Variation In Sexual Size Dimorphism Research Articles

Intraspecific mating system evolution and its effect on complex male secondary sexual traits: Does male-male competition increase selection on size or shape?

Sexual selection is generally held responsible for the exceptional diversity in secondary sexual traits in animals. Mating system evolution is therefore expected to profoundly affect the covariation between secondary sexual traits and mating success. Whereas there is such evidence at the interspecific level, data within species remain scarce. We here investigate sexual selection acting on the exaggerated male fore femur and the male wing in the common and widespread dung flies Sepsis punctum and S.neocynipsea (Diptera: Sepsidae). Both species exhibit intraspecific differences in mating systems and variation in sexual size dimorphism (SSD) across continents that correlates with the extent of male-male competition. We predicted that populations subject to increased male-male competition will experience stronger directional selection on the sexually dimorphic male foreleg. Our results suggest that fore femur size, width and shape were indeed positively associated with mating success in populations with male-biased SSD in both species, which was not evident in conspecific populations with female-biased SSD. However, this was also the case for wing size and shape, a trait often assumed to be primarily under natural selection. After correcting for selection on overall body size by accounting for allometric scaling, we found little evidence for independent selection on any of these size or shape traits in legs or wings, irrespective of the mating system. Sexual dimorphism and (foreleg) trait exaggeration is therefore unlikely to be driven by direct precopulatory sexual selection, but more so by selection on overall size or possibly selection on allometric scaling.

Assessing Rensch’s rule in a newt: Roles of primary productivity and conspecific density in interpopulation variation of sexual size dimorphism

AbstractAimMajor efforts have been devoted to understanding the geographic pattern of sexual size dimorphism (SSD). Rensch's rule posits that SSD increases with body size in male‐biased SSD species and decreases with body size in female‐biased SSD species. This pattern, and its inverse, have been mainly explored at the interspecific level, whereas research at the intraspecific level has been largely neglected. Here, we test whether the allometric pattern of SSD in an urodele amphibian conforms to Rensch's rule and evaluate the relative role of four potential mechanisms: sexual selection, fecundity selection, density‐dependent resource availability and differential plasticity.LocationIberian Peninsula.TaxonLissotriton boscai (Amphibia: Caudata: Salamandridae).MethodsWe used original (field‐based) and published data on body size, courtship behaviour and fecundity parameters and a suite of climatic, ecological and genetic (mitochondrial and nuclear markers) predictors to assess these hypotheses.ResultsThe results showed that SSD increased with increasing female mean body size, supporting the inverse Rensch's rule pattern. Primary productivity‐related variables and female density were among the most relevant ecological predictors of SSD after accounting for genetic structure and capture date.Main conclusionsThis study reveals that the interplay between the density‐dependent resource availability hypothesis and the differential plasticity hypothesis explains the inverse Rensch's rule. We discuss how combining biogeographical and experimental approaches can provide alternative interpretations to the classical sexual and fecundity selection hypotheses on the interpopulation variation in SSD.

A synthesis of major environmental-body size clines of the sexes within arthropod species

Body size at maturity often varies with environmental conditions, as well as between males and females within a species [termed Sexual Size Dimorphism (SSD)]. Variation in body size clines between the sexes can determine the degree to which SSD varies across environmental gradients. We use a meta-analytic approach to investigate whether major biogeographical and temporal (intra-annually across seasons) body size clines differ systematically between the sexes in arthropods. We consider 329 intra-specific environmental gradients in adult body size across latitude, altitude and with seasonal temperature variation, representing 126 arthropod species from 16 taxonomic orders. On average, we observe greater variability in male than female body size across latitude, consistent with the hypothesis that, over evolutionary time, directional selection has acted more strongly on male than female size. In contrast, neither sex exhibits consistently greater proportional changes in body size than the other sex across altitudinal or seasonal gradients, akin to earlier findings for plastic temperature-size responses measured in the laboratory. Variation in the degree to which body size gradients differ between the sexes cannot be explained by a range of potentially influential factors, including environment type (aquatic vs. terrestrial), voltinism, mean species’ body size, degree of SSD, or gradient direction. Ultimately, if we are to make better sense of the patterns (or lack thereof) in SSD across environmental gradients, we require a more detailed understanding of the underlying selective pressures driving clines in body size. Such understanding will provide a more comprehensive hypothesis-driven approach to explaining biogeographical and temporal variation in SSD.

