Sex and environmental sensitivity in blue tit nestlings

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.     

Male‐skewed adult sex ratio,survival, mating opportunity and annual productivity in the Snowy Plover Charadrius alexandrinus

Sex differences in adult mortality may be responsible for male‐skewed adult sex ratios and male‐skewed parental care in some birds. Because a surplus of breeding males has been reported in serially polyandrous populations of Snowy Plover Charadrius alexandrinus, we examined sex ratio, early‐season nesting opportunities, adult survival and annual reproductive success of a Snowy Plover population at Monterey Bay, California. We tested the hypotheses that male adult survival was greater than female survival and that a sex difference in adult survival led to a skewed adult sex ratio, different mating opportunities and different annual productivity between the sexes. Virtually all females left chicks from their first broods to the care of the male and re‐nested with a new mate. As a result, females had time to parent three successful nesting attempts during the lengthy breeding season, whereas males had time for only two successful attempts. Among years, the median population of nesting Plovers was 96 males and 84 females (median difference = 9), resulting in one extra male per eight pairs. The number of potential breeders without mates during the early nesting period each year was higher in males than in females. Adult male survival (0.734 ± 0.028 se) was higher than female survival (0.693 ± 0.030 se) in top‐ranked models. Annually, females parented more successful clutches and fledged more chicks than their first mates of the season. Our results suggest that in C. alexandrinus a sex difference in adult survival results in a male‐skewed sex ratio, which creates more nesting opportunities and greater annual productivity for females than for males.     

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.     

Abundance of Snowy and Wilson's Plovers in the lower Laguna Madre region of Texas

ABSTRACT.   Snowy Plovers ( Charadrius alexandrinus ) and Wilson's Plovers ( C. wilsonia ) are shorebird species of increasing conservation concern, with populations apparently declining in North America. However, estimates of current populations are needed before initiating long-term monitoring or planning. In 2004, we estimated abundance of breeding Snowy and Wilson's plovers in the lower Laguna Madre region of Texas using occupancy abundance estimation. We made repeated visits to survey plots from April to June, recording the number of adults of both species observed and the amount of suitable breeding habitat within each plot. We considered Bayesian occupancy abundance models with and without habitat covariates to explain the abundance of both species. For both Snowy and Wilson's plovers, the number of birds counted in each plot was influenced by the amount of suitable breeding habitat within the plot (Snowy Plover α_habitat= 0.52, SD = 0.10, 95% CI = 0.33–0.71; Wilson's Plover α_habitat= 0.48, SD = 0.12, 95% CI = 0.24–0.71). Using the habitat covariate models for each species, we estimated that 416 adult Snowy Plovers (95% CI = 394–438) and 279 adult Wilson's Plovers (95% CI = 262–296) were present in our study area. Our results illustrate the use of a relatively new method for abundance estimation, and indicate that the lower Laguna Madre region of Texas is an important breeding area for both Snowy and Wilson's plovers. Given the documented and suspected population declines for Snowy and Wilson's plovers, we recommend protection of their breeding habitats along the coast of Texas from development and degradation resulting from unregulated use.     

Brood desertion in Kentish Plover Charadrius alexandrinus: an experimental test of parental quality and remating opportunities

Uniparental male care combined with polyandry is rare in birds, and the best known examples are in shorebirds Charadrii. There are two current hypotheses explaining why males care for the brood, whereas females desert and remate: either males are more capable than females at providing uniparental care (“parental quality hypothesis”) or females gain a greater increase in reproductive success by deserting than do males (“remating opportunity hypothesis”). I experimentally tested both hypotheses in Kentish Plover Charadrius alexandrinus, one of the few avian species in which either parent may desert the brood. By experimentally removing one parent when the chicks hatched, I found that male-tended broods had better survival than female-tended ones, particularly up to 6 days after hatching. It is unlikely that differential brood mortality was caused by chilling of the chicks, since the brooding behaviour of males and females was not different. The results of this study are consistent with the explanation that male-tended broods survived better because males were better able to protect the brood from attacks by conspecifics and predators. The remating opportunity hypothesis was also corroborated because single females acquired new mates faster than did single males. The results of this study suggest that both the better parental capability of males and the greater remating opportunities of females predispose Kentish Plovers for uniparental male care, desertion by the female parent and sequential polyandry.     

