Sexual Size Dimorphism In Birds Research Articles

The macroevolution of sexual size dimorphism in birds

Abstract There is considerable variation of sexual size dimorphism (SSD) in body mass among animal groups, yet the drivers of interspecific variation in SSD are still poorly understood. Possible mechanisms have been suggested, including sexual selection, selection for fecundity in females, niche divergence between sexes, and allometry, yet their relative importance is still poorly understood. Here, we tested predictions of these four hypotheses in different avian groups using a large-scale dataset on SSD of body mass for 4761 species. Specifically, we estimated the probability of transition between male- and female-biased SSD, tested for differences in evolutionary rates of body mass evolution for males and females, and assessed the potential ecological and spatial correlates of SSD. Our results were consistent with the sexual selection, fecundity, and niche divergence hypotheses, but their support varied considerably among avian orders. In addition, we found little evidence that the direction of SSD affected the evolution of male or female body mass, and no relationship was detected between SSD and environmental predictors (i.e. temperature and precipitation seasonality, productivity, species richness, and absolute latitude). These results suggest that avian evolution of SSD is likely to be multifactorial, with sexual selection, fecundity, and niche divergence playing important roles in different avian orders.

Do Closely Related Species Influence Each Other's Sexual Size Dimorphism?

Size differences between males and females (sexual size dimorphism) are often seen in a variety of species. In birds of prey in particular, a phenomenon occurs where the female is larger than the male. One of the main hypotheses attempting to explain sexual size dimorphism in birds of prey is that the female and male differ in size to partition resources, like prey. There is also evidence that predator and prey body size are correlated, so predators of similar size may be in direct competition. It has been shown that when two closely related species interact in the same area, they are likely to be in competition for similar resources, like prey. This study is looking at sexual size dimorphism and closely related species interactions, which has never been looked at before in birds of prey. I am using the subfamily Buteoninae (Buteo Hawks) as my focal group. I will be using sexual size dimorphism ratios, estimates of genetic distance between closely related species and proportion of range overlap between different closely related lineages within the subfamily. I am expecting to see that if species are closely related and inhabit the same area they will have a decreased sexual size dimorphism. This is because both species are likely to be competing for the same resources, and to avoid competition the species will diverge in body size from one another. This will cause the male and female of each species to converge in size, reducing their sexual size dimorphism.

Global geographic patterns of sexual size dimorphism in birds: support for a latitudinal trend?

Sexual size dimorphism (SSD) is widespread among animals, and is a common indication of differential selection among males and females. Sexual selection theory predicts that SSD should increase as one sex competes more fiercely for access to mates, but it is unclear what effect spatial variation in ecology may have on this behavioral process and SSD. Here, we examine SSD across the class Aves in a spatial and phylogenetic framework, and test several a priori hypotheses regarding its relationship with climate. We mapped the global distribution of SSD from published descriptions of body size, distribution, and phylogenetic relationships across 2581 species of birds. We examined correlations between SSD and nine predictor variables representing a priori models of physical geography, climate, and climate variability. Our results show some support for a global latitudinal trend in SSD based on a weak prevalence of species with low or female‐biased SSD in the north, but substantial spatial heterogeneity. While several stronger relationships were observed between SSD and climate predictors within zoogeographical regions, no global relationship emerged that was consistent across multiple methods of analysis. The strong phylogenetic signal and conspicuous lack of support from phylogenetically corrected analyses suggests that any such relationship in birds is likely obscured by the idiosyncratic histories of different lineages. In this manner, our results agree with previous studies in other clades, leading us to conclude that the relationship between climate and SSD is at best complex. This suggests that SSD, and the behavioral dynamics underlying it, may be largely independent of environmental conditions at a global scale.

