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

Abstract

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.