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166 results for “sexual size dimorphism”
Macroevolutionary patterns of sexual size dimorphism among African tree frogs (Family: Hyperoliidae)
<p>Sexual size dimorphism (SSD) is shaped by multiple selective forces that drive the evolution of sex-specific body size, resulting in male or female-biased SSD. Stronger selection on one sex can result in an allometric body-size scaling relationship consistent with Rensch's rule or its converse. Anurans (frogs and toads) generally display female-biased SSD, but there is variation across clades and the mechanisms driving the evolution of SSD remain poorly understood. We investigated these topics in a diverse family of African treefrogs (Hyperoliidae). Hyperoliids display traits considered rare among amphibians, including sexual dichromatism and protogynous sex change. Using phylogenetic comparative methods, we tested if adult ecology, sexual dichromatism, and sex change were predictors of body size or SSD. We also tested whether hyperoliids displayed allometric interspecific body-size-scaling relationships. We found a majority of hyperoliid taxa display female-biased SSD, but that adult ecology and sexual dichromatism are poor predictors of sex-specific body size and SSD. Regardless of the groupings analyzed (partitioned by clades or traits), we found support for isometric body-size scaling. However, we found that sex change is a significant predictor of SSD variation. Species in the <i>Hyperolius viridiflavus </i>complex, which putatively display this trait, show a significant reduction in SSD and tend to be sexually monomorphic in size. Although protogynous sex change needs to be validated for several of these species, we tentatively propose this trait is a novel mechanism influencing anuran body size evolution. Beyond this association, additional factors that shape the evolution of anuran body size and SSD remain elusive.</p>
Sexual size dimorphism is associated with reproductive life history trait differentiation in coexisting sepsid flies
Organismal life histories evolve as syndromes, resulting in correlated evolutionary differentiation of key traits that ultimately aid in discerning species. Reproductive success depends both on the absolute body size of an individual and its size relative to the opposite sex: sexual size dimorphism. In an attempt to further elucidate their coexistence and ecological diversification, we compared standard life history (first reproduction, clutch size, egg size) and associated reproductive trait differentiation of 15 widespread European sepsid fly species (Diptera: Sepsidae) under laboratory common garden conditions. Despite relatively uniform body sizes, sexual dimorphism ranged from female- to male-biased, and development time varied twofold across species. We expected, and found, the abundant and relatively large species (Sepsis cynipsea, punctum, thoracica) with often male-biased SSD to lay larger but fewer eggs and show fast-developing, fast-reproducing life histories with aggressive (coercive) mating behavior characterized by short mating latencies and male conflict. In contrast, the smaller and more dispersed species with female-biased SSD (S. flavimana, orthocnemis, violacea) laid smaller but more eggs, showing a generally slower life history with long and delayed copulation and oviposition, high mating reluctance fostering extensive inter-sexual conflict, and more elaborate male (pre-)copulatory courtship. Two Saltella species were exceptional, being large, developing slowly, nevertheless copulating soon after adult emergence, profusely and briefly. The documented life history differentiation seems partly driven by sexual selection leading to male-biased dimorphism, rather than undetermined ecological selection, which regardless appears insufficient to explain the coexistence and diversification of these sepsid species in European pastoral landscapes.
Data from: revisiting niche divergence hypothesis in dimorphic birds: is diet overlap correlated with sexual size dimorphism?
<p>The evolution of sexual size dimorphism (SSD) is a long-standing topic in evolutionary biology, but there is little agreement on the extent to which SSD is driven by the different selective forces. While sexual selection and fecundity selection have traditionally been proposed as the two leading hypotheses, SSD may also result from natural selection through mechanisms such as sexual niche divergence, which might have reduced resource competition between sexes. Here, we revisited the niche divergence hypothesis by testing the relationship between the sexual overlap in diet and SSD of 56 bird species using phylogenetic comparative analyses. We then assessed how SSD variation relates to the three main hypotheses: sexual selection, fecundity selection, and sexual niche divergence using phylogenetic generalized least squares (PGLS). Then, we compared sexual selection, fecundity selection, and niche divergence selection as SSD drivers through phylogenetic confirmatory path analyses to disentangle the possible causal evolutionary relationships between SSD and the three hypotheses. Phylogenetic generalized least squares showed that SSD was negatively correlated with diet overlap, i. e., the greater the difference in body size between males and females, the less diet overlap. As predicted by sexual selection theory, the difference in body size between sexes was higher in polygynous species. Confirmatory phylogenetic path analyses suggested that the most likely evolutionary path might include mating system as a main driver in SSD and niche divergence as a result of SSD. We found no evidence of a role of fecundity selection in the evolution of female-biased SSD. Our study provides evidence that sexual selection has likely been the main cause of SSD and that dietary divergence is likely an indirect effect of SSD.</p>
Fig. 1 in Size-At-Age Variability And Sexual Dimorphism Of Morphometric Characteristics In The Late Ontogenesis Of The Marsh Frog, Pelophylax Ridibundus (Anura, Ranidae), From Terrytory Of Crimea
Fig. 1. Measurement system for tailess amphibians according to the standard methods.
