Intraspecific Variation In Sexual Size Dimorphism Research Articles

Sexual size dimorphism and male reproductive traits vary across populations of a tropical rainforest dung beetle species (Onthophagus babirussa).

Sexual size dimorphism (SSD) arises when natural selection and sexual selection act differently on males and females. Male‐biased SSD is rarer in insects and usually indicates strong sexual selection pressure on male body size in a species. Patterns of SSD can also vary between populations of species that are exposed to different environmental conditions, such as differing resource availability and diversity. Here, we investigate intraspecific variation in SSD as well as relative investment in precopulatory (horn length) and postcopulatory traits (sperm length and testes weight) in a tropical rainforest dung beetle Onthophagus babirussa across Singapore and Peninsular Malaysia. Overall, three out of four populations displayed significant male‐biased SSD, and SSD was greater in populations with smaller overall body size. Average male body size was similar across all populations while female body size was significantly smaller in Singapore, suggesting that the pronounced SSD may also be due to stronger sexual selection on male body size in Singapore populations. All populations showed significant investment in horns as a weapon likely used in male‐male competition, while postcopulatory traits showed no clear scaling relationship with body size, suggesting a higher priority on precopulatory sexual traits in the mating system of this species.

Environmental drivers of sexual dimorphism in a lizard with alternative mating strategies.

Understanding the relative importance of sexual and natural selection in shaping morphological traits is a long-standing goal of evolutionary ecology. Male-biased sexual size dimorphism (SSD) is typically associated with male-male competition. Similarly, male polymorphisms are considered a consequence of competitive social interactions. This classic paradigm overlooks the fact that environmental factors mediate social interactions and can lead to ecological adaptations. Common side-blotched lizards, Uta stansburiana, are a model system for this paradigm due to well-known rock-paper-scissors social dynamics between male morphs. SSD in this species has been considered primarily a consequence of social interactions, with male size resulting from the number of morphs in each population and female size being constrained through fecundity benefits. We test if the environment explains intraspecific variation in SSD and number of male morphs in U.stansburiana. By compiling data from 49 populations, we show that environmental variables are stronger predictors of SSD than the number of male morphs. Similarly, we show that the environment mediates SSD and potentially contributes to morph loss in colder environments. We propose that the environment favours smaller males in areas of high seasonality. Our results demonstrate the importance of the environment as a mediator of SSD.

A uni- and multivariate analysis approach to reveal sexual size dimorphism in Iranian populations of Hypera postica (Coleoptera: Curculionidae)

Abstract Body size dimorphism between genders is a commonly observed phenomenon in insects, usually manifested in larger female body size. Sexual Size Dimorphism (SSD) varies from species to species, the degree and direction influenced by certain evolutionary pressures. Intraspecific variation in SSD may also occur between populations. The Hypera postica (Gyllenhal, 1813) is a well-known alfalfa plant pest that shows a degree of morphological divergence in its populations. The female alfalfa weevils are very fecund and have a larger body size compared to males. To improve our knowledge on magnitude and direction of SSD in alfalfa weevil, we studied 200 specimens of H. postica from four Iranian populations (Karaj1, Karaj2, Tuyserkan and Jovein). 10 morphological variables from three external anatomic parts (pronotum, elytra and rostrum) and 45 ratio characters were statistically analyzed in order to determine the amount of SSD in Iranian populations. In addition we investigated for morphological divergence pattern in mentioned populations. The results of this study show that a low degree of morphological divergence occurs in Iranian populations. Measured variables indicate that the SSD pattern of H. postica is compatible with the Rensch’s rule, and is related to high fecundity of females and a lack of strong sexual selection. Also it is mentioned that the larger rostrum in females may correspond to its unique role in egg laying.

A test of Rensch's rule in greater horseshoe bat (Rhinolophus ferrumequinum) with female-biased sexual size dimorphism.

Sexual size dimorphism (SSD) is widespread within the animal kingdom. Rensch’s rule describes a relationship between SSD and body size: SSD increases with body size when males are the larger sex, and decreases with body size when females are the larger sex. Rensch’s rule is well supported for taxa that exhibit male-biased SSD but patterns of allometry among taxa with female-biased size dimorphism are mixed, there is evidence both for and against the rule. Furthermore, most studies have investigated Rensch’s rule across a variety of taxa; but among-population studies supporting Rensch’s rule are lacking, especially in taxa that display only slight SSD. Here, we tested whether patterns of intraspecific variation in SSD in greater horseshoe bats conform to Rensch’s rule, and evaluated the contribution of latitude to Rensch’s rule. Our results showed SSD was consistently female-biased in greater horseshoe bats, although female body size was only slightly larger than male body size. The slope of major axis regression of log10 (male) on log10 (female) was significantly different from 1. Forearm length for both sexes of greater horseshoe bats was significantly negatively correlated with latitude, and males displayed a slightly but nonsignificant steeper latitudinal cline in body size than females. We suggest that variation in patterns of SSD among greater horseshoe bat populations is consistent with Rensch’s rule indicating that males were the more variable sex. Males did not have a steeper body size–latitude relationship than females suggesting that sex-specific latitudinal variation in body size may not be an important contributing factor to Rensch’s rule. Future research on greater horseshoe bats might best focus on more comprehensive mechanisms driving the pattern of female-biased SSD variation.

