Environmental effects on sexual size dimorphism of a seed-feeding beetle

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.     

Geographic variation in body size and sexual size dimorphism of a seed-feeding beetle

Body size of many animals varies with latitude: body size is either larger at higher latitudes (Bergmann's rule) or smaller at higher latitudes (converse Bergmann's rule). However, the causes underlying these patterns are poorly understood. Also, studies rarely explore how sexual size dimorphism varies with latitude. Here we investigate geographic variation in body size and sexual size dimorphism of the seed-feeding beetle Stator limbatus, collected from 95 locations along a 38 degrees range in latitude. We examine 14 variables to test whether clines in environmental factors are adequate to explain geographic patterns of body size. We found that body size and sexual size dimorphism of S. limbatus varied considerably with latitude; beetles were smaller but more dimorphic at lower latitudes. Body size was not correlated with a gradient in mean temperature, contrary to the commonly accepted hypothesis that clines are produced by latitudinal gradients in temperature. Instead, we found that three factors were adequate to explain the cline in body size: clinal variation in host plant seed size, moisture (humidity), and seasonality (variance in humidity, precipitation, and temperature). We also found that the cline in sexual size dimorphism was partially explainable by a gradient in moisture, though moisture alone was not sufficient to explain the cline. Other ecological or environmental variables must necessarily contribute to differences in selection on male versus female body size. The main implications of our study are that the sexes differ in the magnitude of clinal variation in body size, creating latitudinal variation in sexual size dimorphism, and that clines in body size of seed beetles are likely influenced by variation in host seed size, water availability, and seasonality.     

The evolution of fecundity is associated with female body size but not female‐biased sexual size dimorphism among frogs

Sexual size dimorphism (SSD) is one of the most common ways in which males and females differ. Male‐biased SSD (when males are larger) is often attributed to sexual selection favouring large males. When females are larger (female‐biased SSD), it is often argued that natural selection favouring increased fecundity (i.e. larger clutches or eggs) has coevolved with larger female body size. Using comparative phylogenetic and multispecies regression model selection approaches, we test the hypothesis that among‐species variation in female fecundity is associated with the evolution of female‐biased SSD. We also ask whether the hypothesized relationship between SSD and fecundity is relaxed upon the evolution of parental care. Our results suggest a strong relationship between the evolution of fecundity and body size, but we find no significant relationship between fecundity and SSD. Similarly, there does not appear to be a relationship between fecundity and the presence or absence of parental care among species. Thus, although female body size and fecundity coevolve, selection for increased fecundity as an explanation for female‐biased SSD is inconsistent with our analyses. We caution that a relationship between female body size and fecundity is insufficient evidence for fecundity selection driving the evolution of female‐biased SSD.     

Population variation in sexual selection and its effect on size allometry in two dung fly species with contrasting sexual size dimorphism

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.     

Evolutionary ecology of egg size and number in a seed beetle: genetic trade-off differs between environments

Abstract In many organisms, large offspring have improved fitness over small offspring, and thus their size is under strong selection. However, due to a trade-off between offspring size and number, females producing larger offspring necessarily must produce fewer unless the total amount of reproductive effort is unlimited. Because differential gene expression among environments may affect genetic covariances among traits, it is important to consider environmental effects on the genetic relationships among traits. We compared the genetic relationships among egg size, lifetime fecundity, and female adult body mass (a trait linked to reproductive effort) in the seed beetle, Stator limbatus , between two environments (host-plant species Acacia greggii and Cercidium floridum ). Genetic correlations among these traits were estimated through half-sib analysis, followed with artificial selection on egg size to observe the correlated responses of lifetime fecundity and female body mass. We found that the magnitude of the genetic trade-off between egg size and lifetime fecundity differed between environments–a strong trade-off was estimated when females laid eggs on C. floridum seeds, yet this trade-off was weak when females laid eggs on A. greggii seeds. Also differing between environments was the genetic correlation between egg size and female body mass–these traits were positively genetically correlated for egg size on A. greggii seeds, yet uncorrelated on C. floridum seeds. On A. greggii seeds, the evolution of egg size and traits linked to reproductive effort (such as female body mass) are not independent from each other as commonly assumed in life-history theory.     

