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.     

The evolution of sexual size dimorphism in the house finch. III. Developmental basis

Sexual size dimorphism of adults proximately results from a combination of sexually dimorphic growth patterns and selection on growing individuals. Yet, most studies of the evolution of dimorphism have focused on correlates of only adult morphologies. Here we examined the ontogeny of sexual size dimorphism in an isolated population of the house finch (Carpodacus mexicanus). Sexes differed in growth rates and growth duration; in most traits, females grew faster than males, but males grew for a longer period. Sexual dimorphism in bill traits (bill length, width, depth) and in body traits (wing, tarsus, and tail length; mass) developed during different periods of ontogeny. Growth of bill traits was most different between sexes during the juvenile period (after leaving the nest), whereas growth of body traits was most sexually dimorphic during the first few days after hatching. Postgrowth selection on juveniles strongly influenced sexual dimorphism in all traits; in some traits, this selection canceled or reversed dimorphism patterns produced by growth differences between sexes. The net result was that adult sexual dimorphism, to a large degree, was an outcome of selection for survival during juvenile stages. We suggest that previously documented fast and extensive divergence of house finch populations in sexual size dimorphism may be partially produced by distinct environmental conditions during growth in these populations.     

Sexual size and shape dimorphism and allometric scaling patterns in head traits in the New Zealand common gecko Woodworthia maculatus

Sexual dimorphism in shape and size is widespread across animal taxa and arises when natural or sexual selection operates differently on the sexes. Male and female common geckos (Woodworthia maculatus; formerly Hoplodactylus maculatus) in New Zealand do not appear to experience different viability selection pressure, nor do males appear to be under intense pre-copulatory sexual selection. It was therefore predicted that this species would be sexually monomorphic with regard to body size and the size and shape of the head. In line with the prediction, there was no sexual difference in head width, depth, or length or in lateral head shape. However, contrary to prediction, males had a larger body and lateral head size than females. This study suggests that males, at least on Maud Island, NZ, might be under stronger pre-copulatory sexual selection than previously recognized and thus have evolved larger heads (i.e. lateral head size) for use in male combat for females. Allometric scaling patterns do not differ between the sexes and suggest that head width and depth are under directional selection whereas lateral head size is under stabilizing selection. Diet ecology – an agent of natural selection common to both sexes – is likely largely responsible for the observed patterns of head size and shape and the lack of sexual dimorphism in them.     

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.     

Male-biased size dimorphism in ichneumonine wasps (Hymenoptera: Ichneumonidae) – the role of sexual selection for large male size

Abstract.  1. Sexual differences in body size are expected to evolve when selection on female and male sizes favours different optima.
2. Insects have typically female-biased size dimorphism that is usually explained by the strong fecundity advantage of larger size in females. However, numerous exceptions to this general pattern have led to the search for selective pressures favouring larger size in males.
3. In this study, the benefits of large size were investigated in males of four species of ichneumonine wasps, a species-rich group of parasitoids, many representatives of which exhibit male-biased size dimorphism.
4. Mating behaviour of all ichneumonine wasps are characterised by pre-copulatory struggles, in the course of which males attempt to override female reluctance to mate. A series of laboratory trials was conducted to study the determinants of male mating success.
5. A tendency was found for larger males as well as those in better condition to be more successful in achieving copulations. Size dimorphism of the species studied, mostly male-biased in hind tibia length but female-biased in body weight, indicates that sexual selection in males favours longer bodies and appendages rather than larger weight.
6. The qualitative similarity of the mating patterns suggests that sexual selection cannot completely explain the considerable among-species differences in sexual size dimorphism.
7. The present study cautions against using various size indices as equivalents for calculating sexual size dimorphism.
8. It is suggested that female reluctance in ichneumonine wasps functions as a mechanism of female mate assessment.     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.     

