Direction Of Sexual Size Dimorphism Research Articles

A Phylogenetic Analysis of Sexual Size Dimorphism in Turtles

Sexual size dimorphism (SSD) is widespread in a variety of vertebrate taxa, and its presence has been associated with a range of evolutionary and ecological factors. Turtles, as a lineage, have been shown to include species that exhibit both forms of SSD—male-biased (male larger) and female-biased (female larger)—as well as species that do not exhibit SSD (male size ≅ female size). Previous work has suggested that factors affecting the direction and magnitude of SSD in turtles may include habitat preference and fecundity; these correlations, however, have not been tested in a phylogenetic context. To provide a framework for evaluating potential correlates with the magnitude and direction of SSD in turtles, we examined the phylogenetic distribution of SSD in this clade. Published sources were surveyed for data on the direction and magnitude of SSD, carapace length, clutch size, egg size, and preferred habitat (terrestrial vs. nonterrestrial). Phylogenies were compiled from multiple published sources to develop a phylogeny for all taxa (n = 82) for which SSD data were assembled. The phylogenetic distribution of SSD was then evaluated using parsimony-based methods. These analyses indicated that the ancestral condition for extant turtles was female-biased SSD. Phylogenetic analyses conducted on subsets of the data (concentrated-changes tests, independent contrasts) indicated that changes in SSD state were not correlated with changes in habitat preference. Both male and female carapace length were found to be correlated with traits related to fecundity (egg size, clutch size), but the degree of dimorphism was not found to be correlated with these traits. A significant negative correlation between egg size and clutch size was also evident in turtles, as predicted by life-history theory.

Sexual size dimorphism in the evolutionary context of facultative paedomorphosis: insights from European newts

BackgroundSexual size dimorphism (SSD) is a key evolutionary feature that has been studied in many organisms. In a wide range of species, this pattern is more complex because of polymorphism within each sex. However, it is not known whether the magnitude and direction of SSD could be affected by alternative developmental trajectories within sexes. Our aim was to test whether an intrasexual polymorphism, facultative paedomorphosis (a process in which the development of somatic and gonadal tissues differs in alternative morphs), could affect SSD variation patterns in European newts.ResultsWe report here the first evidence that SSD varies depending on the paedomorphic or metamorphic ontogenetic pathway. In species with a consistent female-biased SSD, paedomorphosis decreased the SSD level, but did not affect its direction. In species with moderate female-biased SSD or variable SSD patterns, paedomorphosis changed the magnitude, or both the magnitude and the direction, of SSD.ConclusionOur study highlights the importance of developmental processes for shaping SSD patterns in populations in which contrasting life-history pathways evolved. European newts express different SSD patterns depending on their developmental pathway (i.e., metamorphosis versus paedomorphosis), as well as their species and population. These findings emphasize the importance of studying alternative morphotypes, which are found in a wide range of animal groups, to understand the evolution of SSD.

Size-dependent mating strategies and the risk of cannibalism

The role of sexual selection in determining the nature and direction of sexual size dimorphism may depend upon the timing of sexual selection, and this may also influence the variation in male size. For example, selection through sperm competition favours smaller males in the highly sexually size dimorphic orb-weaving spider Nephila edulis, whereas larger males are better able to exclude their smaller rivals from the central hub of the web where mating takes place. We investigate experimentally the role of body size and hub tenure in determining male fertilization success when males of different sizes compete for a single female over a 24-h period that includes a period of darkness. Our results confirm that small and large males obtain similar paternity share but that, in contrast with previous studies, hub tenure does not translate into greater paternity share. Unexpectedly, smaller males are at greater risk of postmating sexual cannibalism than larger males, suggesting that natural selection through sexual cannibalism may place a lower limit on male size.

The Evolution of Condition‐Dependent Sexual Dimorphism

Theory suggests that the net benefit of allocating resources to a sexual trait depends both on the strength of sexual selection on that trait and on individual condition. This predicts a tight coevolution between sexual dimorphism and condition dependence and suggests that these patterns of within-sex and between-sex variation may share a common genetic and developmental basis. Although condition-dependent expression of sexual traits is widely documented, the extent of covariation between condition dependence and sexual dimorphism remains poorly known. I investigated the effects of condition (larval diet quality) on multivariate sexual dimorphism in the fly Telostylinus angusticollis (Neriidae). Condition determined the direction of sexual size dimorphism and modulated sexual shape dimorphism by affecting allometric slopes and/or intercepts of sexually homologous traits in both sexes. Although the greatest responses to condition manipulation were observed in male sexual traits, both sexual and nonsexual traits exhibited substantial variation in the nature and magnitude of condition effects. Nonetheless, condition dependence and sexual dimorphism were remarkably congruent: variation in the strength of condition effects on male traits explained more than 90% of the variation in the magnitude of sexual dimorphism, whether quantified in terms of trait size or allometric slope. The genetic mechanisms that give rise to multivariate sexual dimorphism in body shape thus function in a strongly condition-dependent manner in this species, suggesting a common genetic basis for body shape variation within and between sexes.