All eggs are made equal: meta‐analysis of egg sexual size dimorphism in birds

Sex-biased resource allocation in avian eggs has gained increasing interest. The adaptive explanations of such allocation are often related to life-history strategies of the studied species. In some species, egg sexual size dimorphism (SSD) was suggested to promote future size differences between adults of each sex. In other species, egg SSD was invoked as an adaptive means by which a mother balances sex-specific nestling mortality. According to the first scenario, mothers should produce bigger eggs for the bigger sex, thus across species, adult SSD should be a significant positive predictor of egg SSD. Under the second scenario, mothers should produce bigger eggs for the smaller sex. If different species use contrasting strategies, then a universal expectation is that there should be a significant relationship between the magnitude of adult SSD and the magnitude of egg SSD, irrespective of the direction of those differences. Our aim was to examine whether the direction of egg SSD is predicted by the direction of adult SSD or whether degree of egg SSD is related to degree of adult SSD. To answer that question, we performed meta-analysis of 63 studies, which included information on egg SSD of 65 effect sizes from 51 avian species. We found that across species, adult SSD does not predict egg SSD. More importantly, the observed variation in effect sizes in our data set was largely explained by sampling error (variance). Although adult SSD is undoubtedly a prominent feature of birds, there is little evidence for egg SSD across avian species.

Sex ratio, sex‐specific chick mortality and sexual size dimorphism in birds

It has been suggested that sexual size dimorphism (SSD) may influence sex ratios at different life stages. Higher energy requirements during growth associated with larger body size could lead to a greater mortality of the larger sex and ultimately to an overproduction of the smaller sex. To explore the associations between SSD and hatching and fledging sex ratio we performed a species-level analysis and a phylogenetically controlled analysis, based on 83 bird species. Overall, there was a significant inverse relationship between the degree of SSD and the proportion of males at hatching and fledging. Sex-specific mortality related to SSD showed a weak but persistent negative tendency, suggesting a mortality bias towards the larger sex. These results suggest that changes in relation to SSD may take place mainly at the conception stage, but could be adjusted during growth. However, conclusions should be treated cautiously as these relationships weaken when additional variables are considered.

Measuring sexual size dimorphism in birds

Numerous studies have examined sexual size dimorphism in birds and speculated upon the reasons for its existence. Whilst most studies have focused on individual species or groups of related species, a few have attempted to disentangle the various hypotheses that have been put forward to explain its occurrence. Typical of the latter studies is that by Jehl and Murray (1986), in which they argued that sexual size dimorphism was primarily the result of sexual selection (see also Bennett & Owens 2002). Although some studies have looked at patterns in sexual size dimorphism without calculating a figure to represent the difference (e.g. Amadon 1959), most have examined measurements of birds and used these to calculate such a figure. Traditionally in such studies, measurements used have included wing‐length, culmen‐length, tarsus‐length and mass, although McGillivray (1989) took the sum of 18 skeletal measurements and used their male and female means to determine sexual size dimorphism. Wing‐length has commonly been used to determine sexual size dimorphism, although lack of repeatability of measurements may render it less useful than skeletal measurements like tarsus‐length as in studies of Dunlin Calidris alpina (Blomqvist et al. 1997) and Savannah Sparrows Passerculus sandwichensis (Rising & Somers 1989); however, Gosler et al. (1998) found wing‐length measurements to be more repeatable than other metrics in a group of 27 passerines.

Measuring sexual size dimorphism in birds

Numerous studies have examined sexual size dimorphism in birds and speculated upon the reasons for its existence. Whilst most studies have focused on individual species or groups of related species, a few have attempted to disentangle the various hypotheses that have been put forward to explain its occurrence. Typical of the latter studies is that by Jehl and Murray (1986), in which they argued that sexual size dimorphism was primarily the result of sexual selection (see also Bennett & Owens 2002). Although some studies have looked at patterns in sexual size dimorphism without calculating a figure to represent the difference (e.g. Amadon 1959), most have examined measurements of birds and used these to calculate such a figure. Traditionally in such studies, measurements used have included wing-length, culmen-length, tarsus-length and mass, although McGillivray (1989) took the sum of 18 skeletal measurements and used their male and female means to determine sexual size dimorphism. Wing-length has commonly been used to determine sexual size dimorphism, although lack of repeatability of measurements may render it less useful than skeletal measurements like tarsus-length as in studies of Dunlin Calidris alpina (Blomqvist et al. 1997) and Savannah Sparrows Passerculus sandwichensis (Rising & Somers 1989); however, Gosler et al. (1998) found wing-length measurements to be more repeatable than other metrics in a group of 27 passerines.