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.     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

Seasonal change in sexual size dimorphism of the major cheliped in the hermit crab <Emphasis Type="Italic">Pagurus minutus</Emphasis>

Sexual size dimorphism is a common phenomenon in the animal kingdom, and its seasonal change has been reported in some species that possess traits dimorphic only in males and specialized for male mating success. However, few studies have examined seasonal change in sexual dimorphism of traits possessed by both sexes. Here, we examined the reproductive biology of the hermit crab Pagurus minutus, at a sandflat in the Waka River estuary, Japan, with special reference to seasonal changes in sexual dimorphism of the large claw (major cheliped) size by conducting population and precopulatory guarding-pair sampling. Previous investigation demonstrated that the major cheliped is used as a weapon, and its size, more than body size, determines the winner in male–male contests of this species. We found ovigerous females from November to April, peaking in January, when 80% of females were ovigerous. Sexual size dimorphism of the major cheliped was observed; the degree of dimorphism increased in the reproductive season, when only males possessed an enlarged major cheliped. In addition, in the reproductive season, precopulatory guarding males had a larger body and larger relative size of the major cheliped than did solitary males, although the major cheliped size in guarding males seemed to reach an upper limit. These results suggest that seasonal change in sexual dimorphism of the major cheliped size in P. minutus strongly reflects sexual selection favoring the development of this natural weaponry, and that the degree of the dimorphism might be limited through natural selection.     

蓝尾石龙子的头部两性异形和食性

通过测量头、体大小和胃检研究浙江泰顺产蓝尾石龙子 (Eumeceselegans)个体发育过程中两性异形和食性的变化。蓝尾石龙子成体个体大小和头部大小的两性差异显著 ,雄性大于雌性。不同发育阶段雌性头长与SVL的线性回归斜率无显著差异 ,头宽与SVL线性回归斜率的差异显著 ,成体和SVL <5 0mm幼体头宽随SVL的增长速率显著小于SVL为 5 0 - 6 9mm的幼体。雄性头部相对于SVL呈加速式异速生长。两性比较发现 :雌雄幼体头长和头宽随SVL的增长速率无显著差异 ,SVL <5 0mm幼体特定SVL的头长和头宽无显著的两性差异 ,但SVL为 5 0 - 6 9mm的雄性幼体头长和头宽大于SVL相同的雌性幼体 ;雄性成体头长和头宽随SVL的增长速率显著大于雌性。SVL <5 0mm的雌性幼体头部相对小于SVL为 5 0 - 6 9mm的同性幼体 ,性成熟雌体头部相对小于SVL为 5 0 - 6 9mm的同性幼体。雌性幼体、雄性幼体、雌性成体和雄性成体食物生态位宽度分别为 12 3、 12 5、 4 8和 14 4。雌雄幼体食物生态位重叠度最高 ,雌雄成体食物生态位重叠度次之 ,成体与幼体食物生态位重叠度较小。成体摄入食饵的大小 (用胃内完整食物长度的平均值表示 )和变化范围大于幼体。两性成、幼体摄入的食饵大小差异显著。两性个体摄入的食饵大小均与其SVL呈正相关 ,表明较大     

Environmental influences on the sexual dimorphism in body size of western bobcats

Sexual size dimorphism might be influenced by environmental constraints on sexual selection or by intraspecific competition between males and females. We studied bobcats (Lynx rufus) in collections of museum specimens from western North America to examine these hypotheses. Structural body size was estimated from several measurements of the skull, ln-transformed and indexed through principal components analysis. Sexual dimorphism in body size was estimated from the difference in size index of males and females, and compared to geographic and climatic variables associated with biotic provinces (ecoregions). Of several climatic variables that were associated with bobcat body size, only seasonality of climate was associated with sexual dimorphism. Sexual size dimorphism, longitude, elevation, and seasonality were intercorrelated. As longitude decreased (moving inland from west-coastal ecoregions), sexual dimorphism decreased with the increased elevation and seasonality of continental climates of the Rocky Mountains. We suggest that increased seasonality and the need for fasting endurance by females may place constraints on the degree of sexual dimorphism in bobcats. Sexual dimorphism of body size and sexual size dimorphism of trophic structures (teeth) exhibited a strong positive association over geography, thus indirectly supporting the hypothesis that intrasexual competition for prey could account for the geographic variation in sexual size dimorphism. Thus, both environmental constraints on sexual selection of body size and intersexual competition were supported as possible explanations of the degree of sexual size dimorphism that occurs in populations of bobcats.     