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.     

Proximate causes of sexual size dimorphism in Colorado pikeminnow, a long‐lived cyprinid

Evidence for sexual size dimorphism (SSD) and its possible causes were examined in the endangered Colorado pikeminnow Ptychocheilus lucius, a large, piscivorous, cyprinid endemic to the Colorado River system of North America. Individuals representing 18–24% of the upper Colorado River population were captured, measured, sexed and released in 1999 and 2000. Differing male and female total length‐(L_T) frequency distributions revealed SSD with females having greater mean and maximum sizes than males. Although both sexes exhibit indeterminate post‐maturity growth, growth trajectories differed. The point of trajectory divergence was not established, but slowed male growth might coincide with the onset of maturation. Differing growth rate was the dominant proximate cause of SSD, accounting for an estimated 61% of the observed difference in mean adult L_T. The degree of SSD in adults, however, was also related to two other factors. Evidence suggests males become sexually active at a smaller size and earlier age than females; a 2 year difference, suggested here, accounted for an estimated 12% of the between‐sex difference in mean adult L_T. Temporal shifts in gender‐specific survival accounted for an additional 27% of the observed between‐sex difference in mean adult L_T. Estimated age distributions indicated a higher number of older females than older males and more younger males than younger females in the population during the period of sampling. Dissimilarity of age distributions was an unexpected result because the male : female population sex ratio was 1 : 1 and estimates of long‐term annual survival for adult males and females were equal (88%). Future assessments of SSD in this population are apt to vary depending on the prior history of short‐term gender‐specific survival. Without recognizing SSD, non‐gender‐specific growth curves overestimate mean age of adult females and underestimate mean age of adult males of given L_T. Assuming age 8 years for first reproduction in males and age 10 years for females, the adult male : female ratio was estimated as 1·1 : 1 and mean adult age, or generation time, was estimated as 16·4 years for males and 18·4 years for females.     

TRAPPING AND RINGING OF EGYPTIAN GEESE AND AFRICAN SHELDUCK AT VOGELVLEI,CAPE

Many small plovers Charadrius spp. have sexually monomorphic plumage and cryptic sexual size dimorphism. The objective of our study was to assess the variation in body sizes between male and female plovers breeding in Madagascar. We collected blood samples and data on adult body sizes of four small plovers (Madagascar Plover Charadrius thoracicus, Kittlitz's Plover C. pecuarius, White-fronted Plover C. marginatus and Three-banded Plover C. tricollaris), and used molecular genetic markers to sex the adults. We found significant differences in body size among the four species, and between sexes. Furthermore, individuals from the southern ecoregion tended to be larger than in the western ecoregion. The Madagascar Plover's body size was significantly more dimorphic than the Kittlitz's and White-fronted Plovers. Breeding Malagasy plovers' show significant sexual size dimorphism (SSD): Madagascar Plover females were heavier and had longer wings than males, whereas the males had longer tarsi; in White-fronted Plover only wing length was different between the sexes. Taken together, our work reports SSD in small African plovers that exhibit monomorphic plumage, and we propose that SSD may be more common than currently acknowledged; we term this 'cryptic sexual size dimorphism'. Our results also suggest sexual selection and/or natural selection exert different pressures on body size in different Malagasy plover species.     

Different paths to sexual size dimorphism in two praying mantids,Pseudomantis albofimbriata and Hierodula majuscula

Sexual size dimorphism （SSD） is widespread among diverse animal taxa and has attracted the attention of evolutionary biologists for over a century. SSD is likely to be adaptive and the result of divergent selection on different size optima for males and females, given their different roles in reproduction. The developmental trajectory leading to SSD may help us to understand how selection acts on male and female size. Here, we describe the growth and development of two Australian praying mantids, Pseudomantis albofimbriata and Hierodula majuscula including the number of moults, time to adulthood, size at each moult, and the degree of SSD. While both species exhibit the common pattern of female-biased SSD, the number of moults required for individuals to reach adulthood differed between males and females and between species. Despite their larger adult size, P. albofimbriata females require fewer moults and less time than males to reach adulthood, but are significantly larger than males from the second instar onwards. In contrast, H. majuscula males reached adulthood in fewer moults, and less time than females, however males and females did not differ in size until females went through their final moult into adulthood. H. majuscula also required more time and more moults to reach adulthood than 17. albofimbriata. We discuss these different developmental pathways in light of the existing knowledge of reproductive biology for each species. We also suggest that these differences may relate to the different phenologies that occur in strongly seasonal temperate environments compared with those in the tropics. This study provides evidence that SSD can result from two different patterns of growth and development in closely related species.     

Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata)

Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons ( Morelia spilota ) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geographic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 113–125.     

Macroevolutionary pattern of sexual size dimorphism in geckos corresponds to intraspecific temperature‐induced variation

Many animal lineages exhibit allometry in sexual size dimorphism (SSD), known as ‘Rensch’s rule’. When applied to the interspecific level, this rule states that males are more evolutionary plastic in body size than females and that male‐biased SSD increases with body size. One of the explanations for the occurrence of Rensch’s rule is the differential‐plasticity hypothesis assuming that higher evolutionary plasticity in males is a consequence of larger sensitivity of male growth to environmental cues. We have confirmed the pattern consistent with Rensch’s rule among species of the gecko genus Paroedura and followed the ontogeny of SSD at three constant temperatures in a male‐larger species (Paroedura picta). In this species, males exhibited larger temperature‐induced phenotypic plasticity in final body size than females, and body size and SSD correlated across temperatures. This result supports the differential‐plasticity hypothesis and points to the role phenotypic plasticity plays in generating of evolutionary novelties.     

THE EVOLUTION OF FEMALE-BIASED SEXUAL SIZE DIMORPHISM: A POPULATION-LEVEL COMPARATIVE STUDY IN HORNED LIZARDS (PHRYNOSOMA)

Female-biased sexual size dimorphism is uncommon among vertebrates and traditionally has been attributed to asymmetric selective pressures favoring large fecund females (the fecundity-advantage hypothesis) and/or small mobile males (the small-male advantage hypothesis). I use a phylogenetically based comparative method to address these hypotheses for the evolution and maintenance of sexual size dimorphism among populations of three closely related lizard species (Phrynosoma douglasi, P. ditmarsi, and P. hernandezi). With independent contrasts I estimate evolutionary correlations among female body size, male body size, and sexual size dimorphism (SSD) to determine whether males have become small, females have become large, or both sexes have diverged concurrently in body size during the evolutionary Xhistory of this group. Population differences in degree of SSD are inversely correlated with average male body size, but are not correlated with average female body size. Thus, variation in SSD among populations has occurred predominantly through changes in male size, suggesting that selective pressures on small males may affect degree of SSD in this group. I explore three possible evolutionary mechanisms by which the mean male body size in a population could evolve: changes in size at maturity, changes in the variance of male body sizes, and changes in skewness of male body size distributions. Comparative analyses indicate that population differentiation in male body size is achieved by changes in male size at maturity, without changes in the variance or skewness of male and female size distributions. This study demonstrates the potential of comparative methods at lower taxonomic levels (among populations and closely related species) for studying microevolutionary processes that underlie population differentiation.     

Breeding biology of the Kentish Plover Charadrius alexandrinus in the Sabkhat Al-Fasl Lagoons,Saudi Arabia (Aves: Charadriiformes)

The breeding biology of the Kentish Plover Charadrius alexandrinus was studied in the Sabkhat Al-Fasl Lagoons of Saudi Arabia, where ground temperatures may ex- ceed 55°C in summer. Although halophytic bushes are abundant, this species seems to prefer nesting at exposed sites. Biparental brood care was common: the females were absent in only three out of 24 families. Kentish Plovers attended their nests more than 80% over the full day and more than 90% of the time during day-time, and the number of change-overs increased during the hottest parts of the day which could be due to the possibility that a single parent cannot protect the eggs and itself from overheating.     