Evolution of sexual size dimorphism in birds: test of hypotheses using blue tits in contrasted Mediterranean habitats

Abstract Many hypotheses, either sex-related or environment-related, have been proposed to explain sexual size dimorphism in birds. Two populations of blue tits provide an interesting case study for testing these hypotheses because they live in contrasting environments in continental France and in Corsica and exhibit different degree of sexual size dimorphism. Contrary to several predictions, the insular population is less dimorphic than the continental one but neither the sexual selection hypothesis nor the niche variation hypothesis explain the observed patterns. In the mainland population it is advantageous for both sexes to be large, and males are larger than females. In Corsica, however, reproductive success was greater for pairs in which the male was relatively small, i.e. pairs in which sexual size dimorphism is reduced. The most likely explanation is that interpopulation differences in sexual size dimorphism are determined not by sex-related factors, but by differences in sex-specific reproductive roles and responses to environmental factors. Because of environmental stress on the island as a result of food shortage and high parasite infestations, the share of parents in caring for young favours small size in males so that a reduced sexual size dimorphism is not the target of selection but a by-product of mechanisms that operate at the level of individual sexes.

Detecting Publication Bias in Meta-analyses: A Case Study of Fluctuating Asymmetry and Sexual Selection.

Those familiar with human nature and the publication process acknowledge that biases due to selective reporting of results are likely widespread in all fields of academic inquiry that depend on tools of statistical inference (e.g., see Begg and Berlin 1988; Iyengar and Greenhouse 1988; and the extensive discussion following each). However, although qualitative and quantitative methods exist for assessing the prevalence of selective reporting, and selective reporting has been studied in the medical and social sciences (Begg 1994, and references therein), the issue has received little attention in recent meta-analyses of ecological and evolutionary patterns. Clearly, “if publication bias is present, and if it operates in the same direction for all studies (as is likely), then [meta-analysis] is likely not only to produce biased summary estimates but also to produce estimates which are apparently precise and accurate leading to conclusions which may not only be wrong but appear convincing” (Begg and Berlin 1988, p. 437). To biologists unacquainted with the formal study of publication patterns, the terms “selective reporting” (statistical significance of an outcome influences its likelihood of being reported or published) and “publication bias” (the inflation of average effect size due to selective reporting) may imply a conscious intent to deceive, but this

SEXUAL DIMORPHISM, MATING SYSTEM AND BODY SIZE IN NEW WORLD BLACKBIRDS (ICTERINAE).

Although sexual selection is widely accepted as a primary functional cause of sexual size dimorphism in birds and mammals, results from some comparative studies have cast doubt on this conclusion. Chief among these contradictory results is the widespread association between body size and size dimorphism-large species tend to be more dimorphic than small species. This correlation is not directly predicted by the normal sexual selection scenario, and many hypotheses have been advanced to explain it. This paper reviews these hypotheses and evaluates them using data for the New World blackbirds (Icterinae). In this avian subfamily, (1) body size correlates with the intensity of sexual selection (as measured by mean harem size), and (2) size does not correlate with dimorphism if the effects of mating system are removed. Similar results are obtained when controlling for the confounding influence of phylogeny. Further, body size and mating system are associated with nesting dispersion. These results strongly argue that sexual dimorphism is a product of sexual selection in this subfamily, and suggest that either: (1) large body size itself, or the ecology of large species, promotes the development of coloniality and a polygynous mating system; or (2) polygyny and/or coloniality lead to the evolution of large size in both males and females. None of the other hypotheses examined predict an association between size and mating system, and all predict that size will correlate with dimorphism after the effects of mating system are removed. Thus, none of the other hypotheses seem applicable in this case. These results are compared to those obtained for other avian and mammalian taxa. Difficulties of analysis present in previous studies are discussed. I argue that it is inappropriate to assume that associations between a trait and body size or phylogeny are evidence of nonadaptive evolutionary "constraints."

Sexual Selection, Lek and Arena Behavior, and Sexual Size Dimorphism in Birds

Sexual Selection, Lek and Arena Behavior, and Sexual Size Dimorphism in Birds