Data from: Ecology and biogeography of sexual size dimorphism in squamates

Aim: Sexual size dimorphism (SSD) is widespread in the animal kingdom. The direction and magnitude of SSD differ considerably across taxa, potentially due to different selective forces acting on female and male sizes. We assembled a comprehensive database of mean body sizes of females and males within squamate species. We then tested for associations between the degree and direction of sexual size dimorphism and environmental factors, clutch sizes, reproductive modes, substrate types, and species richness (a common measure of interspecific competition).

Location: Global

Time period: Present

Major taxa studied: Squamata (Reptilia)

Methods: We studied SSD patterns and their correlates for 11,792 squamate species. We also tested the effect of the number of putative competitors on SSD within (~9915 km²) grid cells. We applied phylogenetic path analysis and phylogenetic generalized least squares regression (PGLS) at the species, and applied spatial auto-regressive (SAR) multiple regressions at the assemblage levels.

Results: In general, snake females are larger than males, whereas male lizards are larger, on average, than females. Female squamates in general are larger than males in cold regions, while in warm regions, particularly in deserts, males were usually larger than females. SSD became more female-biased (i.e., larger females) as clutch size increased, and viviparous taxa had more female-biased SSD. There was little relationship between the magnitude of SSD and species richness. Sexual size dimorphism did not vary significantly across substrate types.

Main conclusion: We suspect that the mechanisms driving squamate SSD differ between oviparous and viviparous taxa. The more female-biased SSD in colder regions is likely driven by fecundity selection, while a male bias in warmer regions may be associated with sexual selection. However, we found little evidence to suggest natural selection for substrates, or resource-based competition, affects squamate sexual size dimorphism and suspect the underlying hypotheses may be flawed, and/or that species richness is a poor measure of the intensity of interspecific competition.