Changes In Sexual Size Dimorphism Research Articles

Ecological drivers of sexual size dimorphism in northernchamois.

Male-biased sexual size dimorphism (SSD) is common in ungulates. The dominant scenario for the evolution of ungulate SSD suggests that habitat openness leads to greater SSD by increasing group size and thus sexual selection through male-male competition for mates. At a more proximate level, adaptive changes in SSD may result from the plastic response of individuals to environmental variation. In this study, we used 161,948 body mass data from a seasonally size-dimorphic species, the northern chamois Rupicapra rupicapra, to examine the role of forest cover and other environmental variables in the expression of SSD. Data were collected from individuals hunted in the Austrian Alps, grouped into 28 mountain ranges with different forest cover, geological substrate and population density. Population-specific growth curves were fitted using monomolecular models, and SSD was calculated as the log-transformed ratio of male to female asymptotic body mass. A path model in which environmental factors indirectly influenced SSD via male or female body mass suggested that SSD increased with increasing density via reduced female body mass and decreased on siliceous substrates via reduced male body mass. Forest cover was negatively associated with body mass in both sexes, but not with variation in SSD.

Effects on running speed of changes in sexual size dimorphism at maturity on in the cursorial huntsman spider, Delena cancerides (Sparassidae).

Running speed is a measure of whole-organism performance reflecting relative fitness. For spiders, increased speed translates into enhanced prey capture, mating success and reduced predation risk. In male spiders, leg length increases dramatically with the molt to sexual maturity. To determine how changes in leg length and body mass with sexual maturity influence running performance, we compared allometric and kinematic changes in a species without extreme size sexual dimorphism (SSD): male and female Delena cancerides (Sparassidae) during their penultimate and adult instars. Spiders in each age-sex class were filmed running in the lab, and body morphometrics, maximum velocity, body lengths per second, acceleration, stride length and stride frequency were compared. At maturity, females increase in overall size, whereas male's leg length increases over 30% with little associated increase in body mass or overall size. Adult male legs are similar in length to those of the adult females and maximum velocity did not differ between age-sex classes. However, both male age-classes have higher velocity scaled as body lengths per second than females, due to their lighter mass. Thus, for sparassids spiders without large SSD, lower mass and longer legs translate into lower energetic costs of running distances for males.

Female-biased gape and body-size dimorphism in the New World watersnakes (tribe: Thamnophiini) oppose predictions from Rensch's rule.

Sexual‐size dimorphism (SSD) is ubiquitous across animals and often biased in the direction of larger females in snakes and other ectothermic organisms. To understand how SSD evolves across species, Rensch's rule predicts that in taxa where males are larger, SSD increases with body size. In contrast, where females are larger, SSD decreases with body size. While this rule holds for many taxa, it may be ambiguous for others, particularly ectothermic vertebrates. Importantly, this rule suggests that the outcomes of SSD over phylogenetic time scales depend on the direction of dimorphism predicated on the difference in reproductive efforts between males and females. Here, we examine SSD in the context of Rensch's rule in Thamnophiini, the gartersnakes and watersnakes, a prominent group that in many areas comprises the majority of the North American snake biota. Using a dated phylogeny, measurements of gape, body, and tail size, we show that these snakes do not follow Rensch's rule, but rather female‐biased SSD increases with body size. We in turn find that this allometry is most pronounced with gape and is correlated with both neonate and litter size, suggesting that acquiring prey of increased size may be directly related to fecundity selection. These changes in SSD are not constrained to any particular clade; we find no evidence of phylogenetic shifts in those traits showing SSD. We suggest several ways forward to better understand the anatomical units of selection for SSD and modularity.Open Research Badges This article has been awarded Open Data and Open Materials Badges. All materials and data are publicly accessible via the Open Science Framework at https://doi.org/10.5061/dryad.3pn57h0.

Variability in vertebral numbers does not contribute to sexual size dimorphism, interspecific variability, or phenotypic plasticity in body size in geckos (Squamata: Gekkota: Paroedura).

Body size is a fundamental trait correlated with nearly every aspect of animal life. It is influenced by numerous genetic and non-genetic factors. Despite its central importance, proximate mechanisms of intra- and interspecific variability in body size are still not well understood even in such a largely studied group as reptiles. For our study, we concentrated on the gecko species Paroedura picta. We investigated whether differences in sexual size dimorphism and in final and asymptotic snout-vent length (induced by a range of incubation and rearing temperatures) are correlated with differences in the number of presacral vertebrae. Moreover, we tested whether changes in this number were associated with evolutionary changes in sexual size dimorphism and body size in the genus Paroedura. We found that the variation in the number of presacral vertebrae is very limited both intra- and interspecifically, ranging between 26 and 28 vertebrae with most individuals possessing the modal number of 27. We conclude that changes in the number of vertebrae do not contribute to developmental plasticity or evolutionary changes in body size nor, in contrast to some other squamate lineages, to sexual size dimorphism.