Selection does not favor larger body size at lower temperature in a seed-feeding beetle

Body size of many animals increases with increasing latitude, a phenomenon known as Bergmann's rule (Bergmann clines). Latitudinal gradients in mean temperature are frequently assumed to be the underlying cause of this pattern because temperature covaries systematically with latitude, but whether and how temperature mediates selection on body size is unclear. To test the hypothesis that the "relative" advantage of being larger is greatest at cooler temperatures we compare the fitness of replicate lines of the seed beetle, Stator limbatus, for which body size was manipulated via artificial selection ("Large,"Control," and "Small" lines), when raised at low (22 degrees C) and high (34 degrees C) temperatures. Large-bodied beetles (Large lines) took the longest to develop but had the highest lifetime fecundity, and highest fitness (r(C)), at both low and high temperatures. However, the relative difference between the Large and Small lines did not change with temperature (replicate 2) or was greatest at high temperature (replicate 1), contrary to the prediction that the fitness advantage of being large relative to being small will decline with increasing temperature. Our results are consistent with two previous studies of this seed beetle, but inconsistent with prior studies that suggest that temperature-mediated selection on body size is a major contributor to the production of Bergmann clines. We conclude that other environmental and ecological variables that covary with latitude are more likely to produce the gradient in natural selection responsible for generating Bergmann clines.     

Lifetime selection on adult body size and components of body size in a waterstrider: opposing selection and maintenance of sexual size dimorphism

Abstract.— Sexual size dimorphism (SSD), the difference in body size between males and females, is common in almost all taxa of animals and is generally assumed to be adaptive. Although sexual selection and fecundity selection alone have often been invoked to explain the evolution of SSD, more recent views indicate that the sexes must experience different lifetime selection pressures for SSD to evolve and be maintained. We estimated selection acting on male and female adult body size (total length) and components of body size in the waterstrider Aquarius remigis during three phases of life history. Opposing selection pressures for overall body size occurred in separate episodes of fitness for females in both years and for males in one year. Specific components of body size were often the targets of the selection on overall body size. When net adult fitness was estimated by combining each individual's fitnesses from all episodes, we found stabilizing selection in both sexes. In addition, the net optimum overall body size of males was smaller than that of females. However, even when components of body size had experienced opposing selection pressures in individual episodes, no components appeared to be under lifetime stabilizing selection. This is the first evidence that contemporary selection in a natural population acts to maintain female size larger than male size, the most common pattern of SSD in nature.     

A comparative test of adaptive hypotheses for sexual size dimorphism in lizards

It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well-studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.     

Predicting the evolution of sexual size dimorphism

11 , Evolution 34 : 292–305) equations for predicting the evolution of sexual size dimorphism (SSD) through frequency‐dependent sexual selection, and frequency‐independent natural selection, were tested against results obtained from a stochastic genetic simulation model. The SSD evolved faster than predicted, due to temporary increases in the genetic variance brought about by directional selection. Predictions for the magnitude of SSD at equilibrium were very accurate for weak sexual selection. With stronger sexual selection the total response was greater than predicted. Large changes in SSD can occur without significant long‐term change in the genetic correlation between the sexes. Our results suggest that genetic correlations constrain both the short‐term and long‐term evolution of SSD less than predicted by the Lande model.     

Stronger condition dependence in female size explains altitudinal variation in sexual size dimorphism of a Tibetan frog

The present study investigated altitudinal variation in sexual size dimorphism of a Tibetan frog Nanorana parkeri. Size dimorphism was female‐biased in all populations, although this bias became less at higher altitudes because of a steeper altitudinal decrease in female size than male size. Operational sex ratios, an indicator of the opportunity for sexual selection on larger males, changed independently of altitude. Clutch volume, an indicator of the strength of fecundity selection on larger females, was positively with female size, and tended to decrease approaching high altitudes. Females lived longer and grew more slowly than males, and the mean age in both sexes increased and growth rate decreased altitudinally, although the changes were more rapid in females than males. These results suggest that, relative to males, females (i.e. the sex that typically bears greater reproductive costs and experiences stronger directional selection for larger size to take fecundity advantages) should be more sensitive to environments, attaining a larger size via enhancing growth under favourable lower‐latitude conditions but a smaller size as a result of retarding growth when conditions become harsher at higher altitudes. This supports the condition‐dependence hypothesis with respect to intraspecific variation in sexual size dimorphism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 558–565.     

