on hunting success, with the most important being the prey species hunted. Three types of variables were recognized: (1) lion related, where type of hunt, wind orientation, and the number of adults hunting; (2) prey related, where prey species and herd size; (3) environment related, where moon brightness, and grass height were significant. The sex of the lions had no effect on the overall probability of hunting success. Five second-order interactions significantly influenced the probability of hunting success, with the most important being the interaction between sex and the type of prey. The only significant third-order interaction containing the variable sex, was the inter-relationship with prey species and shrub cover. After removing the over-riding bias of the prey species, greater resolution of the factors affecting success was revealed. The major difference was that group size influences hunting success and concomitantly prey selection, promoting selection for medium-sized ungulates like zebra Equus burchelli and wildebeest Connochaetes taurinus by females, and males mainly capturing buffalo Syncerus caffer. We conclude that in African ecosystems, the hunting success of male and female lions varies with a range of combinations of lion-, prey- and environment-related variables. We demonstrate the hunting ability of male lions, which has perhaps been understated in other studies.
Article
1. Here, I present a new, multifunctional phylogenetics package, phytools, for the R statistical computing environment. 2. The focus of the package is on methods for phylogenetic comparative biology; however, it also includes tools for tree inference, phylogeny input/output, plotting, manipulation and several other tasks. 3. I describe and tabulate the major methods implemented in phytools, and in addition provide some demonstration of its use in the form of two illustrative examples. 4. Finally, I conclude by briefly describing an active web-log that I use to document present and future developments for phytools. I also note other web resources for phylogenetics in the R computational environment.
Article
In the current resurgence of interest in the biological basis of animal behavior and social organization, the ideas and questions pursued by Charles Darwin remain fresh and insightful. This is especially true of The Descent of Man and Selection in Relation to Sex, Darwin's second most important work. This edition is a facsimile reprint of the first printing of the first edition (1871), not previously available in paperback. The work is divided into two parts. Part One marshals behavioral and morphological evidence to argue that humans evolved from other animals. Darwin shoes that human mental and emotional capacities, far from making human beings unique, are evidence of an animal origin and evolutionary development. Part Two is an extended discussion of the differences between the sexes of many species and how they arose as a result of selection. Here Darwin lays the foundation for much contemporary research by arguing that many characteristics of animals have evolved not in response to the selective pressures exerted by their physical and biological environment, but rather to confer an advantage in sexual competition. These two themes are drawn together in two final chapters on the role of sexual selection in humans. In their Introduction, Professors Bonner and May discuss the place of The Descent in its own time and relation to current work in biology and other disciplines.
Article
The ability to grasp and manipulate is often considered a hallmark of hominins and associated with the evolution of their bipedal locomotion and tool use. Yet, many other mammals use their forelimbs to grasp and manipulate objects. Previous investigations have suggested that grasping may be derived from digging behaviour, arboreal locomotion or hunting behaviour. Here, we test the arboreal origin of grasping and investigate whether an arboreal lifestyle could confer a greater grasping ability in musteloid carnivorans. Moreover, we investigate the morphological adaptations related to grasping and the differences between arboreal species with different grasping abilities. We predict that if grasping is derived from an arboreal lifestyle, then the anatomical specializations of the forelimb for arboreality must be similar to those involved in grasping. We further predict that arboreal species with a well-developed manipulation ability will have articulations that facilitate radio-ulnar rotation. We use ancestral character state reconstructions of lifestyle and grasping ability to understand the evolution of both traits. Finally, we use a surface sliding semi-landmark approach capable of quantifying the articulations in their full complexity. Our results largely confirm our predictions, demonstrating that musteloids with greater grasping skills differ markedly from others in the shape of their forelimb bones. These analyses further suggest that the evolution of an arboreal lifestyle likely preceded the development of enhanced grasping ability.
Article
Sexual dimorphism is common in nature and has the potential to increase intraspecific variation in performance and patterns of resource use. We sought to determine whether anadromous threespine stickleback, Gasterosteus aculeatus, exhibit sexual dimorphism in feeding kinematics. We filmed four males and four females consuming live prey in a total of 51 sequences filmed at 500 Hz, then tested for differences in cranial kinematics using a combination of principal component analysis and linear mixed models. We document, for the first time in fishes, divergence between males and females in both the timing of key movements and the magnitude of excursions reached by the hyoid, jaws and neurocranium during prey capture. Some of the largest differences are in jaw protrusion, with males exhibiting faster time to peak jaw protrusion but females exhibiting greater maximum jaw protrusion. Measurements of morphological jaw protrusion on cleared and stained specimens significantly predict jaw protrusion in kinematics. This morphological divergence could reflect ecological divergence between the sexes, or the demands of nest building and territory defense compromising male feeding performance. Remarkably, the morphological jaw protrusion divergence in anadromous males and females is similar to jaw protrusion divergence between ecomorphs in a benthic-limnetic species pair, with limnetics exhibiting female-like patterns of protrusion and benthics exhibiting male-like patterns. These results suggest that sexual dimorphism in feeding functional morphology exists in nature and may have played an important role in the radiation of threespine stickleback.