Dancing drives evolution of sexual size dimorphism in manakins
<p><span>Body size mediates life history, physiology, and inter- and intra-specific interactions. Within species, sexes frequently differ in size, reflecting divergent selective pressures and/or constraints. Both sexual selection and differences in environmentally-mediated reproductive constraints can drive sexual size dimorphism, but empirically testing causes of dimorphism is challenging. Manakins (Pipridae), a family of Neotropical birds comprising ~50 species, exhibit both male- and female-biased size dimorphism and are distributed across gradients of precipitation and elevation. Males perform courtship displays ranging from simple hops to complex aerobatic manoeuvres. </span><span>We tested associations between sexual size dimorphism and (a) agility and (b) environment, analysing morphological, behavioural, and environmental data for 22 manakin species in a phylogenetic framework. </span><span>Sexual dimorphism in mass was most strongly related to agility, with males being lighter than females in species performing more aerial display behaviours. However, wing and tarsus length dimorphism were more strongly associated with environmental variables, suggesting that different sources of selection act on different aspects of body size. These results highlight both the strength of sexual selection in shaping morphology—even atypical patterns of dimorphism—while demonstrating the importance of constraints and ecological consequences of body size evolution.</span></p>
Data for the article entitled: Linking sexual size dimorphism to trophic niche partitioning in a generalist predator
<p>Sexual size dimorphism is a common phenomenon in mammals, and researchers have been trying to demonstrate the evolutionary causes leading to sexual dimorphism. Two main hypotheses emerged: (i) the sexual selection hypothesis and (ii) the sexual competition hypothesis (also called resource partitioning hypothesis). Here, we attempted to link sexual dimorphism in fishers (Pekania pennanti (Erxleben, 1777)) with their fall diet using stable isotope profiling and body and skull measurements. We used the carcasses of 39 fishers which were caught in eastern Québec during fall 2014 by volunteer trappers as well as several potential prey items ranging from small rodents to cervids. We expected minimal niche overlap between sexes, as males should be able to exploit different prey species than females. We also expected to observe an effect of age class (adults vs. juveniles) on trophic niche. As expected, we found great evidence of sexual dimorphism in both body mass and skull measurements: males were heavier and longer than females and had a larger zygomatic and intracanine width and a longer skull. While proportions of prey in diet according to sex and age did not vary greatly, we found some evidence of niche partitioning using Layman's metrics. Indeed, females tended to have a less diversified and more similar diet compared to one another, whereas males showed more diversified and contrasted diets. Despite our limited sample size, our findings provide partial support to the sexual competition hypothesis, as the difference in body and skull size based on sex could have evolved to lessen intraspecific competition in fishers.</p>
Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females
<p><span>Within a species' distribution, populations are often exposed to diverse environments and may thus experience different sources of both natural and sexual selection. These differences are likely to impact the balance between costs and benefits to individuals seeking reproduction, thus entailing evolutionary repercussions. Here, we look into an unusual population (Baltic Sea) of the broadnosed pipefish, <i>Syngnathus typhle</i>, where males do not seem to select females based on size and hypothesise that this pattern may derive from a reduction of direct benefits to the male. We further hypothesise that if larger females do not persistently secure a higher reproductive success, either through pre- or post-copulatory sexual selection, a decrease in sexual size dimorphism in the Baltic population should be apparent, especially when contrasted with a well-studied population, inhabiting similar latitudes (Swedish west coast), where males prefer larger females.</span></p> <p>We found that, in the Baltic population, variation in female quality is low. We were unable to find differences in abortion rates or protein concentration in oocytes produced by females of contrasting sizes. Direct benefits from mating with large partners seem, thus, reduced in the Baltic population. We also found no evidence of any post-copulatory mechanism that could favour larger mothers as embryo development was unrelated to female size. While female size can still be selected through intrasexual competition or fecundity selection, the pressure for large female body size seems to be lower in the Baltic. Accordingly, we found a noticeable decrease in sexual size dimorphism in the Baltic population. We conclude that, although far from negating the significance of other selective process, sexual selection seems to have a decisive role in supporting pipefish sexual size asymmetries.</p>
Figure 1 in Sexual size dimorphism in Rana (Pelophylax) ridibunda ridibunda Pallas, 1771 from a population in Darre-Shahr Township, Ilam Province, western Iran
Figure 1. Map showing the study area in Ilam province, western Iran.
