Intrasexual selection and phylogenetic constraints in the evolution of sexual canine dimorphism in strepsirhine primates

The goals of this study were to analyze the origin and function of sex differences in the size of canine teeth among Malagasy lemurs and other strepsirhine primates. These analyses allowed me to illuminate interactions between different mechanisms of sexual selection and to elucidate constraints on this sexually-selected trait. In contrast to central predictions of sexual selection theory, polygynous lemurs lack both sexual dimorphism in body size and male social dominance, but the degree of sexual dimorphism in the size of their canines is not known. A comparison of male and female canine size in 31 species of lemurs and lorises revealed significant male-biased canine dimorphism in only 6 of 13 polygynous lemur species. This result is in contrast to predictions of a hypothesis that would explain the lack of size dimorphism in lemurs as a result of high viability costs because canine teeth presumably have low maintenance costs and because they are used as weapons in male-male combat. Moreover, because females had significantly larger maxillary canines than males in only one lemur species, female dominance is not generally based on female physical superiority and selective forces favoring female dominance do not constrain sexual canine dimorphism in the sense of a pleiotropic effect. Contrary to predictions of sexual selection theory, species differences in canine dimorphism across strepsirhines were neither associated with differences in mating system, nor with the potential frequency of aggression. Variation in canine dimorphism was also unrelated to differences in body size, but there were significant differences among families, pointing to strong phylogenetic constraints. This study demonstrated that polygynous lemurs are at most subject to weak intrasexual selection on dental traits used in male combat and that traits thought to be under intense sexual selection are strongly influenced by phylogenetic factors.     

Evaluating the “dual selection” hypothesis of canine reduction

A recently proposed model for canine reduction in hominid evolution (the “dual selection” model) suggests that canine reduction occurs as a result for incorporation of the canines into a functional incisal field. Among the evidence used to support this model are patterns of wear and occlusion of the canine teeth, particularly in female anthropoid primates. We examined wear and occlusal patterns of the canine teeth of 311 male and female anthropoid primates. We find no evidence that the canines are typically occluded tip-to-tip, or that they show wear patterns indicating a “gripping and pulling” function during food ingestion and processing. Furthermore, we do not find compelling evidence that the development of the mesial cristid is associated with canine reduction. While we agree that the mechanisms of selective pressures underlying canine reduction need to be investigated, the “dual selection” hypothesis is unsupported by comparative data. © 1996 Wiley-Liss, Inc.     

Correlates of sexual dimorphism in primates: Ecological and size variables

The effects of a series of ecological and size factors on the degree of sexual dimorphism in body weight and canine size were studied among subsets of 70 primate species. Variation in body-weight dimorphism can be almost entirely attributed to body weight (83% of variance R² of weight dimorphism). Much smaller amounts of the variation can be attributed to mating system (R² =6.8%,polygynous species being more dimorphic than monogamous ones) and diet (R² = 2.5%,frugivorous species being more dimorphic than folivorous ones). Habitat (arboreal vs. terrestrial) and activity rhythm (nocturnal vs. diurnal) have only an indirect effect on weight dimorphism. Variation in canine-size dimorphism can be explained in terms of canine size (R² =49%),activity rhythm (R² = 20%,diurnal species being more dimorphic than nocturnal ones), and mating system (R² = 10%).Habitat and diet do not play a significant role in canine-size dimorphism. The unexpectedly high contribution of size to sexual dimorphism coupled with the observation of increased sexual dimorphism with increased size leads us to formulate a new selection model for the evolution of sexual dimorphism. We suggest that if there is selection for size increase, whatever its cause, directional selection in both males and females will lead to an increase in sexual dimorphism based on differences in genetic variance between the sexes. Sexual selection, resource division between the sexes, or lopsided reproductive selection need not play a role in such a model.     

A statistical analysis of the literature on sexual dimorphism in primates

The annotated bibliography on sexual dimorphism in primates compiled by the authors was analysed considering the distribution of entries by keytitles, keywords, kind of periodicals and years of publication. A growing interest in this field was observed especially since the 1970s, but a relative scarcity of basic methodological papers was found. Articles on extant human populations and on living nonhuman primates are much more frequent than works on fossil primates and ancient humans.     

