Sexual selection and sexual size dimorphism in animals

Sexual selection is often considered as a critical evolutionary force promoting sexual size dimorphism (SSD) in animals. However, empirical evidence for a positive relationship between sexual selection on males and male-biased SSD received mixed support depending on the studied taxonomic group and on the method used to quantify sexual selection. Here, we present a meta-analytic approach accounting for phylogenetic non-independence to test how standardized metrics of the opportunity and strength of pre-copulatory sexual selection relate to SSD across a broad range of animal taxa comprising up to 95 effect sizes from 59 species. We found that SSD based on length measurements was correlated with the sex difference in the opportunity for sexual selection but showed a weak and statistically non-significant relationship with the sex difference in the Bateman gradient. These findings suggest that pre-copulatory sexual selection plays a limited role for the evolution of SSD in a broad phylogenetic context.     

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.     

Sexual selection versus alternative causes of sexual dimorphism in teiid lizards

Summary The presence and extent of sexual dimorphisms in body form (size and shape) of adult macroteiid lizards were investigated. Males were significantly larger than females in the temperate species, Cnemidophorus tigris, and in the tropical species, Ameiva ameiva and C. ocellifer. Young adult C. tigris males grew faster than young adult females within and between reproductive seasons. Adult males of all species had larger heads than adult females of the same body size; this difference increased with body size. Moreover, male C. tigris were heavier than females of the same snout-vent length. The causes and consequences of the sexual dimorphisms were also examined. The possible causes of body size are especially numerous, and distinguishing the relative influences of the various causal selection factors on body size is problematical. Nevertheless, observational field data were used to tentatively conclude that intrasexual selection was the cause of larger body size of C. tigris males relative to females because (1) larger males won in male aggressive interactions, (2) the winning males gained access to more females by repelling competitors and by female acceptance, (3) larger males consequently had higher reproductive success, and (4) other hypothetical causes of larger male size were unsupported.     

Sexual selection explains Rensch's rule of allometry for sexual size dimorphism

In 1950, Rensch first described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespread and fundamental allometric relationship is now commonly referred to as 'Rensch's rule'. However, despite numerous recent studies, we still do not have a general explanation for this allometry. Here we report that patterns of allometry in over 5300 bird species demonstrate that Rensch's rule is driven by a correlated evolutionary change in females to directional sexual selection on males. First, in detailed multivariate analysis, the strength of sexual selection was, by far, the strongest predictor of allometry. This was found to be the case even after controlling for numerous potential confounding factors, such as overall size, degree of ornamentation, phylogenetic history and the range and degree of size dimorphism. Second, in groups where sexual selection is stronger in females, allometry consistently goes in the opposite direction to Rensch's rule. Taken together, these results provide the first clear solution to the long-standing evolutionary problem of allometry for sexual size dimorphism: sexual selection causes size dimorphism to correlate with species size.     

Sexual selection, sexual size dimorphism and Rensch's rule in Odonata

Odonata (dragonflies and damselflies) exhibit a range of sexual size dimorphism (SSD) that includes species with male-biased (males > females) or female-biased SSD (males < females) and species exhibiting nonterritorial or territorial mating strategies. Here, we use phylogenetic comparative analyses to investigate the influence of sexual selection on SSD in both suborders: dragonflies (Anisoptera) and damselflies (Zygoptera). First, we show that damselflies have male-biased SSD, and exhibit an allometric relationship between body size and SSD, that is consistent with Rensch's rule. Second, SSD of dragonflies is not different from unit, and this suborder does not exhibit Rensch's rule. Third, we test the influence of sexual selection on SSD using proxy variables of territorial mating strategy and male agility. Using generalized least squares to account for phylogenetic relationships between species, we show that male-biased SSD increases with territoriality in damselflies, but not in dragonflies. Finally, we show that nonagile territorial odonates exhibit male-biased SSD, whereas male agility is not related to SSD in nonterritorial odonates. These results suggest that sexual selection acting on male sizes influences SSD in Odonata. Taken together, our results, along with avian studies (bustards and shorebirds), suggest that male agility influences SSD, although this influence is modulated by territorial mating strategy and thus the likely advantage of being large. Other evolutionary processes, such as fecundity selection and viability selection, however, need further investigation.     