Sexual selection and sexual size dimorphism in animals

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 SSD in a broad phylogenetic context.     

Sex‐specific selection and sexual size dimorphism in the waterstrider Aquarius remigis

We estimated selection on adult body size for two generations in two populations of Aquarius remigis, as part of a long‐term study of the adaptive significance of sexual size dimorphism (SSD). Net adult fitness was estimated from the following components: prereproductive survival, daily reproductive success (mating frequency or fecundity), and reproductive lifespan. Standardized selection gradients were estimated for total length and for thorax, abdomen, genital and mesofemur lengths. Although selection was generally weak and showed significant temporal and spatial heterogeneity, patterns were consistent with SSD. Prereproductive survival was strongly influenced by date of eclosion, but size (thorax and genital lengths in females; total and abdomen lengths in males) played a significant secondary role. Sexual selection favoured smaller males with longer external genitalia in one population. Net adult fitness was not significantly related to body size in females, but was negatively related to size (thorax and total length) in males.     

昆虫体型及性体型二型性的地理变异

体型是昆虫基本的形态特性,它会影响到昆虫几乎所有的生理和生活史特性。同种昆虫不同地理种群在体型上常表现出明显的渐变,导致这些渐变的环境因素包括温度、湿度、光照、寄主植物、种群密度等,并且多种环境因素也会对昆虫种群内个体体型产生影响。雌雄个体的体型存在差异,称性体型二型性。性体型二型性也显示了地理差异。这些差异形成的途径已经得到详细的分析,其形成机制导致多个假说的提出,这些假说又在多种昆虫中得到验证。本文从同一种昆虫不同种群间、同一种群内、雌雄虫个体间3个水平,对种内昆虫体型变异的方式,影响昆虫种群间体型变异和种群内昆虫体型的变异的环境因素,以及昆虫性体型二型性及其地理变异的现象等方面的研究进行了综述,并对未来的相关研究提供了建议。     

Effect of female size on fecundity and egg size in white-spotted charr: comparison between sea-run and resident forms

The relationships between fecundity, egg size and female size of sea-run form were compared with resident form, using white-spotted charr, Salvelinus leucomaenis. Both fecundity and egg size increase with female size. However, the relationship between egg size and female size differed significantly between the resident and sea-run forms. Egg sizes of sea-run and resident were similar even though sea-run fish were much larger. ? 1998 The Fisheries Society of the British Isles     

The interplay between natural and sexual selection in the evolution of sexual size dimorphism in Sceloporus lizards (Squamata: Phrynosomatidae)

Sexual size dimorphism (SSD) evolves because body size is usually related to reproductive success through different pathways in females and males. Female body size is strongly correlated with fecundity, while in males, body size is correlated with mating success. In many lizard species, males are larger than females, whereas in others, females are the larger sex, suggesting that selection on fecundity has been stronger than sexual selection on males. As placental development or egg retention requires more space within the abdominal cavity, it has been suggested that females of viviparous lizards have larger abdomens or body size than their oviparous relatives. Thus, it would be expected that females of viviparous species attain larger sizes than their oviparous relatives, generating more biased patterns of SSD. We test these predictions using lizards of the genus Sceloporus. After controlling for phylogenetic effects, our results confirm a strong relationship between female body size and fecundity, suggesting that selection for higher fecundity has had a main role in the evolution of female body size. However, oviparous and viviparous females exhibit similar sizes and allometric relationships. Even though there is a strong effect of body size on female fecundity, once phylogenetic effects are considered, we find that the slope of male on female body size is significantly larger than one, providing evidence of greater evolutionary divergence of male body size. These results suggest that the relative impact of sexual selection acting on males has been stronger than fecundity selection acting on females within Sceloporus lizards.     