Sexual selection and sexual size dimorphism in animals
<p>Sexual selection is often considered as a critical evolutionary force promoting sexual size dimorphism (SSD) in animals. However, empirical evidence for a positive relationship between sexual selection on males and male-biased SSD received mixed support depending on the studied taxonomic group and on the method used to quantify sexual selection. Here, we present a meta-analytic approach accounting for phylogenetic non-independence to test how standardized metrics of the opportunity and strength of pre-copulatory sexual selection relate to SSD across a broad range of animal taxa comprising up to 95 effect sizes from 59 species. We found that SSD based on length measurements was correlated with the sex difference in the opportunity for sexual selection but showed a weak and statistically non-significant relationship with the sex difference in the Bateman gradient. These findings suggest that pre-copulatory sexual selection plays a limited role for the evolution of sexual size dimorphism in a broad phylogenetic context.</p>
Sexual dimorphism in size and shape of the head in the sea snake Emydocephalus annulatus
<p>In snakes, divergence in head size between the sexes has been interpreted as an adaptation to intersexual niche divergence. By overcoming gape-limitation, a larger head enables snakes of one sex to ingest larger prey items. Under this hypothesis, we do not expect a species that consumes only tiny prey items to exhibit sex differences in relative head size, or to show empirical links between relative head size and fitness-relevant traits such as growth and fecundity. Our field studies on the sea snake <i>Emydocephalus annulatus</i> falsify these predictions. Although these snakes feed exclusively on fish eggs, the heads of female snakes are longer and wider than those of males at the same body length. Individuals with wider heads grew more rapidly, reproduced more often, and produced larger litters. Thus, head shape can affect fitness and can diverge between the sexes even without gape-limitation. Head size and shape may facilitate other aspects of feeding (such as the ability to scrape eggs off coral) and locomotion (hydrodynamics); and a smaller head may advantage the sex that is more mobile, and that obtains its prey in narrow crevices rather than in more exposed situations (i.e., males).</p>
Influence of microhabitat, fecundity, and parental care on the evolution of sexual size dimorphism in Caribbean Eleutherodactylus frogs
<p><span>Rensch's rule suggests that sexual size dimorphism (SSD) increases with species size when males are the larger sex, whereas it decreases when females are the larger sex. However, the process responsible for this pattern remains obscure. SSD can result from sexual selection, such as intra-sexual competition for access to mates, or from natural selection, due to resource partitioning or fecundity selection. We studied SSD in Caribbean <em>Eleutherodactylus</em> frogs using phylogenetic comparative methods to investigate the influence of microhabitat, fecundity, and parental care. Our results show that in Caribbean <em>Eleutherodactylus,</em> females tend to be larger and, contrary to Rensch's rule, dimorphism increases with species size. SSD was not related to microhabitat use. However, SSD was positively correlated with fecundity, mediated by a greater increase in female size. SSD was also influenced by parental care, suggesting that male care promotes larger male size and reduces the female bias in SSD. As suggested for other anurans, female-biased SSD in Caribbean <em>Eleutherodactylus</em> results from fecundity selection, although the magnitude is countered by increased male size in species with paternal care. Our results highlight the importance of considering various selective forces that may act in concert to influence the evolution of sexual size dimorphism.</span></p>
Genetic variation in sexual size dimorphism is associated with variation in sex-specific plasticity in Drosophila
<p><span>The difference in body size between females and males, or sexual size dimorphism (SSD), is ubiquitous, and yet we have a poor understanding of the developmental-genetic mechanisms that generate it, and how these mechanisms may vary within and among species. Such an understanding of the genetic architecture of SSD is important if we are to evaluate alternative models of SSD evolution, but is difficult to describe because SSD is a characteristic of populations, not individuals. Here, we overcome this challenge by using isogenic lineages of <em>Drosophila</em> to measure SSD for 196 genotypes. We demonstrate extensive genetic variation for SSD, primarily driven by higher levels of genetic variation for body size among females than males. While we observe a general increase in SSD with sex-averaged body size (pooling for sex) among lineages, the vast majority of variation in SSD is independent of sex-averaged body size, and shows a strong genetic correlation with sex-specific plasticity, such that increased female-biased SSD is associated with increased body-size plasticity in females. Our data are consistent with the condition-dependence hypothesis of sexual dimorphism, and suggest that SSD in <em>Drosophila</em> is a consequence of selection on the developmental-genetic mechanisms that regulate the plasticity of body size. </span></p>
Data and code for: Sex-specific trait architecture in a sexually size dimorphic spider