Sexual dimorphism in weight among the primates: The relative impact of allometry and sexual selection

In this paper, we examine allometric and sexual-selection explanations for interspecific differences in the amount of sexual dimorphism among 60 primate species. Based on evidence provided by statistical analyses, we reject Leutenegger and Cheverud’s [(1982). Int. J. Primatol.3:387-402] claim that body size alone is the major factor in the evolution of sexual dimorphism. The alternative proposed here is that sexual selection due to differences in the reproductive potential of males and females is the primary cause of sexual dimorphism. In addition, we propose that the overall size of a species determines whether the dimorphism will be expressed as size dimorphism,rather than in some other form.     

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as ‘Rensch''s rule’. While most researchers attribute Rensch''s rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch''s rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch''s rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.     

Adult sex ratio,sexual dimorphism and sexual selection in a Mesozoic reptile

The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index −0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.     

Sexual size dimorphism in shorebirds, gulls, and alcids: the influence of sexual and natural selection

Abstract. Charadrii (shorebirds, gulls, and alcids) have an unusual diversity in their sexual size dimorphism, ranging from monomorphism to either male-biased or female-biased dimorphism. We use comparative analyses to investigate whether this variation relates to sexual selection through competition for mates or natural selection through different use of resources by males and females. As predicted by sexual selection theory, we found that in taxa with socially polygynous mating systems, males were relatively larger than females compared with less polygynous species. Furthermore, evolution toward socially polyandrous mating systems was correlated with decreases in relative male size. These patterns depend on the kinds of courtship displays performed by males. In taxa with acrobatic flight displays, males are relatively smaller than in taxa in which courtship involves simple flights or displays from the ground. This result remains significant when the relationship with mating system is controlled statistically, thereby explaining the enigma of why males are often smaller than females in socially monogamous species. We did not find evidence that evolutionary changes in sexual dimorphism relate to niche division on the breeding grounds. In particular, biparental species did not have greater dimorphism in bill lengths than uniparental species, contrary to the hypothesis that selection for ecological divergence on the breeding grounds has been important as a general explanation for patterns of bill dimorphism. Taken together, these results strongly suggest that sexual selection has had a major influence on sexual size dimorphism in Charadrii, whereas divergence in the use of feeding resources while breeding was not supported by our analyses.     

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.     

Sexual dimorphism in epicuticular compounds despite similar sexual selection in sex role‐reversed seed beetles

Sexual selection imposed by mating preferences is often implicated in the evolution of both sexual dimorphism and divergence between species in signalling traits. Epicuticular compounds (ECs) are important signalling traits in insects and show extensive variability among and within taxa. Here, we investigate whether variation in the multivariate EC profiles of two sex role‐reversed beetle species, Megabruchidius dorsalis and Megabruchidius tonkineus, predicts mate attractiveness and mating success in males and females. The two species had highly distinct EC profiles and both showed significant sexual dimorphism in ECs. Age and mating status in both species were also distinguishable by EC profile. Males and females of both species showed significant association between their EC profile and attractiveness, measured both as latency to mating and as success in mate‐choice trials. Remarkably, the major multivariate vector describing attractiveness was correlated in both species, both sexes, and in both choice and no‐choice experiments such that increased attractiveness was in all cases associated with a similar multivariate modification of EC composition. Furthermore, in both sexes this vector of attractiveness was associated with more male‐like EC profiles, as well as those characterizing younger and nonvirgin individuals, which might reflect a general preference for individuals of high condition in both sexes. Despite significant sexual selection on EC composition, however, we found no support for the proposition that sexual selection is responsible for divergence in ECs between these species.     