Macroevolutionary patterns of bumblebee body size: detecting the interplay between natural and sexual selection

Bumblebees and other eusocial bees offer a unique opportunity to analyze the evolution of body size differences between sexes. The workers, being sterile females, are not subject to selection for reproductive function and thus provide a natural control for parsing the effects of selection on reproductive function (i.e., sexual and fecundity selection) from other natural selection. Using a phylogenetic comparative approach, we explored the allometric relationships among queens, males, and workers in 70 species of bumblebees (Bombus sp.). We found hyperallometry in thorax width for males relative to workers, indicating greater evolutionary divergence of body size in males than in sterile females. This is consistent with the hypothesis that selection for reproductive function, most probably sexual selection, has caused divergence in male size among species. The slope for males on workers was significantly steeper than that for queens on workers and the latter did not depart from isometry, providing further evidence of greater evolutionary divergence in male size than female size, and no evidence that reproductive selection has accelerated divergence of females. We did not detect significant hyperallometry when male size was regressed directly on queen size and our results thus add the genus Bombus to the increasing list of clades that have female-larger sexual size dimorphism and do not conform to Rensch's rule when analyzed according to standard methodology. Nevertheless, by using worker size as a common control, we were able to demonstrate that bumblee species do show the evolutionary pattern underlying Rensch's rule, that being correlated evolution of body size in males and females, but with greater evolutionary divergence in males.     

Resurrecting the differential mortality model of sexual size dimorphism

In some animal groups males may be several times smaller than females. One of the hypotheses proposed to explain the evolution of this extreme sexual size dimorphism (SSD) is the differential mortality model (DMM), which is based on the assumption that when males are the searching sex, higher male mortality relaxes male–male contest competition, leading to the adaptive evolution of early‐maturing, small males that are favoured by viability selection. Evidence for the main prediction of this model, i.e. that there is a negative relationship between differential mortality and SSD, has remained elusive. Using sex differences in pitfall trap catches – a proxy of sex differences in mobility and mortality – across 40 spider species, and using the evolutionary comparative method, we found significant negative relationships between differential mortality and SSD for three size traits. Thus, the DMM can still explain part of the observed variance in SSD.     

A biogeographic reversal in sexual size dimorphism along a continental temperature gradient

The magnitude and direction of sexual size dimorphism (SSD) varies greatly across the animal kingdom, reflecting differential selection pressures on the reproductive and/or ecological roles of males and females. If the selection pressures and constraints imposed on body size change along environmental gradients, then SSD will vary geographically in a predictable way. Here, we uncover a biogeographical reversal in SSD of lizards from Central and North America: in warm, low latitude environments, males are larger than females, but at colder, high latitudes, females are larger than males. Comparisons to expectations under a Brownian motion model of SSD evolution indicate that this pattern reflects differences in the evolutionary rates and/or trajectories of sex‐specific body sizes. The SSD gradient we found is strongly related to mean annual temperature, but is independent of species richness and body size differences among species within grid cells, suggesting that the biogeography of SSD reflects gradients in sexual and/or fecundity selection, rather than intersexual niche divergence to minimize intraspecific competition. We demonstrate that the SSD gradient is driven by stronger variation in male size than in female size and is independent of clutch mass. This suggests that gradients in sexual selection and male–male competition, rather than fecundity selection to maximize reproductive output by females in seasonal environments, are predominantly responsible for the gradient.     

Evolution of sexual size dimorphism in grouse and allies (Aves: Phasianidae) in relation to mating competition,fecundity demands and resource division

Sexual size dimorphism (SSD) is often assumed to be driven by three major selective processes: (1) sexual selection influencing male size and thus mating success, (2) fecundity selection acting on females and (3) inter‐sexual resource division favouring different size in males and females to reduce competition for resources. Sexual selection should be particularly strong in species that exhibit lek polygyny, since male mating success is highly skewed in such species. We investigated whether these three selective processes are related to SSD evolution in grouse and allies (Phasianidae). Male‐biased SSD increased with body size (Rensch’s rule) and lekking species exhibited more male‐biased SSD than nonlekking ones. Directional phylogenetic analyses indicated that lekking evolved before SSD, but conclusions were highly dependent on the body size traits and chosen model values. There was no relationship between SSD and male display agility, nor did resource division influence SSD. Although clutch mass increased with female body size it was not related to the degree of SSD. Taken together, the results are most consistent with the hypothesis that lekking behaviour led to the evolution of male‐biased SSD in Phasianidae.