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.

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.

Sex‐specific selective pressures on body mass in the greater white‐toothed shrew, Crocidura russula

The direction, intensity and shape of viability-, sexual- and fecundity selection on body mass were investigated in a natural population of the greater white-toothed shrew (Crocidura russula), combining parentage assignment through molecular techniques and mark-recapture data over several generations. A highly significant stabilizing viability selection was found in both sexes, presumably stemming from the constraints imposed by their insectivorous habits and high metabolic costs. Sexual selection, directional in both sexes, was twice as large in males than in females. Our results suggest that body mass matters in this context by facilitating the acquisition and defense of a breeding territory. No fecundity selection could be detected. The direction of sexual size dimorphism (SSD) was in agreement with the observed pattern of selective pressures: males were heavier than females, because of stronger sexual selection. SSD intensity, however, was low compared with other mammals, because of the low level of polygyny, the active role of females in territory defense and the intensity of stabilizing viability selection.

Clinal variation in body size and sexual dimorphism in an Indian fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

Geographic variation in body size and sexual dimorphism of the short-nosed fruit bat (Cynopterus sphinx) was investigated in peninsular India. Bats were sampled at 12 localities along a 1200 km latitudinal transect that paralleled the eastern flanks of the Western Ghats. The geographic pattern of variation in external morphology of C. sphinx conforms to the predictions of Bergmann's Rule, as indicated by a steep, monotonic cline of increasing body size from south to north. This study represents one of the first conclusively documented examples of Bergmann's Rule in a tropical mammal and confirms that latitudinal clines in body size are not exclusively restricted to temperate zone homeotherms. Body size was indexed by a multivariate axis derived from principal components analysis of linear measurements that summarize body and wing dimensions. Additionally, length of forearm was used as a univariate index of structural size to examine geographic variation in a more inclusive sample of bats across the latitudinal transect. Multivariate and univariate size metrics were strongly and positively correlated with body mass, and exhibited highly concordant patterns of clinal variation. Stepwise multiple regression on climatological variables revealed that increasing size of male and female C. sphinx was associated with decreasing minimum temperature, increasing relative humidity, and increasing seasonality. Although patterns of geographic size variation were highly concordant between the sexes, C. sphinx also exhibited a latitudinal cline in the magnitude and direction of sexual size dimorphism. The size differential reversed direction across the latitudinal gradient, as males averaged larger in the north, and females averaged larger in the south. The degree of female-biased size dimorphism across the transect was negatively correlated with body size of both sexes. Canonical discriminant analysis revealed that male- and female-biased size dimorphism were based on contrasting sets of external characters. Available data on geographic variation in the degree of polygyny in C. sphinx suggests that sexual selection on male size may play a role in determining the geographic pattern of sexual size dimorphism.

Clinal variation in body size and sexual dimorphism in an Indian fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

Geographic variation in body size and sexual dimorphism of the short-nosed fruit bat (Cynopterus sphinx) was investigated in peninsular India. Bats were sampled at 12 localities along a 1200 km latitudinal transect that paralleled the eastern flanks of the Western Ghats. The geographic pattern of variation in external morphology of C. sphinx conforms to the predictions of Bergmann's Rule, as indicated by a steep, monotonic cline of increasing body size from south to north. This study represents one of the first conclusively documented examples of Bergmann's Rule in a tropical mammal and confirms that latitudinal clines in body size are not exclusively restricted to temperate zone homeotherms. Body size was indexed by a multivariate axis derived from principal components analysis of linear measurements that summarize body and wing dimensions. Additionally, length of forearm was used as a univariate index of structural size to examine geographic variation in a more inclusive sample of bats across the latitudinal transect. Multivariate and univariate size metrics were strongly and positively correlated with body mass, and exhibited highly concordant patterns of clinal variation. Stepwise multiple regression on climatological variables revealed that increasing size of male and female C. sphinx was associated with decreasing minimum temperature, increasing relative humidity, and increasing seasonality. Although patterns of geographic size variation were highly concordant between the sexes, C. sphinx also exhibited a latitudinal cline in the magnitude and direction of sexual size dimorphism. The size differential reversed direction across the latitudinal gradient, as males averaged larger in the north, and females averaged larger in the south. The degree of female-biased size dimorphism across the transect was negatively correlated with body size of both sexes. Canonical discriminant analysis revealed that male- and female-biased size dimorphism were based on contrasting sets of external characters. Available data on geographic variation in the degree of polygyny in C. sphinx suggests that sexual selection on male size may play a role in determining the geographic pattern of sexual size dimorphism.