昆虫的雌雄二型现象

对发生雌雄二型现象的昆虫类群、生态因子及进化进行了概括总结;还特别介绍了长足虻科昆虫雌雄二型的相关方面;并简要讨论了雌雄二型与性选择的关系。     

Sexual selection on male size drives the evolution of male‐biased sexual size dimorphism via the prolongation of male development

Sexual size dimorphism (SSD) arises when the net effects of natural and sexual selection on body size differ between the sexes. Quantitative SSD variation between taxa is common, but directional intraspecific SSD reversals are rare. We combined micro‐ and macroevolutionary approaches to study geographic SSD variation in closely related black scavenger flies. Common garden experiments revealed stark intra‐ and interspecific variation: Sepsis biflexuosa is monomorphic across the Holarctic, while S. cynipsea (only in Europe) consistently exhibits female‐biased SSD. Interestingly, S. neocynipsea displays contrasting SSD in Europe (females larger) and North America (males larger), a pattern opposite to the geographic reversal in SSD of S. punctum documented in a previous study. In accordance with the differential equilibrium model for the evolution of SSD, the intensity of sexual selection on male size varied between continents (weaker in Europe), whereas fecundity selection on female body size did not. Subsequent comparative analyses of 49 taxa documented at least six independent origins of male‐biased SSD in Sepsidae, which is likely caused by sexual selection on male size and mediated by bimaturism. Therefore, reversals in SSD and the associated changes in larval development might be much more common and rapid and less constrained than currently assumed.     

Sexual dimorphism in steppe tortoises (Testudo horsfieldii): influence of the environment and sexual selection on body shape and mobility

Selective forces shape sexes differently, with male body proportions facing strong selection to enhance mate searching and male-to-male combat traits, and female fitness being influenced by the ability to assimilate large amounts of nutrients necessary for vitellogenesis (and/or gestation), and their ability to carry the eggs or embryos. We evaluated the sexual dimorphism of body proportion of more than 800 wild steppe tortoises (Testudo horsfieldii) in Uzbekistan. The thick, well-developed shell offers protection from predators but pronounced digging habits probably also constrain body shape (e.g. a shell that is dorso-ventrally flattened, although round from a dorsal view helps to penetrate into, and move within the soil). Thus, in this species, natural selection might favour a heavy and flat shell that is 'closed' with small openings for appendages. In males, these environmental influences appear to be countered by sexual selection. Compared to females, they weigh less (absolutely and relative to shell dimensions), have longer legs, have shell structure allowing wider movements for their legs, and they walk faster. Males were also able to right themselves more quickly than females did in experimental tests. This quick righting ability is critical because intra-sexual combats frequently result in males being flipped onto their backs and becoming prone to hyperthermia or predation. Females are heavily built, with wide shells (relative to male shells), which may provide space for carrying eggs. From our results, a number of simple hypotheses can be tested on a wide range of chelonian species.     

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.     

Primate models for australopithecine sexual dimorphism

Several different models of sexual dimorphism in the South African australopithecines are compared with sexual dimorphism in the living primates. Australopithecine dimorphism is placed in an evolutionary context, and contrasting trends in the hominid and pongid lineages are shown. Evidence suggesting that the australopithecines were an extremely polytypic taxon is presented, and a high level of both inter- and intra-population variation is indicated.     