Canine tip wear in male and female anthropoids

One component of the “dual selection hypothesis” (Greenfield [1992a] Year. Phys. Anthropol. 35:153–185) is that the tips of female canines are commonly blunted and more frequently so than those of conspecific males. Data derived from two randomly selected age-graded samples of Macaca fascicularis (n = 70) and Colobus badius (n = 59) show that at least 80% of the females exhibit tip blunting on one or both canines and that frequencies of blunting are far greater than those of conspecific males in both jaws. Sexual dimorphism in mandibular canine morphology and wear and other recently critiqued aspects of the “dual selection hypothesis” (Plavcan and Kelley [1996] Am. J. Phys. Anthropol. 99:379–387.) are also discussed. Am J Phys Anthropol 107:87–97. © 1998 Wiley-Liss, Inc.     

Canine size, shape, and bending strength in primates and carnivores

Anthropoid primates are well known for their highly sexually dimorphic canine teeth, with males possessing canines that are up to 400% taller than those of females. Primate canine dimorphism has been extensively documented, with a consensus that large male primate canines serve as weapons for intrasexual competition, and some evidence that large female canines in some species may likewise function as weapons. However, apart from speculation that very tall male canines may be relatively weak and that seed predators have strong canines, the functional significance of primate canine shape has not been explored. Because carnivore canine shape and size are associated with killing style, this group provides a useful comparative baseline for primates. We evaluate primate maxillary canine tooth size, shape and relative bending strength against body size, skull size, and behavioral and demographic measures of male competition and sexual selection, and compare them to those of carnivores. We demonstrate that, relative to skull length and body mass, primate male canines are on average as large as or larger than those of similar sized carnivores. The range of primate female canine sizes embraces that of carnivores. Male and female primate canines are generally as strong as or stronger than those of carnivores. Although we find that seed-eating primates have relatively strong canines, we find no clear relationship between male primate canine strength and demographic or behavioral estimates of male competition or sexual selection, in spite of a strong relationship between these measures and canine crown height. This suggests either that most primate canines are selected to be very strong regardless of variation in behavior, or that primate canine shape is inherently strong enough to accommodate changes in crown height without compromising canine function.     

Intrasexual competition and body weight dimorphism in anthropoid primates

Body weight dimorphism in anthropoid primates has been thought to be a consequence of sexual selection resulting from male-male competition for access to mates. However, while monogamous anthropoids show low degrees of weight dimorphism, as predicted by the sexual selection hypothesis, polygynous anthropoids show high variation in weight dimorphism that is not associated with measures of mating system or sex ratio. This observation has led many to debate the role of other factors such as dietary constraints, predation pressure, substrate constraints, allometric effects, and phylogeny in the evolution of anthropoid weight dimorphism. Here, we re-evaluate variation in adult body weight dimorphism in anthropoids, testing the sexual selection hypothesis using categorical estimates of the degree of male-male intrasexual competition (“competition levels”). We also test the hypotheses that interspecific variation in body weight dimorphism is associated with female body weight and categorical estimates of diet, substrate use, and phylogeny. Weight dimorphism is strongly associated with competition levels, corroborating the sexual selection hypothesis. Weight dimorphism is positively correlated with increasing female body weight, but evidence suggests that the correlation reflects an interaction between overall size and behavior. Arboreal species are, on average, less dimorphic than terrestrial species, while more frugivorous species tend to be more dimorphic than folivorous or insectivorous species. Several alternative hypotheses can explain these latter results. Weight dimorphism is correlated with taxonomy, but so too are competition levels. We suggest that most taxonomic correlations of weight dimorphism represent “phylogenetic niche conservatism”; however, colobines show consistently low degrees of weight dimorphism for reasons that are not clear. Am J Phys Anthropol 103:37–68, 1997. © 1997 Wiley-Liss, Inc.     

Cranial ontogeny and sexual dimorphism in two new world monkeys: Alouatta caraya (Atelidae) and Cebus apella (Cebidae)

Pattern of skull development and sexual dimorphism was studied in Cebus apella and Alouatta caraya using univariate, bivariate, and multivariate statistics. In both species, sexual dimorphism develops because the common growth trajectory in males extends and because of differences in growth rates between sexes. The expectation that the ontogenetic bases of adult dimorphism vary interspecifically is well substantiated by this study. A. caraya exhibits transitional dimorphism in its subadult stage, although the condylobasal length, zygomatic breadth, and rostrum length are strongly dimorphic in the final adult stage, being greater in males. Most cranial measurements in C. apella exhibit significant dimorphism in the adult stage, being strongly influenced by a faster rate of growth in males. Sexual dimorphism is also evidenced through sex differences in growth rates in several cranial measurements. These results also indicate that different ontogenetic mechanisms are acting in C. apella and A. caraya and reveal differences in the way through which neotropical primates attain adult sexual dimorphism. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Sexual size dimorphism in parasitoid wasps

Sexual dimorphism in body length and proportion of overlap between the ranges of body length for males and females were estimated for 361 species of parasitoid wasps from 21 families. In most species, females are generally larger than males, though the range of male and female sizes overlap. Species in the family Ichneumonidae differ significantly from species in other families in three ways: (1) ichneumonids on average are larger, (2) in most species, females are generally smaller than males, and (3) on average, proportion overlap between the ranges of body length for males and females is greater. At present, there is a paucity of life history data on parasitoid wasp species for which size dimorphism is known. Thus it is not clear why ichneumonids differ from species in other families. Possible evolutionary explanations for variation in dimorphism among parasitoid wasp species are discussed.     

