Sexual size dimorphism is not associated with the evolution of parental care in frogs

Sex differences in parental care are thought to arise from differential selection on the sexes. Sexual dimorphism, including sexual size dimorphism (SSD), is often used as a proxy for sexual selection on males. Some studies have found an association between male‐biased SSD (i.e., males larger than females) and the loss of paternal care. While the relationship between sexual selection on males and parental care evolution has been studied extensively, the relationship between female‐biased SSD (i.e., females larger than males) and the evolution of parental care has received very little attention. Thus, we have little knowledge of whether female‐biased SSD coevolves with parental care. In species displaying female‐biased SSD, we might expect dimorphism to be associated with the evolution of paternal care or perhaps the loss of maternal care. Here, drawing on data for 99 extant frog species, we use comparative methods to evaluate how parental care and female‐biased SSD have evolved over time. Generally, we find no significant correlation between the evolution of parental care and female‐biased SSD in frogs. This suggests that differential selection on body size between the sexes is unlikely to have driven the evolution of parental care in these clades and questions whether we should expect sexual dimorphism to exhibit a general relationship with the evolution of sex differences in parental care.     

DISENTANGLING THE CONTRIBUTION OF SEXUAL SELECTION AND ECOLOGY TO THE EVOLUTION OF SIZE DIMORPHISM IN PINNIPEDS

The positive relationship between sexual size dimorphism (SSD) and harem size across pinnipeds is often cited as a textbook example of sexual selection. It assumes that female aggregation selected for large male size via male–male competition. Yet, it is also conceivable that SSD evolved prior to polygyny due to ecological forces. We analyzed 11 life‐history traits in 35 pinniped species to determine their coevolutionary dynamics and infer their most likely evolutionary trajectories contrasting these two hypotheses. We find support for SSD having evolved prior to changes in the mating system, either as a consequence of niche partitioning during aquatic foraging or in combination with sexual selection on males to enforce copulations on females. Only subsequently did polygyny evolve, leading to further coevolution as the strength of sexual selection intensified. Evolutionary sequence analyses suggest a polar origin of pinnipeds and indicate that SSD and polygyny are intrinsically linked to a suite of ecological and life‐history traits. Overall, this study calls for the inclusion of ecological variables when studying sexual selection and argues for caution when assuming causality between coevolving traits. It provides novel insights into the role of sexual selection for the coevolutionary dynamics of SSD and mating system.     

Sexual size dimorphism in seabirds: sexual selection, fecundity selection and differential niche-utilisation

Seabirds exhibit a range of sexual size dimorphism (SSD) that includes both male-biased (males>females) and female-biased SSD (males相似文献   

Variance in male reproductive success and sexual size dimorphism in pinnipeds: testing an assumption of sexual selection theory

相似文献   

Evolution of sexual size dimorphism in grouse and allies (Aves: Phasianidae) in relation to mating competition,fecundity demands and resource division

Sexual size dimorphism (SSD) is often assumed to be driven by three major selective processes: (1) sexual selection influencing male size and thus mating success, (2) fecundity selection acting on females and (3) inter‐sexual resource division favouring different size in males and females to reduce competition for resources. Sexual selection should be particularly strong in species that exhibit lek polygyny, since male mating success is highly skewed in such species. We investigated whether these three selective processes are related to SSD evolution in grouse and allies (Phasianidae). Male‐biased SSD increased with body size (Rensch’s rule) and lekking species exhibited more male‐biased SSD than nonlekking ones. Directional phylogenetic analyses indicated that lekking evolved before SSD, but conclusions were highly dependent on the body size traits and chosen model values. There was no relationship between SSD and male display agility, nor did resource division influence SSD. Although clutch mass increased with female body size it was not related to the degree of SSD. Taken together, the results are most consistent with the hypothesis that lekking behaviour led to the evolution of male‐biased SSD in Phasianidae.     