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 selection: Another Darwinian process

Why was sexual selection so important to Darwin? And why was it de-emphasized by almost all of Darwin's followers until the second half of the 20th century? These two questions shed light on the complexity of the scientific tradition named “Darwinism”. Darwin's interest in sexual selection was almost as old as his discovery of the principle of natural selection. From the beginning, sexual selection was just another “natural means of selection”, although different from standard “natural selection” in its mechanism. But it took Darwin 30 years to fully develop his theory, from the early notebooks to the 1871 book The Descent of Man, and Selection in Relation to Sex. Although there is a remarkable continuity in his basic ideas about sexual selection, he emphasized increasingly the idea that sexual selection could oppose the action of natural selection and be non adaptive. In time, he also gave more weight to mate choice (especially female choice), giving explicit arguments in favor of psychological notions such as “choice” and “aesthetic sense”. But he also argued that there was no strict demarcation line between natural and sexual selection, a major difficulty of the theory from the beginning. Female choice was the main reason why Alfred Russel Wallace, the co-discoverer of the principle of natural selection, engaged in a major controversy with Darwin about sexual selection. Wallace was suspicious about sexual selection in general, trying to minimize it by all sorts of arguments. And he denied entirely the existence of female choice, because he thought that it was both unnecessary and an anthropomorphic notion. This had something to do with his spiritualist convictions, but also with his conception of natural selection as a sufficient principle for the evolutionary explanation of all biological phenomena (except for the origin of mind). This is why Wallace proposed to redefine Darwinism in a way that excluded Darwin's principle of sexual selection. The main result of the Darwin–Wallace controversy was that most Darwinian biologists avoided the subject of sexual selection until at least the 1950 s, Ronald Fisher being a major exception. This controversy still deserves attention from modern evolutionary biologists, because the modern approach inherits from both Darwin and Wallace. The modern approach tends to present sexual selection as a special aspect of the theory of natural selection, although it also recognizes the big difficulties resulting from the inevitable interaction between these two natural processes of selection. And contra Wallace, it considers mate choice as a major process that deserves a proper evolutionary treatment. The paper's conclusion explains why sexual selection can be taken as a test case for a proper assessment of “Darwinism” as a scientific tradition. Darwin's and Wallace's attitudes towards sexual selection reveal two different interpretations of the principle of natural selection: Wallace's had an environmentalist conception of natural selection, whereas Darwin was primarily sensitive to the element of competition involved in the intimate mechanism of any natural process of selection. Sexual selection, which can lack adaptive significance, reveals this exemplarily.     

Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.     

Sexual selection and canine dimorphism in New World monkeys

Social and ecological factors are important in shaping sexual dimorphism in Anthropoidea, but there is also a tendency for body-size dimorphism and canine dimorphism to increase with increased body size (Rensch's rule) (Rensch: Evolution Above the Species Level. London: Methuen, 1959.) Most ecologist interpret Rensch's rule to be a consequence of social and ecological selective factors that covary with body size, but recent claims have been advanced that dimorphism is principally a consequence of selection for increased body size alone. Here we assess the effects of body size, body-size dimorphism, and social structure on canine dimorphism among platyrrhine monkeys. Platyrrhine species examined are classified into four behavioral groups reflecting the intensity of intermale competition for access to females or to limiting resources. As canine dimorphism increases, so does the level of intermale competition. Those species with monogamous and polyandrous social structures have the lowest canine dimorphism, while those with dominance rank hierarchies of males have the most canine dimorphism. Species with fission-fusion social structures and transitory intermale breeding-season competition fall between these extremes. Among platyrrhines there is a significant positive correlation between body size and canine dimorphism However, within levels of competition, no significant correlation was found between the two. Also, with increased body size, body-size dimorphism tends to increase, and this correlation holds in some cases within competition levels. In an analysis of covariance, once the level of intermale competition is controlled for, neither molar size nor molar-size dimorphism accounts for a significant part of the variance in canine dimorphism. A similar analysis using body weight as a measure of size and dimorphism yields a less clear-cut picture: body weight contributes significantly to the model when the effects of the other factors are controlled. Finally, in a model using head and body length as a measure of size and dimorphism, all factors and the interactions between them are significant. We conclude that intermale competition among platyrrhine species is the most important factor explaining variations in canine dimorphism. The significant effects of size and size dimorphism in some models may be evidence that natural (as opposed to sexual) selection also plays a role in the evolution of increased canine dimorphism.