灰椋鸟身体大小和内脏器官形态的两性差异

鸟类性二态现象广泛存在,比如身体大小、羽色等,性二态很可能是自然选择和性选择共同作用的结果。为了探索和更好地了解雀形目鸟类身体大小性二态的进化,在2019年繁殖季节早期研究了灰椋鸟(Sturnus cineraceus)野外种群身体大小和内脏器官形态的两性差异。结果表明,除嘴宽外,其他身体特征参数均雄性显著大于雌性,表现出雄性偏向的身体大小二态性。内脏器官大小两性间无显著差异。灰椋鸟是聚群生活的鸟类,雌雄鸟常一起觅食,食性相似,雌雄鸟内脏器官和消化道形态差异不显著,暗示食性分化在灰椋鸟身体大小性二态进化中的作用并不明显;雄鸟体型较大的原因可能是其在巢址竞争、配偶保护中适应进化的结果。本研究明确了灰椋鸟身体大小的两性差异,对于该物种身体大小性二态进化的确切原因,尚需更多研究。     

Multivariate morphometries and sexual dimorphism in the orb-web spider Metellina segmentata (Clerck, 1757) (Araneae, Metidae)

Sexual dimorphism in body size and leg length was investigated in a common orb-weaving spider of Ireland and northern Europe, Metellina segmentata (Clerck, 1757) (Araneae, Metidae). Univariate and multivariate analyses of sexual dimorphism revealed that a greater proportion of between sex variation (sexual dimorphism) was attributable to variation in shape than in size. Significant differences were found in the scores for males and females for the first two principal components. PCI (shape) accounted for 44.25% of the variation and PC2 (size) 13.01% of the variation. Although M. segmentata has been attributed with minimal sexual size dimorphism, females were markedly heavier, possibly a reflection of differential reproductive investment between the sexes, but males had markedly longer legs and broader prosoma. The results are discussed with regard to existing theories of natural and sexual selection, particularly those concerning sexual cannibalism and differential life history traits in males and females. Models that attempt to explain the evolution of sexual size dimorphism in spiders and of the web builders in particular, fail to account for the multivariate nature of dimorphism, especially with respect to shape.     

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.     

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.     

Function‐related Drivers of Skull Morphometric Variation and Sexual Size Dimorphism in a Subterranean Rodent,Plateau Zokor (Eospalax baileyi)

Sexual dimorphism is prevalent in most living organisms. The difference in size between sexes of a given species is generally known as sexual size dimorphism (SSD). The magnitude of the SSD is determined by Rensch's rule where size dimorphism increases with increasing body size when the male is the larger sex and decreases with increasing average body size when the female is the larger sex. The unique underground environment that zokors (Eospalax baileyi) live under in the severe habitat of the Qinghai‐Tibetan Plateau (QTP) could create SSD selection pressures that may or may not be supported by Rensch's rule, making this scientific question worthy of investigation. In this study, we investigated the individual variation between sexes in body size and SSD of plateau zokors using measurements of 19 morphological traits. We also investigated the evolutionary mechanisms underlying SSD in plateau zokors. Moreover, we applied Rensch's rule to all extant zokor species. Our results showed male‐biased SSD in plateau zokors: The body‐ and head‐related measurements were greater in males than in females. Linear regression analysis between body length, body weight, and carcass weight showed significant relationships with some traits such as skull length, lower incisor length, and tympanic bulla width, which might support our prediction that males have faster growth rates than females. Further, the SSD pattern corroborated the assumption of Rensch's rule in plateau zokors but not in the other zokor species. Our findings suggest that the natural underground habitat and behavioral differences between sexes can generate selection pressures on male traits and contribute to the evolution of SSD in plateau zokors.     

Sexual size dimorphism and assortative mating in Carolina Wrens

ABSTRACT.   Sexual size dimorphism (SSD) may be due to sexual and natural selection, but identifying specific mechanisms that generate such dimorphism in a species is difficult. I examined SSD in Carolina Wrens ( Thryothorus ludovicianus ) by examining (1) the degree of SSD in the population and between pairs using five morphometrics, (2) assortative mating patterns based on size and age, and (3) relationships between size and longevity. Analysis revealed that males were significantly larger than females in all body measurements. For example, mass, bill, and wing measurements yielded a canonical variable that permitted separation of the sexes and linear classification functions correctly determined the sex of 95% (238/250) of all wrens measured. No evidence was found to suggest that SSD was related to resource partitioning. However, assortative mating trends based on morphometrics (e.g., wing length), positive associations between longevity and morphometrics (e.g., wing length in females and body size in males), and intense male-male contests for territorial resources year-round provide evidence that sexual selection may contribute to SSD in Carolina Wrens.     