A comparative study on the evolution of reversed size dimorphism in monogamous waders

Sexual dimorphism in size is common in birds. Males are usually larger than females, although in some taxa reversed size dimorphism (RSD) predominates. Whilst direct dimorphism is attributed to sexual selection in males giving greater reproductive access to females, the evolutionary causes of RSD are still unclear. Four different hypotheses could explain the evolution of RSD in monogamous birds: (1) The ‘energy storing’ hypothesis suggests that larger females could accumulate more reserves at wintering or refuelling areas to enable an earlier start to egg laying. (2) According to the ‘incubation ability’ hypothesis, RSD has evolved because large females can incubate more efficiently than small ones. (3) The ‘parental role division’ hypothesis suggests that RSD in monogamous waders has evolved in species with parental role division and uniparental male care of the chicks. It is based on the assumption that small male size facilitates food acquisition in terrestrial habitats where chick rearing takes place and that larger females can accumulate more reserves for egg laying in coastal sites. (3) The ‘display agility’ hypothesis suggests that small males perform better in acrobatic displays presumably involved in mate choice and so RSD may have evolved due to female preference for agile males. I tested these hypotheses in monogamous waders using several comparative methods. Given the current knowledge of the phylogeny of this group, the evolutionary history of waders seems only compatible with the hypothesis that RSD has evolved as an adaptation for increasing display performance in males. In addition, the analysis of wing shape showed that males of species with acrobatic flight displays had wings with higher aspect ratio (wing span/²wing area) than non-acrobatic species, which probably increases flight manoeuvrability during acrobatic displays. In species with acrobatic displays males also had a higher aspect ratio than females although no sexual difference was found in non-acrobatic species. These results suggest that acrobatic flight displays could have produced changes in the morphology of some species and suggest the existence of selection favouring higher manoeuvrability in species with acrobatic flight displays. This supports the validity of the mechanisms proposed by the ‘display agility’ hypothesis to explain the evolution of RSD in waders.     

鳄蜥的两性异形

通过比较鳄蜥的体型与头部大小等特征的差异,研究了鳄蜥的两性异形情况.结果表明:性成熟鳄蜥个体体色差异显著,成年雄性头胸部腹面呈鲜红色或浅蓝色,而雌性为浅黄色或淡红色;成年雄性头部显著大于成年雌性(头长(HL),P＜0.001;头宽(HW),P＜0.001),成年雌性腹部长(AL)显著大干成年雄性(P=0.018);而成...     

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.     

Ontogeny and the evolution of adult body size dimorphism in apes

This analysis investigates the ontogeny of body size dimorphism in apes. The processes that lead to adult body size dimorphism are illustrated and described. Potential covariation between ontogenetic processes and socioecological variables is evaluated. Mixed-longitudinal growth data from 395 captive individuals (representing Hylobates lar [gibbon], Hylobates syndactylus [siamang], Pongo pygmaeus [orangutan], Gorilla gorilla [gorilla], Pan paniscus [pygmy chimpanzee], and Pan troglodytes [“common” chimpanzee]) form the basis of this study. Results illustrate heterogeneity in the growth processes that produce ape dimorphism. Hylobatids show no sexual differentiation in body weight growth. Adult body size dimorphism in Pongo can be largely attributed to indeterminate male growth. Dimorphism in African apes is produced by two different ontogenetic processes. Both pygmy chimpanzees (Pan paniscus) and gorillas (Gorilla gorilla) become dimorphic primarily through bimaturism (sex differences in duration of growth). In contrast, sex differences in rate of growth account for the majority of dimorphism in common chimpanzees (Pan troglodytes). Diversity in the ontogenetic pathways that produce adult body size dimorphism may be related to multiple evolutionary causes of dimorphism. The lack of sex differences in hylobatid growth is consistent with a monogamous social organization. Adult dimorphism in Pongo can be attributed to sexual selection for indeterminate male growth. Interpretation of dimorphism in African apes is complicated because factors that influence female ontogeny have a substantial effect on the resultant adult dimorphism. Sexual selection for prolonged male growth in gorillas may also increase bimaturism relative to common chimpanzees. Variation in female growth is hypothesized to covary with foraging adaptations and with differences in female competition that result from these foraging adaptations. Variation in male growth probably corresponds to variation in level of sexual selection. © 1995 Wiley-Liss, Inc.     

