Interspecific variation in paper wasp body size supports the converse Bergmann’s rule

Abstract

Variation in body size is a conspicuous phenomenon and much scientific interest has been devoted to study the processes affecting this trait. Clines of body size across geographical areas have been detected, supporting ecogeographical rules that describe spatial patterns of phenotypic differences driven by environmental variation. In social insects, the causes of body size variation have been best studied at the within-species level, even though interspecific variation is far more striking. This is the case of Polistes paper wasps, a model genus for sociobiological and ecological studies. Here, we examined whether environmental variation predicts interspecific differences in the body size of New World Polistes, a monophyletic clade widespread through the Americas. Dried museum specimens comprising females from 37 species and males from 28 species were measured. In a phylogenetically controlled analysis, body size was associated with latitude, environmental temperature, and environmental seasonality. Larger species were more likely at low latitudes, in warmer climates, or in environments with mild seasonality. There was no evidence for a phylogenetic signal in body size. These findings support a pattern consistent with the converse Bergmann’s rule, thus suggesting that the limited time for foraging and growth in environments with strong seasonality (or higher latitudes) selects for faster development and maturation at smaller sizes. If so, the evolutionary lability of body size reported here might have contributed to the geographical spread and species diversification in this genus.