Rensch’s rule inverted – female‐driven gigantism in nine‐spined stickleback Pungitius pungitius

1. Allometric scaling of sexual size dimorphism (SSD) with body size is a commonplace occurrence in intraspecific or interspecific comparisons. Typically, SSD increases with body size when males, and decreases when females are the larger sex--a pattern known as Rensch's rule. Intraspecific studies of Rensch's rule in vertebrates are extremely scarce. 2. In an allometric SSD-body size relationship, the sex with the larger body size variation is the driver of size divergence whereas the other sex is following it owing to correlational selection. Hence, one can test which sex is responsible for the observed body size divergence within this framework. 3. Nine-spined stickleback (Pungitius pungitius) provides an excellent model to study intraspecific variation in SSD owing to the large interpopulation variation in mean body size. Using data on body size variation in 11 nine-spined stickleback populations covering the full known size range of the species, we investigated: (i) whether variation in SSD scales allometrically with mean body size across the populations; (ii) which sex is driving the allometric relationship and (iii) whether the observed pattern is likely to have a genetic component. In addition, we analysed the size dependency of female reproductive output. 4. We found strong support for an inverse of Rensch's rule: level of female-biased SSD increased with increasing mean size while females were the more variable sex. Results from a common garden experiment supported the pattern found in the wild. Females from giant populations had 2-3 times larger reproductive output than normal-sized females. 5. The fact that females were the more variable sex indicates that the evolution of gigantism in nine-spined sticklebacks is driven by females, and the 2-3 times larger reproductive output per clutch of giant vs. normal-sized females suggests fecundity selection to have an important role in it. Our results oppose the commonly held view that males drive the evolution of SSD as a result of sexual selection favouring larger males.

SEX-SPECIFIC SELECTION AND INTRASPECIFIC VARIATION IN SEXUAL SIZE DIMORPHISM

Sexual size dimorphism (SSD) is thought to evolve due to sex differences in selection on body size, but it is largely unknown whether intraspecific variation in SSD reflects differences in sex-specific selection among populations. We addressed this question by comparing viability selection between two island populations of the brown anole lizard (Anolis sagrei) that differ in the magnitude of male-biased SSD. On both islands, females experienced stabilizing selection favoring intermediate size whereas males experienced directional selection favoring larger size. Thus, sex-specific selection matched the overall pattern of male-biased SSD, but population differences in the magnitude of SSD were not associated with local differences in selection. Rather, population differences in SSD appear to result from underlying differences in the environmental potential for a rapid growth, coupled with sex-specific phenotypic plasticity. Males grew more slowly on the island with low SSD whereas growth of females did not differ between islands. Both sexes had substantially lower mass per unit length on the island with low SSD, suggesting that they were in a relatively poorer energetic condition. We propose that this energetic constraint disproportionately impacts growth of males due to their greater absolute energy requirements, thus driving intraspecific variation in SSD.

Male body size varies with latitude in a temperate lizard

To examine the evolution of sexual size dimorphism (SSD), interspecific studies are often performed to generate hypotheses for the origin and maintenance of SSD. Although these methods are invaluable to our understanding of the evolution of SSD, they often quantify SSD for a species based on few populations. We found a significant sex-specific latitudinal cline in Plestiodon fasciatus (L., 1758) (= Eumeces fasciatus (L., 1758)), a species that was previously considered to be monomorphic for body size. Male body size significantly increased with increasing latitude, whereas female body size was relatively constant. Our findings argue for the importance of increased understanding and appreciation of intraspecific variation in SSD. We suggest that a more integrated approach to SSD be employed, where both intraspecific and interspecific variation is considered. We provide a foundation for posing hypotheses of the causes and consequences of SSD in P. fasciatus and perhaps other members of the species group.