SEXUAL SELECTION AND THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN THE WATER STRIDER,AQUARIUS REMIGIS

Sexual size dimorphism (SSD) is often attributed to sexual selection, particularly when males are the larger sex. However, sexual selection favoring large males is common even in taxa where females are the larger sex, and is therefore not a sufficient explanation of patterns of SSD. As part of a more extensive study of the evolution of SSD in water striders (Heteroptera, Gerridae), we examine patterns of sexual selection and SSD in 12 populations of Aquarius remigis. We calculate univariate and multivariate selection gradients from samples of mating and single males, for two sexually dimorphic traits (total length and profemoral width) and two sexually monomorphic traits (mesofemoral length and wing form). The multivariate analyses reveal strong selection favoring larger males, in spite of the female-biased SSD for this trait, and weaker selection favoring aptery and reduced mesofemoral length. Selection is weakest on the most dimorphic trait, profemoral width, and is stabilizing rather than directional. The pattern of sexual selection on morphological traits is therefore not concordant with the pattern of SSD. The univariate selection gradients reveal little net selection (direct + indirect) on any of the traits, and suggest that evolution away from the plesiomorphic pattern of SSD is constrained by antagonistic patterns of selection acting on this suite of positively correlated morphological traits. We hypothesize that SSD in A. remigis is not in equilibrium, a hypothesis that is consistent with both theoretical models of the evolution of SSD and our previous studies of allometry for SSD. A negative interpopulation correlation between the intensity of sexual selection and the operational sex ratio supports the hypothesis that, as in several other water strider species, sexual selection in A. remigis occurs through generalized female reluctance rather than active female choice. The implications of this for patterns of sexual selection are discussed.     

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 selection uncouples the evolution of brain and body size in pinnipeds

The size of the vertebrate brain is shaped by a variety of selective forces. Although larger brains (correcting for body size) are thought to confer fitness advantages, energetic limitations of this costly organ may lead to trade-offs, for example as recently suggested between sexual traits and neural tissue. Here, we examine the patterns of selection on male and female brain size in pinnipeds, a group where the strength of sexual selection differs markedly among species and between the sexes. Relative brain size was negatively associated with the intensity of sexual selection in males but not females. However, analyses of the rates of body and brain size evolution showed that this apparent trade-off between sexual selection and brain mass is driven by selection for increasing body mass rather than by an actual reduction in male brain size. Our results suggest that sexual selection has important effects on the allometric relationships of neural development.     

MALE CONTEST COMPETITION AND THE COEVOLUTION OF WEAPONRY AND TESTES IN PINNIPEDS

Male reproductive success is influenced by competitive interactions during precopulatory and postcopulatory selective episodes. Consequently, males can gain reproductive advantages during precopulatory contest competition by investing in weaponry and during postcopulatory sperm competition by investing in ejaculates. However, recent theory predicts male expenditure on weaponry and ejaculates should be subject to a trade‐off, and should vary under increasing risk and intensity of sperm competition. Here, we provide the first comparative analysis of the prediction that expenditure on weaponry should be negatively associated with expenditure on testes mass. Specifically, we assess how sexual selection influences the evolution of primary and secondary sexual traits among pinnipeds (seals, sea lions, and walruses). Using recently developed comparative methods, we demonstrate that sexual selection promotes rapid divergence in body mass, sexual size dimorphism (SSD), and genital morphology. We then show that genital length appears to be positively associated with the strength of postcopulatory sexual selection. However, subsequent analyses reveal that both genital length and testes mass are negatively associated with investment in precopulatory weaponry. Thus, our results are congruent with recent theoretical predictions of contest‐based sperm competition models. We discuss the possible role of trade‐offs and allometry in influencing patterns of reproductive trait evolution in pinnipeds.     