SEX‐SPECIFIC SELECTION AND INTRASPECIFIC VARIATION IN SEXUAL SIZE DIMORPHISM

Sexual size dimorphism (SSD) is thought to evolve due to sex differences in selection on body size, but it is largely unknown whether intraspecific variation in SSD reflects differences in sex‐specific selection among populations. We addressed this question by comparing viability selection between two island populations of the brown anole lizard (Anolis sagrei) that differ in the magnitude of male‐biased SSD. On both islands, females experienced stabilizing selection favoring intermediate size whereas males experienced directional selection favoring larger size. Thus, sex‐specific selection matched the overall pattern of male‐biased SSD, but population differences in the magnitude of SSD were not associated with local differences in selection. Rather, population differences in SSD appear to result from underlying differences in the environmental potential for a rapid growth, coupled with sex‐specific phenotypic plasticity. Males grew more slowly on the island with low SSD whereas growth of females did not differ between islands. Both sexes had substantially lower mass per unit length on the island with low SSD, suggesting that they were in a relatively poorer energetic condition. We propose that this energetic constraint disproportionately impacts growth of males due to their greater absolute energy requirements, thus driving intraspecific variation in SSD.     

Mate Choice by Chemical Cues in Western Redback (Plethodon vehiculum) and Dunn's (P. dunni) Salamanders

Western redback and Dunn's salamanders (Plethodon vehiculum and Plethodon dunni, respectively) can distinguish between potential mates by using chemical cues. In laboratory choice tests, adult males of both species showed significant discrimination between chemical cues of gravid females over non-gravid females of equal body size. Furthermore, males of both species differentiated the odour of paired gravid females that differed by ? 5 mm snout-vent length (SVL). Given that clutch size is related to female body size in these species, adult males may be able to distinguish between females via cues that signal potentially high female reproductive success. In choice tests, P. vehiculum females did not discriminate between two relatively large males that differed by ? 5 mm SVL. However, females of P. vehiculum did discriminate between two relatively small males that differed by the same amount. Apparently, P. vehiculum females ranked males by both absolute and relative body size using chemical cues. This pattern could reflect a female preference for large males or that females avoid mating with the smallest males.     

山地麻蜥个体发育过程中头部两性异形和食性的变化

研究了山地麻蜥(Eremias brenchleyi)个体发育过程中头部两性异形和食性的变化．成体个体大小(SVL)无显著的两性差异,但雄体具有较大的头部(头长和头宽)．头部两性异形在孵出幼体就已存在,成体头部两性异形比幼体(包括孵出幼体)更为显著,雄性较大的头部与其头部随SVL的增长速率大于雌性有关．两性头部总体上随SVL呈异速增长,表现为个体发育过程中头长和头宽与SVL的线性回归方程斜率有显著的变化．孵出幼体有相对较大的头部,这种形态特征是胚胎优先保证生态学意义更为显著的头部生长的结果,有利于孵出幼体的早期生存和生长．相对头部大小在个体发育过程中有显著的变化．不同性别和大小的山地麻蜥摄入食物的种类及各种食物在摄入食物中所占的比例有一定程度的差别,食物生态位宽度和重叠度因此有一定的差别．然而,没有直接的证据表明头部两性异形能导致两性食物生态位的明显分离,并有利于减缓两性个体对食物资源的竞争。     

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.     

Variation and Sexual Dimorphism of Body Size in the Plateau Brown Frog along an Altitudinal Gradient

Variation in body size and sexual size dimorphism(SSD) can have important consequences for animal ecology, behavior, population dynamics and the evolution of life-history traits. Organisms are expected to be larger in colder climate(i.e., Bergmann's rule) and SSD varies with body size(i.e., Rensch's rule). However, the underlying mechanisms are still elusive. The plateau brown frog(Rana kukunoris), a medium-sized anuran species with femalebiased SSD, is endemic to the Qinghai-Tibetan Plateau(QTP). From 1797 m(Maoxiang'ping) to 3453 m(Heihe'qiao) in the eastern margin of the QTP, we surveyed 10 populations of R. kukunoris and collected phalanges and snout vent length(SVL) data for 258 adult individuals(199 males versus 59 females). Based on these data, we explored how body size and SSD varying along the altitudinal gradient and examined the corresponding effects of temperature. We found body size to be larger at higher altitude for males but not for females, with likely effects from the temperature on the variation in male body size. Sex differences in growth rates may be the main cause of the variation in SSD. Our results suggested that only males follow the Bergmann's rule and variation in SSD of R. kukunoris do not support the Rensch's rule and its inverse. Therefore, the variations of body size can be different between sexes and the applicability of both Bergmann's rule and Rensch's rule should depend on species and environment where they live.     

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.