Seasonal change in sexual size dimorphism of the major cheliped in the hermit crab Pagurus minutus

AbstractSexual size dimorphism is a common phenomenon in the animal kingdom, and its seasonal change has been reported in some species that possess traits dimorphic only in males and specialized for male mating success. However, few studies have examined seasonal change in sexual dimorphism of traits possessed by both sexes. Here, we examined the reproductive biology of the hermit crab Pagurus minutus, at a sandflat in the Waka River estuary, Japan, with special reference to seasonal changes in sexual dimorphism of the large claw (major cheliped) size by conducting population and precopulatory guarding‐pair sampling. Previous investigation demonstrated that the major cheliped is used as a weapon, and its size, more than body size, determines the winner in male–male contests of this species. We found ovigerous females from November to April, peaking in January, when 80% of females were ovigerous. Sexual size dimorphism of the major cheliped was observed; the degree of dimorphism increased in the reproductive season, when only males possessed an enlarged major cheliped. In addition, in the reproductive season, precopulatory guarding males had a larger body and larger relative size of the major cheliped than did solitary males, although the major cheliped size in guarding males seemed to reach an upper limit. These results suggest that seasonal change in sexual dimorphism of the major cheliped size in P. minutus strongly reflects sexual selection favoring the development of this natural weaponry, and that the degree of the dimorphism might be limited through natural selection.

Ontogeny of sexual size dimorphism and environmental quality in Guatemalan children

Human growth data from Guatemalan school children were analyzed to test the hypothesis that the degree of sexual size dimorphism (SSD) in height is reduced for people living under more adverse environmental quality. The sample consists of 2,560 girls and 3,262 boys, 6-16.99 years of age, from the two major Guatemalan ethnic groups, Maya of very low socioeconomic status (SES) and Ladino of high, middle, and low SES. SES was estimated by questionnaire and ethnographic observation. All data are from the Longitudinal Study of Child Development of the Universidad del Valle de Guatemala. Significance of SSD was tested within each whole year age category (e.g., 6.0-6.9 years) by SES for Ladinos and Maya. Ethnic groups were then compared for each age category to determine whether the SSD values were significantly different between groups. Statistically significant height SSD was found for 10 of the 11 age categories for the high SES Ladinos, 8 of the 11 age categories for middle SES Ladinos, 3 of the 11 low SES Ladino age categories, and 0 of the 11 very low SES Maya age categories. For all SES and ethnic groups SSD tended to decrease between 6 and 11 years of age and then increase after age 12 years. A poor quality environment for growth and development, as estimated by SES, was found to reduce or eliminate statistically significant SSD. Patterns of biological maturation before and after puberty also seem to contribute to age changes in SSD.

Allometry of sexual size dimorphism in domestic dog.

BackgroundThe tendency for male-larger sexual size dimorphism (SSD) to scale with body size – a pattern termed Rensch's rule – has been empirically supported in many animal lineages. Nevertheless, its theoretical elucidation is a subject of debate. Here, we exploited the extreme morphological variability of domestic dog (Canis familiaris) to gain insights into evolutionary causes of this rule.Methodology/Principal FindingsWe studied SSD and its allometry among 74 breeds ranging in height from less than 19 cm in Chihuahua to about 84 cm in Irish wolfhound. In total, the dataset included 6,221 individuals. We demonstrate that most dog breeds are male-larger, and SSD in large breeds is comparable to SSD of their wolf ancestor. Among breeds, SSD becomes smaller with decreasing body size. The smallest breeds are nearly monomorphic.Conclusions/SignificanceSSD among dog breeds follows the pattern consistent with Rensch's rule. The variability of body size and corresponding changes in SSD among breeds of a domestic animal shaped by artificial selection can help to better understand processes leading to emergence of Rensch's rule.