The evolution of aquatic feeding in seals: insights from Enaliarctos (Carnivora: Pinnipedimorpha), the oldest known seal

The development of pierce‐feeding and loss of oral processing represented major adaptations for underwater feeding in marine mammals. We examined the evolution of pierce‐feeding and its association with changes in tooth spacing and tooth size to determine whether pierce‐feeding was practiced by the earliest known pinnipeds. Data on crown size and spacing in postcanine dentition were collected and 1) analysed by principal components analysis (PCA) to determine the tooth morphospace of arctoid carnivores, 2) analysed by least squares (LS) regression and phylogenetic independent contrasts (PIC) to determine what morphological variables were associated with increases in tooth spacing, and 3) used to reconstruct the evolution of feeding related traits within a phylogenetic context. The PCA analysis revealed that within arctoid carnivores, the greatest differences in morphospace were associated with pierce‐feeding, and the early‐diverging seal Enaliarctos was placed within the pinniped morphospace. Increased tooth spacing within Pinnipedia is a result of decreased postcanine crown size. When the evolution of dental characters is reconstructed, ‘enaliarctines’ were found to represent an intermediate stage in evolution between ‘fissiped’ and pinniped carnivores. They retained the limited tooth spacing of terrestrial carnivores, possessed postcanine crown lengths intermediate in size between pinnipeds and fissipeds, and possessed reduced heterodonty characteristic of crown pinnipeds. Our study indicated that pierce‐feeding evolved early within pinnipeds. This suggested either that pierce‐feeding evolved prior to the loss of mastication, or that pierce‐feeding evolved at the same time as loss of mastication, and well before simplification of the dentition was completed.     

The interplay between natural and sexual selection in the evolution of sexual size dimorphism in Sceloporus lizards (Squamata: Phrynosomatidae)

Sexual size dimorphism (SSD) evolves because body size is usually related to reproductive success through different pathways in females and males. Female body size is strongly correlated with fecundity, while in males, body size is correlated with mating success. In many lizard species, males are larger than females, whereas in others, females are the larger sex, suggesting that selection on fecundity has been stronger than sexual selection on males. As placental development or egg retention requires more space within the abdominal cavity, it has been suggested that females of viviparous lizards have larger abdomens or body size than their oviparous relatives. Thus, it would be expected that females of viviparous species attain larger sizes than their oviparous relatives, generating more biased patterns of SSD. We test these predictions using lizards of the genus Sceloporus. After controlling for phylogenetic effects, our results confirm a strong relationship between female body size and fecundity, suggesting that selection for higher fecundity has had a main role in the evolution of female body size. However, oviparous and viviparous females exhibit similar sizes and allometric relationships. Even though there is a strong effect of body size on female fecundity, once phylogenetic effects are considered, we find that the slope of male on female body size is significantly larger than one, providing evidence of greater evolutionary divergence of male body size. These results suggest that the relative impact of sexual selection acting on males has been stronger than fecundity selection acting on females within Sceloporus lizards.     

Male Infanticide in the Golden Snub-Nosed Monkey (<Emphasis Type="Italic">Rhinopithecus roxellana</Emphasis>), a Seasonally Breeding Primate

In nonhuman primates, infanticide by adult males can occur when the leader male is ousted from a one-male, multifemale group, or when male dominance rank changes within a multimale, multifemale group. According to the sexual selection hypothesis, this behavior may be adaptive if perpetrators increase their reproductive success by killing unrelated, unweaned infants, thus shortening the interbirth interval of the mother, and then siring her next infant. Under an alternative hypothesis, infanticide is a byproduct of aggressive male–male competition and these predictions do not hold. Direct observations of the context surrounding infanticide in free-ranging primate populations that allow a test of these predictions are rare. Here, we document four cases of male infanticide and report paternity data for a group of golden snub-nosed monkeys (Rhinopithecus roxellana) at Shennongjia, China. Three cases of infanticide by new leader males supported the predictions of the sexual selection hypothesis, while another provides partial support for the sexual selection hypothesis, but can also be explained via a nonadaptive hypothesis. In this latter case, a male from an all-male group killed an infant during an aggressive episode that appeared to be accidental, as it took place 7 mo before a male takeover happened, and the perpetrator did not obtain any reproductive advantage. We conclude that most male infanticide events in golden snub-nosed monkeys are consistent with the adaptive selection sexual hypothesis.     