<p class="MsoNormal">Sexual dimorphism, or sex-specific trait expression, may evolve when selection favours different optima for the same trait between sexes, i.e., under antagonistic selection. Intra-locus sexual conflict exists when the sexually dimorphic trait under antagonistic selection is based on genes shared between sexes. A common assumption is that the presence of sexual-size dimorphism (SSD) indicates that sexual conflict has been, at least partly, resolved via decoupling of the trait architecture between sexes. However, whether and how decoupling of the trait architecture between sexes has been realised often remains unknown. We tested for differences in architecture of adult body size between sexes in a species with extreme SSD, the African hermit spider (<em>Nephilingis cruentata</em>), where adult female body size greatly exceeds that of males. Specifically, we estimated the sex-specific importance of genetic and maternal effects on adult body size among individuals that we laboratory-reared for up to eight generations. Quantitative genetic model estimates indicated that size variation in females is to a larger extent explained by direct genetic effects than by maternal effects, but in males to a larger extent by maternal than by genetic effects. We conclude that this sex-specific body-size architecture enables body-size evolution to proceed much more independently than under a common architecture to both sexes.</p>
Data for: Size rather than complexity of sexual ornaments prolongs male metamorphosis and explains sexual size dimorphism in sepsid flies
<p><span>Male sexual ornaments often evolve rapidly and are thought to be costly, thus contributing to sexual size dimorphism. However, little is known about their developmental costs, and even less about costs associated with structural complexity. Here, we quantified the size and complexity of three morphologically elaborate sexually dimorphic male ornaments that starkly differ across sepsid fly species (Diptera: Sepsidae). Male forelegs range from being unmodified, like in most females, to being adorned with spines and large cuticular protrusions. The 4th abdominal sternites are either unmodified or are converted into complex de novo appendages. Male genital claspers range from small and simple to large and complex (e.g. bifurcated). </span><span>We tracked the development of 18 sepsid species from egg to adult to determine larval feeding and pupal metamorphosis times of both sexes. We then statistically explored whether pupal and adult body size, ornament size, and/or ornament complexity are correlated with sex-specific development times. Larval growth and foraging periods of male and female larvae did not differ, but the time spent in the pupal stage was ca. 5% longer for sepsid males despite emerging 9% smaller than females on average. Surprisingly, we found no evidence that sexual trait complexity prolongs pupal development beyond some effects of trait size. </span><span>Evolving more complex traits thus does not incur developmental costs.</span></p>
Sex differences in the behavioural traits across ontogenetic stages in a sexually-size dimorphic spider
<p><span>Selection acts differently on females and males due to differences in potential reproductive rates, driving the evolution of sex differences in traits, including growth and behaviour. Additionally, selection pressures vary during an individual's ontogeny, with growth being crucial in early developmental stages and reproduction during adulthood, leading to age- and sex-specific behavioural strategies. In this study, we investigated a sexually-size dimorphic spider, the raft spider (<em>Dolomedes fimbriatus</em>), where females are substantially larger than males. We repeatedly observed spiders from juvenile to adult stages, examining boldness, voracity towards prey, and probability to attack. Our findings revealed that females exhibited greater boldness, voracity, and probability to back-attack the simulated attacker compared to males. Notably, the observed behaviours changed during ontogeny, with sex differences in the magnitudes and directions of change, indicating distinct life history strategies between sexes. Moreover, we detected positive associations between body mass or age and behavioural traits, supporting a proposed positive feedback loop between assets and behaviour. While mass and age were not significant confounding predictors in the analyses of sex differences in behaviour, some collinearity was present between sex, mass, and age, so that their effects on behavioural differences between sexes cannot be conclusively disentangled. Repeatability of behaviours was low but significant for boldness and probability to attack, with similar estimates between sexes. These results underscore the importance of considering sex-specific life history strategies in behavioural trait studies. </span></p>
Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females
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Data from: Divergence of sexual size dimorphism between wild and hatchery chum salmon under intensive Japanese hatchery programs
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Viviparity is associated with larger female size and higher sexual size dimorphism in a reproductively bimodal lizard
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Sexual selection and sexual size dimorphism in animals
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Sexual dimorphism of head, teeth, flipper, and body size in northern elephant seals (<em>Mirounga angustirostris</em>) throughout ontogeny
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