Seasonal changes in sexual size dimorphism in northern chamois

In many polygynous mammals, sexual size dimorphism (SSD) is thought to have evolved through sexual selection, because larger males prevail in male–male combat and secure access to estrous females. SSD is often correlated with higher age-specific mortality of males than of females, possibly because males have higher nutritional requirements and riskier growth and reproductive tactics. In adult chamois Rupicapra rupicapra, sexual dimorphism in skeletal size was about 5%, but dimorphism in body mass was highly seasonal. Males were about 40% heavier than females in autumn but only 4% heavier in spring. For a given skeletal size, males were heavier than females only in autumn. Chamois sexual dimorphism appears mainly due to greater summer accumulation of fat and muscle mass by males than by females. Male mass declines rapidly during the rut. Limited dimorphism in skeletal size combined with substantial but seasonal dimorphism in mass has not been reported in other sexually dimorphic ungulates. Seasonal changes in mass allow males to achieve large size for the rut by accumulating body resources during summer. The use of these resources over the rut may reduce mortality associated with sustaining a large size over the winter.

Sexual size dimorphism decreases with temperature in a blowfly, Chrysomya megacephala (Fabricius) (Diptera: Calliphoridae)

1. There is wide intra-specific variation in sexual size dimorphism (SSD). Much of this variation is probably as a result of sexual differences in the selective pressure on body size. However, environmental variables could affect males and females differently, causing variation in SSD. 2. We examined the effects of two temperatures (20 and 30 °C) on SSD in six populations of the blowfly, Chrysomya megacephala. 3. We found that body size increased with temperature in all the populations studied, and the sexes differed in phenotypic plasticity of body size in response to rearing temperature. This created substantial temperature-induced variation in SSD (i.e. sex × temperature interaction). Males were often smaller than females, but the degree of dimorphism was smaller at the higher temperature (30 °C) and larger at the lower temperature (20 °C). This change in SSD was not because of a gender difference in the effect of temperature on development time. Further studies should address whether this variation can be produced by adaptive canalisation of one sex against variation in temperature, or whether it may be a consequence of non-adaptive developmental differences between the sexes. 4. Although most studies assume that the magnitude of SSD is fixed within a species, the present study demonstrates that rearing temperature can generate considerable intra-specific variation in the degree of SSD.

Seasonal changes in body size, sexual size dimorphism and sex ratio in relation to mating system in an adult odonate community

Seasonal environments impose developmental time constraints on insects which can be reflected in body size and sex ratio. By tracking these two aspects in recently emerged adults of 10 species of an odonate community in a number of lakes, we investigated whether (a) body size in both sexes decreased as the flight season progressed and whether this led to seasonal changes in sexual size dimorphism (SSD); (b) SSD patterns were related to mating systems; (c) biases in sex ratio could be explained by mortality rates associated with the largest sex (e.g. in species with male-biased SSD, a female-biased sex ratio; in species with female-biased SSD, a male-biased sex ratio). Our results indicated that adults in most species, but not all, tend to reach a smaller body size as the season progressed. However, the opposite pattern was found in a few species. Predictions about the relation between SSD and mating systems were confirmed: a female-biased SSD in nonterritorial species and monomorphism for territorial species. However, predictions of biases in sex ratio according to SSD were not met in all species. Interestingly, changes in body size and SSD along the season were lake-specific in two species in which these patterns could be examined. These results, although partially supportive of environmental and sexual selection patterns acting on size and sex ratio as documented in other odonate species, indicate that we are still far from understanding seasonal constrains in these animals.

The evolution of sexual size dimorphism in cottontail rabbits (Sylvilagus, Leporidae)

In mammals, ‘female-biased’ sexual size dimorphism (SSD), in which females are larger than males, is uncommon. In the present study, we examined Sylvilagus, a purported case of female-biased SSD, for evolutionary correlations among species between SSD, body-size, and life-history variables. We find that: (1) although most species are female-biased, the degree and direction of SSD vary more than was previously recognized and (2) the degree of SSD decreases with increasing body size. Hence, Sylvilagus provides a new example, unusual for a female-biased taxon, in which allometry for SSD is consistent with ‘Rensch's Rule’. As a corollary to Rensch's Rule, we observe that changes in SSD in Sylvilagus are typically associated with larger, more significant changes in males than females. Female-biased SSD could be produced by selection for larger females, smaller males, or both. Although larger female size may be related to high fecundity and the extremely rapid fetal and neonatal growth in Sylvilagus, we find little evidence for a correlation between SSD and various fecundity-related traits in among-species comparisons. Smaller male size may confer greater reproductive success through greater mobility and reduced energetic requirements. We propose that a suite of traits (female dispersion, large male home ranges, reduced aggression, and a promiscuous mating system) has favoured smaller males and thus influenced the evolution of SSD in cottontails.