Male Demography,Female Mating Behavior,and Infanticide in Wild Patas Monkeys (Erythrocebus patas)

Infanticide by males has been hypothesized to be a naturally selected behavioral strategy that increases the infanticidal male's reproductive success. The sexual selection hypothesis has been challenged via alternative, nonadaptive hypotheses that dispute its empirical and theoretical bases. Two of the most widely recognized alternatives are the social pathology hypothesis, in which infanticide results from overcrowding or recent human disturbance, and the generalized aggression hypothesis, in which infanticide is an epiphenomenon of increased male aggression. We report the first case of infanticide in wild, seasonally breeding patas monkeys (Erythrocebus patas) living at a low population density in a stable habitat, conditions which do not support the social pathology hypothesis. Its exceptional occurrence is consistent with the sexual selection hypothesis: over a 7-year period the infanticidal male was the only one of 13 resident males that was not present during the actual conception season but was present during the following birth season. Also consistent with this hypothesis, mothers were differentially targeted for male aggression, which increased sevenfold during the days surrounding the infanticide and then decreased to baseline levels after the infanticide. Aggression targeted at mothers does not support the generalized aggression hypothesis. As predicted by the sexual selection hypothesis, females began soliciting mating immediately after the infanticide, despite its occurrence in the nonconceptive season.     

Climbing to reach females: Romeo should be small

The race for reaching mates by the time they are receptive, or sexual selection by scramble competition, has received little attention. We argue that smaller males are favored in species in which the male must climb to reach females located in high habitat patches. This new explanation we term the "gravity hypothesis" of sexual size dimorphism (SSD). We show that a simple biomechanical model of animal movement predicts that: (1) selection should favor a comparatively smaller size in the searching sex when searching involves climbing; and (2) this effect should be stronger in larger species than in smaller species. In reaching high habitats, smaller, faster searchers will be favored either through sexual selection by scramble competition and/or by escaping predation easier by running faster on vertical surfaces. Different spider species are found at a wide range of heights. We compiled a dataset of spider taxa and arranged their habitats according to four height categories, ranked from soil surface to trees. We show that, after controlling for phylogeny, both predictions of the gravity hypothesis of SSD are met. Thus, it appears that the constraint imposed by gravity on climbing males is a selective factor in determining male dwarfism.     

Sexual selection accounts for the geographic reversal of sexual size dimorphism in the dung fly, sepsis punctum (Diptera: Sepsidae)

Sexual size dimorphism (SSD) varies widely across and within species. The differential equilibrium model of SSD explains dimorphism as the evolutionary outcome of consistent differences in natural and sexual selection between the sexes. Here, we comprehensively examine a unique cross-continental reversal in SSD in the dung fly, Sepsis punctum. Using common garden laboratory experiments, we establish that SSD is male-biased in Europe and female-biased in North America. When estimating sexual (pairing success) and fecundity selection (clutch size of female partner) on males under three operational sex ratios (OSRs), we find that the intensity of sexual selection is significantly stronger in European versus North American populations, increasing with male body size and OSR in the former only. Fecundity selection on female body size also increases strongly with egg number and weakly with egg volume, however, equally on both continents. Finally, viability selection on body size in terms of intrinsic (physiological) adult life span in the laboratory is overall nil and does not vary significantly across all seven populations. Although it is impossible to prove causality, our results confirm the differential equilibrium model of SSD in that differences in sexual selection intensity account for the reversal in SSD in European versus North American populations, presumably mediating the ongoing speciation process in S. punctum.     

Lack of Evolution of Sexual Size Dimorphism in Heteromyidae (Rodentia): The Influence of Resource Defense and the Trade-Off between Pre- and Post-Copulatory Trait Investment