Seasonal Variation in Sexual Size Dimorphism in the Wrinkle-lipped Free-tailed Bat (Chaerephon plicatus Buchannan, 1800) Population in the Khao Chong Phran Non-hunting Area, Thailand

Seasonal Variation in Sexual Size Dimorphism in the Wrinkle-lipped Free-tailed Bat (Chaerephon plicatus Buchannan, 1800) Population in the Khao Chong Phran Non-hunting Area, Thailand

Ecological and biological studies on five-lined skink, Trachylepis (= Mabuya) quinquetaeniata inhabiting two different habitats in Egypt.

This study was carried out to study the diet type, genetic, morphometric and histological variations between two natural populations of Trachylepis quinquetaeniata inhabiting Abu-Rawash, Embaba, North Giza and Garfas-Senoures, El-Faiyum habitats. In this study, morphometric examination showed that Trachylepis quinquetaeniata is a typically ground-dwelling Skink and seems to be frequently associated with grass and below shrubs. Geographic variation in sexual size dimorphism (SSD) and head size dimorphism (HSD) with greater dimensions in Garfas-Senoures, El-Faiyum compared with Abu-Rawash, Embaba, North Giza might be correlated with spatial food availability and abundance or micro-evolutionary change or phenotypic plasticity. Also, the increase of body size and mass in individuals from Garfas-Senoures, El-Faiyum might be an adaptation to lower ambient temperature and hypoxia. Diet analysis showed that the bulk of gut contents were beetles, plant bugs, hymenopterans and dipterans. The remainder of the plant material was probably ingested secondarily. Moreover, the increase in the activity of Ldh in liver tissues of T. quinquetaeniata inhabiting Abu-Rawash, Embaba, North Giza could be confirmed by the significant increase in the accumulation of the total lipids in liver and muscle tissues than that in the other population. As well as, a marked degeneration, necrosis and desquamation of spermatogoneal cells lining seminiferous tubules in testes of T. quinquetaeniata inhabiting Garfas-Senoures, El-Faiyum than that in the other population inhabiting Abu-Rawash, Embaba, North Giza that could be attributed to the bioaccumulation of soil heavy metals in testes. Higher carbohydrate accumulation in the former lizards' population was confirmed by the high availability and diversity of diet in El-Faiyum habitat. Thus we concluded that T. quinquetaeniata inhabiting Abu-Rawash, Embaba, North Giza is more active, energetic and adaptable in its habitat than T. quinquetaeniata inhabiting Garfas-Senoures, El-Faiyum.

Spatial variation in sexual size dimorphism of the American black bear (Ursus americanus) in eastern North America

Spatial patterns of sexual size dimorphism for the American black bear (Ursus americanus) from 12 localities in eastern United States were investigated. Forty-four measurements from 371 (187 males and 184 females) adult specimens (≥4.5 years old) were recorded using digital calipers. Males were larger than females for all characters except braincase width and length of m3 for specimens from Virginia. The degree of sexual size dimorphism (SSD) for characters across localities ranged from 4.1–22.5%, with most being from 6.6–17.0%. For 39 of the 44 characters (88.8%), there was no statistically significant difference in degree of SSD across localities, indicating it was approximately the same at each locality. This was further supported by results of principal component analysis. Results followed a similar pattern reported for U. americanus in western North America, which supports a growing generalization for medium-sized and large-sized terrestrial carnivores of a uniform pattern of sexual size variation across geographic space; this is likely due to bioenergetic factors associated with growth and reproduction, with selective pressure responsible for size dimorphism acting uniformly throughout the range of the species.

Spatial Variation In Sexual Size Dimorphism of the American Black Bear (Ursus americanus) In Eastern North America

Spatial Variation In Sexual Size Dimorphism of the American Black Bear (<i>Ursus americanus</i>) In Eastern North America

Sexual size dimorphism in a landlocked Pacific salmon in relation to breeding habitat features

Many animal species exhibit size dimorphism between sexes. Sexual selection, whereby male–male competition favors larger body sizes, has been considered a likely cause of sexual size dimorphism. Habitat features in breeding areas could affect the outcome of male–male competition, yet few attempts have been made to relate breeding habitat features with interpopulation variation in sexual size dimorphism. In this study, we examined interpopulation variation in sexual size dimorphism by studying the landlocked amago salmon (Oncorhynchus masou ishikawae) at a microgeographic scale. We found that female body size was independent of stream size but that male body size decreased with smaller stream sizes. A likely explanation is that the relationship between reproductive success and the size of males is influenced by the availability of refuges that are only available to small-bodied males. Sexual differences in body size increased with decreasing stream sizes, supporting the hypothesis that the reproductive success of larger males is reduced in smaller streams. In contrast, the maturation-length threshold increased with stream size for both sexes. The stream-size-based interpopulation variation in sexual size dimorphism and size at maturity in landlocked amago salmon may therefore have arisen through a combination of sexual and natural selection.