Can sexual selection drive female life histories? A comparative study on Galliform birds

Sexual selection has been identified as a major evolutionary force shaping male life history traits but its impact on female life history evolution is less clear. Here we examine the impact of sexual selection on three key female traits (body size, egg size and clutch size) in Galliform birds. Using comparative independent contrast analyses and directional discrete analyses, based on published data and a new genera-level supertree phylogeny of Galliform birds, we investigated how sexual selection [quantified as sexual size dimorphism (SSD) and social mating system (MS)] affects these three important female traits. We found that female body mass was strongly and positively correlated with egg size but not with clutch size, and that clutch size decreased as egg size increased. We established that SSD was related to MS, and then used SSD as a proxy of the strength of sexual selection. We found both a positive relationship between SSD and female body mass and egg size and that increases in female body mass and egg size tend to occur following increases in SSD in this bird order. This pattern of female body mass increases lagging behind changes in SSD, established using our directional discrete analysis, suggests that female body mass increases as a response to increases in the level of sexual selection and not simply through a strong genetic relationship with male body mass. This suggests that sexual selection is linked to changes in female life history traits in Galliformes and we discuss how this link may shape patterns of life history variation among species.

Maternal Inheritance and Rapid Evolution of Sexual Size Dimorphism: Passive Effects or Active Strategies?

Adaptive evolution is often strongly influenced by maternal inheritance that transfers the parental strategies across generations. The consequences of maternal effects for the offspring generation depend on the between-generation similarity in environments and on the evolved sensitivity of the offspring's ontogeny to maternal effects. When these factors differ between sons and daughters, maternal effects can influence the evolution of sexual dimorphism. The establishment of house finch populations across western Montana during the last 30 years was accompanied by rapid evolutionary change in sexual size dimorphism. Here I show that traits that changed the most across generations were most influenced by maternal effects in males but not females. Maternal effects differentially affected sons' and daughters' survival; greater maternal effects were commonly associated with higher survival of sons, especially when maternal and offspring environments were similar. Stronger maternal effects extended preselection phenotypic variance in morphological traits of males, thereby producing some locally adaptive phenotypes and lessening juvenile mortality. Thus, the observed sex-specific maternal effects and their contribution to the evolution of sexual size dimorphism are likely a passive consequence of the distinct sensitivity of sons and daughters to maternal adaptations to breeding in ecologically distinct parts of the house finch's expanding range.

Maternal Inheritance and Rapid Evolution of Sexual Size Dimorphism: Passive Effects or Active Strategies?

Adaptive evolution is often strongly influenced by maternal inheritance that transfers the parental strategies across generations. The consequences of maternal effects for the offspring generation depend on the between‐generation similarity in environments and on the evolved sensitivity of the offspring’s ontogeny to maternal effects. When these factors differ between sons and daughters, maternal effects can influence the evolution of sexual dimorphism. The establishment of house finch populations across western Montana during the last 30 years was accompanied by rapid evolutionary change in sexual size dimorphism. Here I show that traits that changed the most across generations were most influenced by maternal effects in males but not females. Maternal effects differentially affected sons' and daughters' survival; greater maternal effects were commonly associated with higher survival of sons, especially when maternal and offspring environments were similar. Stronger maternal effects extended preselection phenotypic variance in morphological traits of males, thereby producing some locally adaptive phenotypes and lessening juvenile mortality. Thus, the observed sex‐specific maternal effects and their contribution to the evolution of sexual size dimorphism are likely a passive consequence of the distinct sensitivity of sons and daughters to maternal adaptations to breeding in ecologically distinct parts of the house finch’s expanding range.

Directional changes in sexual size dimorphism in shorebirds, gulls and alcids.

The Charadrii (shorebirds, gulls and alcids) are one of the most diverse avian groups from the point of view of sexual size dimorphism, exhibiting extremes in both male-biased and female-biased dimorphism, as well as monomorphism. In this study we use phylogenetic comparative analyses to investigate how size dimorphism has changed over evolutionary time, distinguishing between changes that have occurred in females and in males. Independent contrasts analyses show that both body mass and wing length have been more variable in males than in females. Directional analyses show that male-biased dimorphism has increased after inferred transitions towards more polygynous mating systems. There have been analogous increases in female-biased dimorphism after transitions towards more socially polyandrous mating systems. Changes in dimorphism in both directions are attributable to male body size changing more than female body size. We suggest that this might be because females are under stronger natural selection constraints related to fecundity. Taken together, our results suggest that the observed variation in dimorphism of Charadrii can be best explained by male body size responding more sensitively to variable sexual selection than female body size.