One paradoxical finding in some mammals is the presence of male–male intrasexual competition in the absence of sexual size dimorphism. It has been a major goal of evolutionary biologists for over a century to understand why some species in which large males can monopolize multiple mates while excluding smaller competitors, exhibit little or no sexual dimorphism. In this paper I examine three of the main hypotheses that have been proposed to explain this conundrum using as study case the Heteromyidae, a rodent family with subtle sexual size dimorphism. Using a phylogenetic comparative approach, I address the potential influence of (1) fecundity selection, (2) covariation between pre- and post-copulatory traits, and (3) environmental constraints (resource shortage) in explaining patterns of body size and sexual size dimorphism (SSD) across 62 heteromyid species. Baculum size, a proxy of the strength of post-copulatory sexual selection, and SSD were negatively correlated suggesting that heteromyid rodents balance their reproductive investment between pre- and post-copulatory traits, which may prevent the evolution of extensive SSD. Results also support a role for resource competition in moderating SSD. The amount of SSD correlated negatively with latitude. This can be explained if high productivity relaxes the level of intrasexual competition among females, leading to more male-biased dimorphism since forces acting on both sexes are not cancelled. In line with this argument, territorial species exhibited a higher dimorphism in comparison with social species. No support was found for the fecundity selection hypothesis. Overall, this study provides insight into the factors driving observed patterns of sexual dimorphism in this iconic group and highlights the need to consider a broader framework beyond sexual selection for better understanding the evolution of dimorphism in this family.     

Population variation in sexual selection and its effect on size allometry in two dung fly species with contrasting sexual size dimorphism

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.     

Ecological divergence and sexual selection drive sexual size dimorphism in new world pitvipers (Serpentes: Viperidae)

Hypotheses for the origin and maintenance of sexual size dimorphism (SSD) fall into three primary categories: (i) sexual selection on male size, (ii) fecundity selection on female size and (iii) ecological selection for gender‐specific niche divergence. We investigate the impact of these forces on SSD evolution in New World pitvipers (Crotalinae). We constructed a phylogeny from up to eight genes (seven mitochondrial, one nuclear) for 104 species of NW crotalines. We gathered morphological and ecological data for 82 species for comparative analyses. There is a strong signal of sexual selection on male size driving SSD, but less evidence for fecundity selection on female size across lineages. No support was found for allometric scaling of SSD (Rensch's rule), nor for directional selection for increasing male size (the Fairbairn–Preziosi hypothesis) in NW crotalines. Interestingly, arboreal lineages experience higher rates of SSD evolution and a pronounced shift to female‐biased dimorphism. This suggests that fecundity selection on arboreal females exaggerates ecologically mediated dimorphism, whereas sexual selection drives male size in terrestrial lineages. We find that increasing SSD in both directions (male‐ and female‐biased) decreases speciation rates. In NW crotalines, it appears that increasing magnitudes of ecologically mediated SSD reduce rates of speciation, as divergence accumulates within species among sexes, reducing adaptive divergence between populations leading to speciation.     

A comparative test of adaptive hypotheses for sexual size dimorphism in lizards

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.     

Patterns of sexual size dimorphism in stingless bees: Testing Rensch’s rule and potential causes in highly eusocial bees (Hymenoptera: Apidae,Meliponini)

Eusocial insects offer a unique opportunity to analyze the evolution of body size differences between sexes in relation to social environment. The workers, being sterile females, are not subject to selection for reproductive function providing a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other kinds of natural selection. Patterns of sexual size dimorphism (SSD) and testing of Rensch's rule controlling for phylogenetic effects were analyzed in the Meliponini or stingless bees. Theory predicts that queens may exhibit higher selection for fecundity in eusocial taxa, but contrary to this, we found mixed patterns of SSD in Meliponini. Non‐Melipona species generally have a female‐biased SSD, while all analyzed species of Melipona showed a male‐biased SSD, indicating that the direction and magnitude of the selective pressures do not operate in the same way for all members of this taxon. The phylogenetic regressions revealed that the rate of divergence has not differed between the two castes of females and the males, that is, stingless bees do not seem to follow Rensch's rule (a slope >1), adding this highly eusocial taxon to the various solitary insect taxa not conforming with it. Noteworthy, when Melipona was removed from the analysis, the phylogenetic regressions for the thorax width of males on queens had a slope significantly smaller than 1, suggesting that the evolutionary divergence has been larger in queens than males, and could be explained by stronger selection on female fecundity only in non‐Melipona species. Our results in the stingless bees question the classical explanation of female‐biased SSD via fecundity and provide a first evidence of a more complex determination of SSD in highly eusocial species. We suggest that in highly eusocial taxa, additional selection mechanisms, possibly related to individual and colonial interests, could influence the evolution of environmentally determined traits such as body size.