A biogeographic reversal in sexual size dimorphism along a continental temperature gradient

The magnitude and direction of sexual size dimorphism (SSD) varies greatly across the animal kingdom, reflecting differential selection pressures on the reproductive and/or ecological roles of males and females. If the selection pressures and constraints imposed on body size change along environmental gradients, then SSD will vary geographically in a predictable way. Here, we uncover a biogeographical reversal in SSD of lizards from Central and North America: in warm, low latitude environments, males are larger than females, but at colder, high latitudes, females are larger than males. Comparisons to expectations under a Brownian motion model of SSD evolution indicate that this pattern reflects differences in the evolutionary rates and/or trajectories of sex‐specific body sizes. The SSD gradient we found is strongly related to mean annual temperature, but is independent of species richness and body size differences among species within grid cells, suggesting that the biogeography of SSD reflects gradients in sexual and/or fecundity selection, rather than intersexual niche divergence to minimize intraspecific competition. We demonstrate that the SSD gradient is driven by stronger variation in male size than in female size and is independent of clutch mass. This suggests that gradients in sexual selection and male–male competition, rather than fecundity selection to maximize reproductive output by females in seasonal environments, are predominantly responsible for the gradient.     

Sexual dimorphism of the human mandible and its association with dental development

The present study investigates whether the human mandible is sexually dimorphic during early postnatal development and whether early dimorphic features persist during subsequent ontogeny. We also examine whether mandibular dimorphism is linked to dimorphism of dental development. Dense CT-derived mandibular meshes of 84 females and 75 males, ranging from birth to adulthood, were analyzed using geometric morphometric methods. On the basis of the specimen's chronological ages and mineralization stages of the deciduous and permanent teeth, we compute dental age as proxy for dental development by the additive conjoint measurement method. By birth, males have, on average, more advanced age-specific shapes than females. However, sex differences decrease quickly as females catch up via a different association between shape and size. This leads to an almost complete reduction of sexual dimorphism between the ages of 4 and 14. From puberty to adulthood, males are characterized by allometric shape changes while the shape of the female mandible continues to change even after size has ceased to increase. Dimorphism of dental maturation becomes visible only at puberty. Sexual dimorphism, concentrated at the ramus and the mental region during the earliest ontogenetic stages and again at adulthood, is not associated with the development of the teeth. At puberty there is a simultaneous peak in size increase, shape development, and dental maturation likely controlled by the surge of sex hormones with a dimorphic onset age. We argue that the infant and adult dimorphism of the mental region may be associated with the development of supralaryngeal structures.     

Sexual dimorphism and allometry of external morphology in Oreochromis mossambicus

Sexual dimorphism in growth of conventional morphometric characters was investigated in juveniles and young adults (size range: 31 to 91 mm) of Oreochromis mossambicus . A closely associated set of traits was identified that shows sexually dimorphic growth, which was positively allometric in the males. These traits correspond to two different morphological complexes: jaw structure and anal/dorsal fins. The best sex discriminators among this set of traits were premaxilla width, anal fin height and snout length. These findings may be explained in terms of intra– and inter–sexual selection acting together and favouring males with strong and large mouths and high dorsal and anal fins, traits that are important in agonistic displays (jaw and fins), fighting and nest digging (jaw).     

Sexual size dimorphism and timing of spring migration in birds

Sexually selected traits are limited by selection against those traits in other fitness components, such as survival. Thus, sexual selection favouring large size in males should be balanced by higher mortality of larger males. However, evidence from red-winged blackbirds (Agelaius phoeniceus) indicates that large males survive better than small males. A survival advantage to large size could result from males migrating north in early spring, when harsh weather favours large size for energetic reasons. From this hypothesis we predicted that, among species, sex differences in body size should be correlated with sex differences in timing of spring migration. The earlier males migrate relative to females, the larger they should be relative to females. We tested this prediction using a comparative analysis of data collected from 30 species of passerine birds captured on migration. After controlling for social mating system, we found that sexual size dimorphism and difference in arrival dates of males and females were significantly positively correlated. This result is consistent with the hypothesis that selection for survival ability promotes sexual size dimorphism (SSD), rather than opposes SSD as is the conventional view. If both natural selection and sexual selection favour large adult males, then limits to male size must be imposed before males become adults.