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.

THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN THE HOUSE FINCH. V. MATERNAL EFFECTS

The phenotype of a mother and the environment that she provides might differentially affect the phenotypes of her sons and daughters, leading to change in sexual size dimorphism. Whereas these maternal effects should evolve to accommodate the adaptations of both the maternal and offspring generations, the mechanisms by which this is accomplished are rarely known. In birds, females adjust the onset of incubation (coincident with the first egg or after all eggs are laid) in response to the environment during breeding, and thus, indirectly, determine the duration of offspring growth. In the two house finch (Carpodacus mexicanus) populations that breed at the extremes of the species' distribution (Montana and Alabama), females experience highly distinct climatic conditions during nesting. We show that in close association with these conditions, females adjusted jointly the onset of incubation and the sequence in which they produced male and female eggs and consequently modified the growth of sons and daughters. The onset of incubation in newly breeding females closely tracked ambient temperature in a pattern consistent with the maintenance of egg viability. Because of the very different climates in Montana and Alabama, females in these populations showed the opposite patterns of seasonal change in incubation onset and the opposite sex bias in egg-laying order. In females with breeding experience, incubation onset and sex bias in laying order were closely linked regardless of the climatic variation. In nests in which incubation began with the onset of egg laying, the first-laid eggs were mostly females in Montana, but mostly males in Alabama. Because in both populations, male, but not female, embryos grew faster when exposed to longer incubation, the sex-bias produced highly divergent sizes of male and female juveniles between the populations. Overall, the compensatory interaction between the onset of incubation and the sex-biased laying order achieved a compromise between maternal and offspring adaptations and contributed to rapid morphological divergence in sexual dimorphism between populations of the house finch breeding at the climatic extremes of the species range.

Population consequences of maternal effects: sex-bias in egg-laying order facilitates divergence in sexual dimorphism between bird populations

Abstract When costs and benefits of raising sons and daughters differ between environments, parents may be selected to modify their investment into male and female offspring. In two recently colonized environments, breeding female house finches (Carpodacus mexicanus) modified the sex and growth of their offspring in relation to the order in which eggs were laid in a clutch. Here we show that, in both populations, these maternal effects strongly biased frequency distribution of tarsus size of fully grown males and females and ultimately produced population divergence in this trait. Although in each population, male and female offspring show a wide range of growth patterns, maternal modifications of sex-ratio in relation to egg-laying order resulted in under-representation of the morphologies that were selected against and over-representation of morphologies that were favoured by the local selection on juveniles. The result of these maternal adjustments was fast phenotypic change in sexual size dimorphism within and between populations. Maternal manipulations of offspring morphologies may be especially important at the initial stages of population establishment in the novel environments and may have facilitated recent colonization of much of North America by the house finch.

Predicting the evolution of sexual size dimorphism

Abstract Lande’s (1980b, Evolution34: 292–305) equations for predicting the evolution of sexual size dimorphism (SSD) through frequency-dependent sexual selection, and frequency-independent natural selection, were tested against results obtained from a stochastic genetic simulation model. The SSD evolved faster than predicted, due to temporary increases in the genetic variance brought about by directional selection. Predictions for the magnitude of SSD at equilibrium were very accurate for weak sexual selection. With stronger sexual selection the total response was greater than predicted. Large changes in SSD can occur without significant long-term change in the genetic correlation between the sexes. Our results suggest that genetic correlations constrain both the short-term and long-term evolution of SSD less than predicted by the Lande model.

Human fertility variation, size-related obstetrical performance and the evolution of sexual stature dimorphism.

In several animal species, change in sexual size dimorphism is a correlated response to selection on fecundity. In humans, different hypotheses have been proposed to explain the variation of sexual dimorphism in stature, but no consensus has yet emerged. In this paper, we evaluate from a theoretical and an empirical point of view the hypothesis that the extent of sexual dimorphism in human populations results from the interaction between fertility and size-related obstetric complications. We first developed an optimal evolutionary model based on extensive simulations and then we performed a comparative analysis for a total set of 38 countries worldwide. Our optimization modelling shows that size-related mortality factors do indeed have the potential to affect the extent of sexual stature dimorphism. Comparative analysis using generalized linear modelling supports the idea that maternal death caused by deliveries and complications of pregnancy (a variable known to be size related) could be a key determinant explaining variation in sexual stature dimorphism across populations. We discuss our results in relation to other hypotheses on the evolution of sexual stature dimorphism in humans.