Sexual dimorphism in relation to current selection in the house finch

Abstract.— Sexual dimorphism is thought to have evolved in response to selection pressures that differ between males and females. Our aim in this study was to determine the role of current net selection in shaping and maintaining contemporary sexual dimorphism in a recently established population of the house finch ( Carpodacus mexicanus ) in Montana. We found strong differences between sexes in direction of selection on sexually dimorphic traits, significant heritabilities of these traits, and a close congruence between current selection and observed sexual dimorphism in Montana house finches. Strong directional selection on sexually dimorphic traits and similar intensities of selection in each sex suggested that sexual dimorphism arises from adaptive responses in males and females, with both sexes being far from their local fitness optimum. This pattern is expected when a recently established population experiences continuous immigration from ecologically distinct areas of a species range or as a result of widely fluctuating selection pressures, as found in our study. Strong and sexually dimorphic selection pressures on heritable morphological traits, in combination with low phenotypic and genetic covariation among these traits during growth, may have accounted for close congruence between current selection and observed sexual dimorphism in the house finch. This conclusion is consistent with the profound adaptive population divergence in sexual dimorphism that accompanied very successful colonization of most of the North America by the house finch over the last 50 years.     

The evolution of sexual dimorphism in parasitic cuckoos: sexual selection or coevolution?

Sexual dimorphism is ubiquitous in animals and can result from selection pressure on one or both sexes. Sexual selection has become the predominant explanation for the evolution of sexual dimorphism, with strong selection on size-related mating success in males being the most common situation. The cuckoos (family Cuculidae) provide an exceptional case in which both sexes of many species are freed from the burden of parental care but where coevolution between parasitic cuckoos and their hosts also results in intense selection. Here, we show that size and plumage differences between the sexes in parasitic cuckoos are more likely the result of coevolution than sexual selection. While both sexes changed in size as brood parasitism evolved, we find no evidence for selection on males to become larger. Rather, our analysis indicates stronger selection on parasitic females to become smaller, resulting in a shift from dimorphism with larger females in cuckoos with parental care to dimorphism with larger males in parasitic species. In addition, the evolution of brood parasitism was associated with more cryptic plumage in both sexes, but especially in females, a result that contrasts with the strong plumage dimorphism seen in some other parasitic birds. Examination of the three independent origins of brood parasitism suggests that different parasitic cuckoo lineages followed divergent evolutionary pathways to successful brood parasitism. These results argue for the powerful role of parasite-host coevolution in shaping cuckoo life histories in general and sexual dimorphism in particular.     

Development of intraspecific size variation in black coucals,white-browed coucals and ruffs from hatching to fledging

Most studies on sexual size dimorphism address proximate and functional questions related to adults, but sexual size dimorphism usually develops during ontogeny and developmental trajectories of sexual size dimorphism are poorly understood. We studied three bird species with variation in adult sexual size dimorphism: black coucals (females 69% heavier than males), white-browed coucals (females 13% heavier than males) and ruffs (males 70% heavier than females). Using a flexible Bayesian generalized additive model framework (GAMM), we examined when and how sexual size dimorphism developed in body mass, tarsus length and bill length from hatching until fledging. In ruffs, we additionally examined the development of intrasexual size variation among three morphs (Independents, Satellites and Faeders), which creates another level of variation in adult size of males and females. We found that 27–100% of the adult inter- and intrasexual size variation developed until fledging although none of the species completed growth during the observational period. In general, the larger sex/morph grew more quickly and reached its maximal absolute growth rate later than the smaller sex/morph. However, when the daily increase in body mass was modelled as a proportion, growth patterns were synchronized between and within sexes. Growth broadly followed sigmoidal asymptotic models, however only with the flexible GAMM approach, residual distributions were homogeneous over the entire observation periods. These results provide a platform for future studies to relate variation in growth to selective pressures and proximate mechanisms in these three species, and they highlight the advantage of using a flexible model approach for examining growth variation during ontogeny.