Resource availability and sexual size dimorphism: differential effects of prey abundance on the growth rates of tropical snakes

Summary Broad phylogenetic patterns in sexual size dimorphism (SSD) are shaped by sex differences in net selection pressures (e.g. sexual selection, fecundity selection, survival selection), but environmental and ecological factors can also affect the expression of SSD. Discussions of proximate ecological influences on SSD have focused on niche divergence; for example, increase in a prey type used by only one sex can elevate growth rates of that sex but not the other. Food limitation also can generate spatial and temporal variation in SSD. Under restricted prey abundance, curtailed growth may mask SSD even if the optimal size is greater for one sex than the other. Because an increase in food availability elicits increased feeding and growth by the sex that benefits more from increased body size, variation in prey abundance can generate variation in SSD. We used mark‐recapture methods to study growth rates relative to prey (frog) abundance in two species of sexually dimorphic colubrid snake species in tropical Australia. In slatey‐grey snakes (Stegonotus cucullatus), a species in which larger body size enhances reproductive output in both sexes (because larger males win combat bouts, and larger females produce more/heavier eggs), increased abundance of frogs caused equivalent increases in growth rates in both sexes and hence did not affect SSD. In keelbacks (Tropidonophis mairii), a species in which larger size enhances reproductive output in females more than males (reflecting a lack of male–male combat), increased abundance of frogs elicited higher growth rates of females only. Thus, SSD in keelbacks was modified by prey abundance. Our results show that the magnitude of sex differences in adult body size can be influenced by proximate environmental factors and support the hypothesis of sex‐specific targets for maximum feeding rates. A lay summary is available for this article.

Intraspecific variation in body size and sexual size dimorphism, and a test of Rensch's rule in bats

AbstractThe magnitude and direction of sexual size dimorphism (SSD) may vary considerably within and among taxa, and the primary causes of such variation have not been thoroughly elucidated. For example, the effect of abiotic factors is frequently attributed to explain intra‐ and interspecific variation in SSD. Rensch's rule, which states that males vary more in size than females when body size increases, has rarely been tested in bats. Therefore, whether bats follow Rensch's rule remains unclear, particularly when females are larger than males. We investigated whether four bat species presented SSD, as well as whether their body sizes varied within each sex across localities, testing the hypothesis that intraspecific SSD varies substantially depending of sampling localities. We finally examined whether bats followed Rensch's rule by simultaneously using intraspecific and interspecific approaches. Although SSD was not observed for most bat species within each locality, the females of three of the four captured species exhibited differences in body size between particular localities. Usually the females varied more in size than did males across localities, mostly exhibiting a female‐biased SSD. Significant differences in SSD were observed (i.e. mean values of the sexual dimorphism index), even though Rensch's rule was not followed.

Sexual size dimorphism in musteloids: An anomalous allometric pattern is explained by feeding ecology.

Rensch's rule states that sexual size dimorphism (SSD) increases with body size in taxa where males are larger, and decreases when females are larger. The dominant explanation for the trend is currently that competitive advantage for males is greater in larger individuals, whereas female size is constrained by the energetics of rearing offspring. This rule holds for a variety of vertebrate taxa, and opposing trends are rare. We examine the allometry of SSD within the Musteloidea and demonstrate a hypo‐allometry contrary to Rensch's rule, with lower SSD associated with larger body size. We provide evidence that feeding ecology is involved. Where diet promotes group‐living, the optimal strategy for the males of larger species is often not to attempt to defend access to multiple females, obviating any competitive advantage of relatively greater size. We conclude that the effect of feeding ecology on mating systems may be a hitherto neglected factor explaining variation in SSD.

The effect of seed traits on geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Acanthoscelides macrophthalmus.

