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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In birds with bi-parental care, the provisioning link between prey capture and delivery to dependent offspring is regarded as often symmetric between the mates. However, in raptors, the larger female usually broods and feeds the nestlings, while the smaller male provides food for the family, assisted by the female in the latter part of the nestling period, if at all. Prey items are relatively large and often impossible for nestlings to handle without extended maternal assistance. We video-recorded prey delivery and handling in nests of a raptor with a wide diet, the Eurasian kestrel Falco tinnunculus, and simultaneously observed prey transfer from male to female outside the nest. The male selectively allocated larger items, in particular birds and larger mammals, to the female for further processing and feeding of nestlings, and smaller items, in particular lizards and smaller mammals, directly to the nestlings for unassisted feeding. Hence, from the video, the female appeared to have captured larger prey than the male, while in reality no difference existed. The female's size-biased interception of the male's prey provisioning line would maximize the male's foraging time, and maximize the female's control of the allocation of food between her own need and that of the offspring. The male would maximize his control of food allocation by capturing smaller prey. This conflict would select for larger dominant females and smaller energy-efficient males, and induce stronger selection the longer the female depends on the male for self-feeding, as a proportion of the offspring dependence period. [ABSTRACT FROM AUTHOR]

Generally in birds, the classic sex roles of male competition and female choice result in females providing most offspring care while males face uncertain parentage. In less than 5% of species, however, reversed courtship sex roles lead to predominantly male care and low extra-pair paternity. These role-reversed species usually have reversed sexual size dimorphism and polyandry, confirming that sexual selection acts most strongly on the sex with the smaller parental investment and accordingly higher potential reproductive rate. We used parentage analyses and observations from three field seasons to establish the social and genetic mating system of pheasant coucals, Centropus phasianinus, a tropical nesting cuckoo, where males are much smaller than females and provide most parental care. Pheasant coucals are socially monogamous and in this study males produced about 80% of calls in the dawn chorus, implying greater male sexual competition. Despite the substantial male investments, extra-pair paternity was unusually high for a socially monogamous, duetting species. Using two or more mismatches to determine extra-pair parentage, we found that 11 of 59 young (18.6%) in 10 of 21 broods (47.6%) were not sired by their putative father. Male incubation, starting early in the laying sequence, may give the female opportunity and reason to seek these extra-pair copulations. Monogamy, rather than the polyandry and sex-role reversal typical of its congener, C. grillii, may be the result of the large territory size, which could prevent females from monopolising multiple males. The pheasant coucal's exceptional combination of classic sex-roles and male-biased care for extra-pair young is hard to reconcile with current sexual selection theory, but may represent an intermediate stage in the evolution of polyandry or an evolutionary remnant of polyandry. [ABSTRACT FROM AUTHOR]

When environmental conditions exert sex-specific selection on offspring, mothers should benefit from biasing their sex allocation towards the sex with the highest fitness in a given environment. Yet, studies show mixed support for such adaptive strategies in vertebrates, which may be due to mechanistic constraints and/or weak selection on facultative sex allocation. In an attempt to disentangle these alternatives, we quantified sex-specific fitness returns and sex allocation (sex ratio and sex-specific mass at birth) according to maternal factors (body size, age, birth date, and litter size), habitat, and year in a viviparous snake with genotypic sex determination. We used data on 106 litters from 19 years of field survey in two nearby habitats occupied by the meadow viper Vipera ursinii ursinii in south-eastern France. Maternal reproductive investment and habitat quality had no differential effects on the growth and survival of sons and daughters. Sex ratio at birth was balanced despite a slight female-biased mortality before birth. No sexual mass dimorphism between offspring was evident. Sex allocation was almost random apart for a trend towards more male-biased litters as females grew older, which could be explained by an inbreeding avoidance strategy. Thus, a weak selection for facultative sex allocation seems sufficient to explain the almost equal sex allocation in the meadow viper. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In birds and mammals with sexual size dimorphism (SSD), the larger sex is typically more sensitive to adverse environmental conditions, such as food shortage, during ontogeny. However, some recent studies of altricial birds have found that the larger sex is less sensitive, apparently because large size renders an advantage in sibling competition. Still, this effect is not an inevitable outcome of sibling competition, because several studies of other species of altricial birds have found the traditional pattern. We investigated if the sexes differ in environmental sensitivity during ontogeny in the blue tit, a small altricial bird with c. 6% SSD in body mass (males larger than females). We performed a cross-fostering and brood size manipulation experiment during 2 years to investigate if the sexes were differently affected as regards body size (body mass, tarsus and wing length on day 14 after hatching) and pre-fledging survival. We also investigated if the relationship between body size and post-fledging survival differed between the sexes. Pre-fledging mortality was higher in enlarged than in reduced broods, representing poor and good environments, respectively, but the brood size manipulation did not affect the mortality rate of males and females differently. In both years, both males and females were smaller on day 14 after hatching in enlarged as compared to reduced broods. In one of the years, we also found significant Sex × Experiment interactions for body size, such that females were more affected by poor environmental conditions than that of males. Body size was positively correlated with post-fledging survival, but we found no interactive effects of sex and morphological traits on survival. We conclude that in the blue tit, females (the smaller sex) are more sensitive to adverse environmental conditions which, in our study, was manifest in terms of fledgling size. A review of published studies of sex differences in environmental sensitivity in sexually size-dimorphic altricial birds suggests that the smaller sex is more sensitive than the larger sex in species with large brood size and vice versa. [ABSTRACT FROM AUTHOR]