Sexual size dimorphism in a Drosophila clade, the D. obscura group

The Drosophila obscura clade consists of about 41 species, of which 20 were used for analyses of wing and thorax length. Our primary goal was to investigate the magnitude of sexual size dimorphism (SSD) of these traits within this clade and to test Rensch's Rule [when females are larger than males, SSD (e.g., female/male ratio) should decrease with body size]. Our secondary goal was methodological and involved evaluating for these flies alternative measures of SSD (female/male ratio, female/male absolute difference, female/male relative difference), developing a bootstrap method to estimate the magnitude of intraspecific variation in SSD, and applying a new method of estimating allometric relationships that is phylogenetically based and incorporates error variance in both traits. All indices of SSD were strongly correlated for both size traits. Nevertheless, female/male ratio is the best index here: it is easily interpretable and essentially independent of size. For both traits, SSD (F/M) varied interspecifically, showed a strong phylogenetic signal, but did not differ for the main phylogenetic subgroups or correlate with latitude. Factors underlying variation in SSD in this clade are elusive and might include genetic drift. SSD (wing) tended to decrease with increasing size, as predicted by Rensch's Rule, though not consistently so. SSD (thorax) was unrelated to size. However, analysis of published data for thorax length of Drosophila spp. ( N=42) with a larger size range showed that SSD decreased significantly with increasing size (consistent with Rensch's Rule), suggesting our ability to detect SSD-size relations in the D. obscura data may be limited by low statistical power.

Allometry for Sexual Size Dimorphism: Testing Two Hypotheses for Rensch’s Rule in the Water StriderAquarius remigis

Within any given clade, male size and female size typically covary, but male size often varies more than female size. This generates a pattern of allometry for sexual size dimorphism (SSD) known as Rensch's rule. I use allometry for SSD among populations of the water strider Aquarius remigis (Hemiptera, Gerridae) to test the hypothesis that Rensch's rule evolves in response to sexual selection on male secondary sexual traits and an alternative hypothesis that it is caused by greater phenotypic plasticity of body size in males. Comparisons of three populations reared under two temperature regimes are combined with an analysis of allometry for genital and somatic components of body size among 25 field populations. Contrary to the sexual-selection hypothesis, genital length, the target of sexual selection, shows the lowest allometric slope of all the assayed traits. Instead, the results support a novel interpretation of the differential-plasticity hypothesis: that the traits most closely associated with reproductive fitness (abdomen length in females and genital length in males) are "adaptively canalized." While this hypothesis is unlikely to explain Rensch's rule among species or higher clades, it may explain widespread patterns of intraspecific variation in SSD recently documented for many insect species.

Sexual size dimorphism within species increases with body size in insects

Studies examining interspecific differences in sexual size dimorphism (SSD) typically assume that the degree of sexual differences in body size is invariable within species. This work was conducted to assess validity of this assumption. As a result of a systematic literature survey, datasets for 158 insect species were retrieved. Each dataset contained adult or pupal weights of males and females for two or more different subsets, typically originating from different conditions during immature development. For each species, an analysis was conducted to examine dependence of SSD on body size, the latter variable being used as a proxy of environmental quality. A considerable variation in SSD was revealed at the intraspecific level in insects. The results suggest that environmental conditions may strongly affect the degree, though not the direction of SSD within species. In most species, female size appeared to be more sensitive to environmental conditions than male size: with conditions improving, there was a larger relative increase in female than male size. As a consequence, sexual differences in size were shown to increase with increasing body size in species with female‐biased SSD (females were the larger sex in more than 80% of the species examined). The results were consistent across different insect orders and ecological subdivisions. Mechanisms leading to intraspecific variation in SSD are discussed. This study underlines the need to consider intraspecific variation in SSD in comparative studies.

Growth and Sexual Size Dimorphism in Northern Water Snakes (Nerodia sipedon)

Understanding the evolutionary basis for sexual size dimorphism (SSD) in a species first requires determining the extent to which observed SSD is a result of sex differences in growth or in size at maturity, as opposed to being a consequence of factors such as differential survival or catchability of the sexes. We used data from a nine-year study to assess how these factors contribute to female-biased SSD in Nerodia sipedon in eastern Ontario. Females grew substantially faster and larger than males, although both sexes approached their asymptotic size at the same rate. Smaller individuals of both sexes grew faster, and snakes grew faster in years with higher mean air temperatures during the activity season. Variation in growth rate was unrelated to relative fat content of individuals in the spring. We found no evidence that either the sex or size of individuals affected their probability of capture. However, comparisons of estimated ages of males and females, and a null growth-based analysis, both indicated that large, old males were overrepresented in capture samples, probably as a result of differential survival of males and females. The overrepresentation of large males caused us to slightly underestimate SSD caused by sex differences in growth. A comparison of sex-specific growth patterns in our population of N. sipedon with those in two other N. sipedon populations suggests that intraspecific variation in SSD in this species may be primarily a consequence of differences between populations in selection pressures on males.