Explaining large‐scale patterns of variation in body size has been considered a central question in ecology and evolutionary biology because several life‐history traits are directly linked to body size. For ectothermic organisms, little is known about what processes influence geographic variation in body size. Changes in body size and sexual size dimorphism (SSD) have been associated with environmental variables, particularly for Bruchinae insects, which feed exclusively on seeds during the larval stage. However, the effect of important seed traits on body size variation has rarely been investigated, and whether SSD varies substantially among populations within bruchine species is poorly known. Using the seed‐feeding beetle Acanthoscelides macrophthalmus infesting its host plant Leucaena leucocephala, we investigated whether specific seed traits (hardness, size, water content, carbon/nitrogen ratio, and phenolic content) were determinant in generating geographic variation in body size and SSD of A. macrophthalmus. We also examined the relationships between body size and SSD with latitude and altitude. The body size of both sexes combined was not related to latitude, altitude, and any of the physical and chemical seed traits. However, the female body size tended to vary more in size than the males, generating significant variation in SSD in relation to latitude and altitude. The females were the larger sex at higher latitudes and at lower altitudes, precisely where seed water content was greater. Therefore, our results suggest that water content was the most important seed trait, most severely affecting the females, promoting geographic variation in SSD of A. macrophthalmus.

Sexual size dimorphism and Rensch's rule in Canidae

The size variation between males and females of a species is a phenomenon known as sexual size dimorphism (SSD). The observed patterns of variation in SSD among species has led to the formulation of Rensch's rule, which establishes that, in species showing a male size bias, SSD increases with an increase in the body size of the species. However, for species in which there is a female size bias, the SSD would decrease when the body size of the species increases. In the present study, we examined the variation in body size and SSD of 33 species of canids from estimates of body mass and body length. We studied its relationship with life-history characteristics and tested Rensch's rule using phylogenetic generalized least squares and phylogenetic reduced major axis regressions, respectively. We observed the existence of correlation between body mass and body length, although the SSDs from these estimators are uncorrelated. SSD did not show the pattern predicted by Rensch's rule. SSD also did not show any correlation with life-history traits. It is likely that the low SSD observed in canids is related to the monogamy observed in the family, which is a rare situation in mammals.

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

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

Parasite diversity of disease-bearing rodents of Southeast Asia: habitat determinants and effects on sexual size dimorphism and life-traits

We investigated a causal chain of relationships between habitat specialization and parasite species richness in rodent communities in Southeast Asia, and the consequences for variation in immune investment (using spleen size), the degree of sexual competition (using testes) and sexual size dimorphism (SSD). We used data gathered on rodents, their habitat specialization and their parasites (macro- and micro-parasites) in Southeast Asian landscapes. The results supported the hypotheses that parasite diversity drives the evolution of host life-traits and sexual selection. Firstly host habitat specialization explained the variation in parasite species richness. Secondly high parasite species richness was linked to host immune investment, using the relative spleen size of rodents. Thirdly according to the potential costs associated with immune investment, the relative spleen size was found to be negatively correlated with the relative size of testes among rodents. Fourthly, a positive relationship between male-biased SSD and parasite species richness was observed supporting the role of parasitism in sexual selection. Finally, the variation in SSD was positively associated with the degree of habitat specialization. Highest values of female-biased SSD were associated with habitat specialization, whereas highest values of male-biased SSD concerned synanthropic or generalist rodent species. These results, also correlative, will help to facilitate selection of the species that should be thoroughly investigated at the population level to better understand the selective effects of parasites on rodent life-history and behavior.

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.

Phenotypic Variation and Sexual Size Dimorphism in Dichroplus elongatus (Orthoptera: Acrididae).

Patterns of body size evolution are of particular interest because body size can affect virtually all the physiological and life history traits of an organism. Sexual size dimorphism (SSD), a difference in body size between males and females, is a widespread phenomenon in insects. Much of the variation in SSD is genetically based and likely due to differential selection acting on males and females. The importance of environmental variables and evolutionary processes affecting phenotypeic variation in both sexes may be useful to gain insights into insect ecology and evolution. Dichroplus elongatus Giglio-Tos is a South American grasshopper widely distributed throughout Argentina, Uruguay, most of Chile, and southern Brazil. In this study, we analyzed 122 adult females of D. elongatus collected in eight natural populations from central-east Argentina. Females show large body size variation among the analyzed populations and this variation exhibits a strong relationship with fecundity. Our results have shown that larger females were more fecund than smaller ones. We found that ovariole number varied along a latitudinal gradient, with higher ovariole numbers in populations from warmer locations. A considerable female-biased SSD was detected. SSD for three analyzed morphometric traits scaled isometrically. However, SSD for thorax length displayed a considerable variation across the studied area, indicating a larger relative increase in female size than in male size in warmer environmental conditions.