Sperm competition and small size advantage for males of the golden orb-web spider Nephila edulis

Abstract Sexual selection, through female choice and/or male–male competition, has influenced the nature and direction of sexual size dimorphism in numerous species. However, few studies have examined the influence of sperm competition on size dimorphism. The orb-web spider Nephila edulis has a polygamous mating system and extreme size dimorphism. Additionally, the frequency distribution of male body size is extremely skewed with most males being small and few large. The duration of copulation, male size and sexual cannibalism have been identified as the significant factors determining patterns of sperm precedence in spiders. In double mating trials, females were assigned to three treatments: either they mated once with both males or the first or the second male was allowed to mate twice. Paternity was strongly associated with the duration of copulation, independent of mating order. Males that were allowed to mate twice not only doubled the duration of copulation but also their paternity. Small males had a clear mating advantage, they copulated longer than large males and fertilized more eggs. Males of different sizes used different tactics to mate. Large males were more likely to mate through a hole they cut into the web, whereas small males approached the female directly. Furthermore, small males usually mated at their first attempt but large males required several attempts before mating took place. There was no obvious female reaction towards males of different sizes.

COSTS OF REPRODUCTION INFLUENCE THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN SNAKES.

Most explanations for sex differences in adult body size have focused on differences between the two sexes in the degree to which larger body size enhances reproductive success. For example, the general tendency for females to grow larger than males has been attributed to production of larger litters by larger females (Darwin 1871; Trivers 1972; but see Shine 1988), and the correlation between large male size and mating systems with male-male competition has led many authors to conclude that larger male size enhances male reproductive success in such systems (Darwin 1871; Clutton-Brock et al. 1977; Shine 1978; Berry and Shine 1980). Because each of these hypotheses deals only with the relationship between body size and reproductive success within one sex, no prediction can be made about the relative sizes of the two sexes (Greenwood and Adams 1987; Shine 1987). The degree of size dimorphism is presumably determined by the relative intensities of selection for large size in the two sexes, as well as the degree of genetic correlation between the sexes for alleles associated with body size (Arnold 1985). It is entirely likely that large size enhances reproductive success in both sexes, but more strongly in one than in the other. In order to predict the actual direction or degree of sexual size dimorphism within a population, we need data on the effects of body size on reproductive success in both sexes. Such data are rarely available (Clutton-Brock 1988), so the balance between selective forces acting on conspecific males and females has rarely been evaluated. A comparative analysis of sexual size dimorphism and mating systems in snakes reveals that females grow larger than males in most species, but not in species that show male-male combat behavior (Shine 1978). The same pattern has been noted in many other animals, including other reptiles, and generally has been attributed to sexual selection for large male body size in taxa with male-male combat (Shine 1979; Berry and Shine 1980; Fitch 1981). Nonetheless, there are many exceptions to this generalization, and the most puzzling of these are species in which male-male combat occurs but in which females attain larger sizes than males. Species that fail to conform to the usual pattern may tell us a great deal about the factors responsible for those general patterns. In this paper, we use detailed demographic and ecological data on one of these atypical species (the adder Vipera berus) to test among potential explanations for female-larger dimorphism in a species with male-male combat. Our analyses suggest that sex differences in costs of reproduction may strongly influence optimal body sizes in each sex, and hence may affect the direction of sexual size dimorphism. We focus here on the potential costs and benefits of larger body size in terms of reproductive success, especially, fecundity selection in females and sexual selection in males.

Growth, maturity, and sexual dimorphism in the wood turtle, Clemmys insculpta

Growth in the wood turtle (Clemmys insculpta) is described using the von Bertalanffy growth model and nonlinear regression. Growth in both sexes is similar until about 160 mm plastron length. Males grew at a faster rate after this size. Males appear to mature at a larger size and later age than females, although age-specific body size is highly variable. Sexual size dimorphism, in carapace length, is pronounced, with males attaining mean body sizes significantly larger (1.07–1.10 times) than those of adult females. Plastron length is an inappropriate measure of sexual size dimorphism because of the development of plastral concavity in males. Females tend to predominate in samples, possibly owing to differential maturity of the sexes. The direction of sexual size dimorphism may be maintained by intrasexual competition among males for access to females. Data for several adults captured at intervals of from 9 to 20 years support the concept of indeterminate growth.

Generation glands and sexual size dimorphism in the Cape Crag Lizard, Pseudocordylus microlepidotus

The Cape Crag Lizard, Pseudocordylus microlepidotus, is unique among cordylid lizards in that generation glands are not restricted to the femoral region, but also occur on the dorsum and in the inguinal region. We analysed 274 museum specimens of P. microlepidotus, collected at various localities in the Western Cape Province, South Africa, and found that generation glands ontogenetically first differentiate in the femoral region, then on the back along the vertebral line, and finally, in males only, in the inguinal region. In both sexes, generation glands start to differentiate before sexual maturity is reached, but males develop significantly more glands than females. Males have larger heads than females, but the sexes do not differ significantly in overall body size. High scar frequencies were recorded for both sexes and the sex ratio for the total sample was close to 1:1. The functional significance of the additional generation glands is discussed in context of the social structure of the species as inferred from the degree and direction of sexual size dimorphism and scar frequency data. Sexual size dimorphism recorded for P. microlepidotus is of a similar nature as that recorded for other cordylids occurring at high altitudes in the winter rainfall area of South Africa.