Mate search plays a central role in hypotheses for the adaptive significance of extreme female-biased sexual size dimorphism (SSD) in animals. Spiders (Araneae) are the only free-living terrestrial taxon where extreme SSD is common. The “gravity hypothesis” states that small body size in males is favoured during mate search in species where males have to climb to reach females, because body length is inversely proportional to achievable speed on vertical structures. However, locomotive performance of males may also depend on relative leg length. Here we examine selection on male body size and leg length during mate search in the highly dimorphic orb-weaving spiderArgiope aurantia, using a multivariate approach to distinguish selection targeted at different components of size. Further, we investigate the scaling relationships between male size and energy reserves, and the differential loss of reserves. Adult males do not feed while roving, and a size-dependent differential energy storage capacity may thus affect male performance during mate search. Contrary to predictions, large body size was favoured in one of two populations, and this was due to selection for longer legs. Male size was not under selection in the second population, but we detected direct selection for longer third legs. Males lost energy reserves during mate search, but this was independent of male size and storage capacity scaled isometrically with size. Thus, mate search is unlikely to lead to selection for small male size, but the hypothesis that relatively longer legs in male spiders reflect a search-adapted morphology is supported. [ABSTRACT FROM AUTHOR]

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle. [ABSTRACT FROM AUTHOR]

The general female bias in body size of animals is usually attributed to fecundity selection. While many studies have demonstrated a positive relationship between body size and fecundity, the most common interpretation of fecundity selection is that larger females have larger abdomens and can hold more eggs, yet the relationship between abdomen size and fecundity has rarely been examined. For the waterstrider, Aquarius remigis, we find a significant relationship between body size and fecundity and demonstrate that the target of fecundity selection is abdomen size. Thus, larger females have higher fecundities because they have larger abdomens and not because of their total size per se. The rate at which fecundity increases with increasing abdomen size exceeds that which would be expected due to a simple volume constraint and suggests that other factors, such as increased ability to obtain resources, may contribute to the increase in fecundity with body size. Selection intensities estimated from our data indicate that fecundity selection could be a significant selective force on both total and abdomen lengths. Previous studies have found that abdomen size increased faster than body size and thus, larger females had relatively larger abdomens. The relationship of abdomen length and thorax length in A. remigis is hypoallometric and indicates that larger females have relatively smaller abdomens. We hypothesize that this may reflect conservation of abdomen size in females developing under poor conditions. Finally, while egg size is not directly related to body size, we find a trade-off between egg size and number when female abdomen length is held constant, suggesting that selection on egg size may influence abdomen length only indirectly through its effects on fecundity. [ABSTRACT FROM AUTHOR]

Populations of Diaptomus leptopus (Copepoda: Calanoida) and other calanoid copepods exhibit varying degrees of sexual size dimorphism. We examined whether intraspecific or interspecific variation in dimorphism could be explained by allometry, and we examined the relationship between adult size attained and development rate to determine any relationship between the two. We compared the degree of sexual size dimorphism in D. leptopus and in other calanoid copepods inhabiting temporary and permanent habitats. Allometry did not explain variation in sexual size dimorphism within or among populations or among species. Permanence of habitat affected the degree of dimorphism: dimorphism was greater within and among species inhabiting temporary environments. Non-significant differences in development rate were found among populations and significant differences were found between sexes of D. leptopus when reared under identical laboratory conditions: males developed more rapidly than females but there was no general relationship between development rate and adult size. Potential adaptive hypotheses to explain the differences between populations inhabiting temporary and permanent habitats are discussed. [ABSTRACT FROM AUTHOR]