Developmental plasticity affects sexual size dimorphism in an anole lizard

Summary While developmental plasticity has been shown to contribute to sexual size dimorphism (SSD) in laboratory studies, its role in shaping SSD variation in wild vertebrate populations is unclear. Here we use a field study and a laboratory experiment to show that resource availability influences the degree of SSD among insular populations of Anolis sagrei lizards in the Bahamas. Total amounts of food biomass explained variation in male, but not female, body size on six Bahamian islands, giving rise to significant differences in SSD. Laboratory experiments on a captive colony of A. sagrei confirmed that variation in SSD was mediated by the effects of prey biomass on developmental plasticity in males, but not females. Indeed, males grew faster and attained larger sizes as adults under high‐food treatments than under restricted diets, whereas adult females retained similar body sizes under both conditions. Our results indicate that the amount of food available can influence intersexual variation in body size within a vertebrate species. Sex‐specific developmental plasticity may be favoured if it allows individuals to take advantage of varying levels of food opportunities offered by different habitats, by reducing competition between the sexes. As such, plasticity in response to food availability may have played a role in the invasion success of A. sagrei. This study adds to our growing understanding of the effect of resource availability in shaping SSD in reptiles and lends further support to the condition‐dependent hypothesis, according to which the larger sex should display greater plasticity in growth in response to environmental conditions.

Effect of genomic deficiencies on sexual size dimorphism through modification of developmental time in Drosophila melanogaster.

Sexual size dimorphism (SSD), a difference in body size between sexes, is common in many taxa. In insects, females are larger than males in >70% of all taxa in most orders. The fruit fly, Drosophila melanogaster is one prominent model organism to investigate SSD since its clear and representative female-biased SSD and its growth regulation are well studied. Elucidating the number and nature of genetic elements that can potentially influence SSD would be helpful in understanding the evolutionary potential of SSD. Here, we investigated the SSD pattern caused by artificially introduced genetic variation in D. melanogaster, and examined whether variation in SSD was mediated by the sex-specific modification of developmental time. To map the genomic regions that had effects on sexual wing size and/or developmental time differences (SDtD), we reanalyzed previously published genome-wide deficiency mapping data to evaluate the effects of 376 isogenic deficiencies covering a total of ~67% of the genomic regions of the second and third chromosomes of D. melanogaster. We found genetic variation in SSD and SDtD generated by genomic deficiencies, and a negative genetic correlation between size and development time. We also found SSD and SDtD allometries that are not qualitatively congruent, which however overall at best only partly help in explaining the patterns found. We identified several genomic deficiencies with the tendency to either exaggerate or suppress SSD, in agreement with quantitative genetic null expectations of many loci with small effects. These novel findings contribute to a better understanding of the evolutionary potential of sexual dimorphism.

Andrew meets Rensch: sexual size dimorphism and the inverse of Rensch's rule in Andrew's toad (Bufo andrewsi).

Variation in sexual size dimorphism (SSD) is a widespread phenomenon and is commonly attributed to variation in sex-specific patterns of selection. According to Rensch's rule, SSD increases with increasing body size when males are the larger sex, and decreases when females are the larger sex. Using data from 17 populations of Andrew's toad (Bufo andrewsi), we tested whether the patterns of SSD conform to Rensch's rule. Using field experiments, we also evaluated the hypothesis that sexual selection favours large male body size and that fecundity selection favours large female body size. The results revealed that the degree of SSD increased with increasing mean size in females, consistent with the inverse of Rensch's rule. Although experiments revealed evidence for a large-male mating advantage, selection for large male size was weak at best, and hence unlikely to be an important source of variation in SSD. However, fecundity selection favouring large females was evident, and likely to explain the observed inverse of Rensch's rule. After correcting male and female body size for age differences, the patterns of SSD remained the same, suggesting that the intra- and interpopulational variation in SSD is not driven by sex differences in age structure. Hence, these findings suggest that the strong fecundity selection favouring large females drives the evolution of female-biased SSD in B. andrewsi, providing an explanation for the inverse of Rensch's rule. As such, the study provides an important addition to the small body of literature that uses an intraspecific approach to demonstrate the inverse of Rensch's rule.