Body Size Evolution in Burying Beetles (Staphylinidae: Silphinae: Nicrophorus)

Body size is an important component of burying beetle (genus Nicrophorus) life history, affecting competitive interactions and resource use. Currently, there is no comprehensive analysis of what drives these differences in size and how body size is distributed within the genus and across its geographic range. We used a large dataset of body size measurements and geographical data to evaluate the relative importance of phylogeny, biogeography, and ecology in explaining body size variation in burying beetles. Mean body size distribution among species is broad (4.15-10.97 mm pronotal width) and skewed, with more small and medium-bodied species than large species. We found evidence of phylogenetic signal in the evolution of body size across the genus, although only one instance of sister species both being giants and no instances of sister species being both small. However, the phylogenetic analysis does not explain the evolution of extremes in Nicrophorus body size. Areas with higher species richness have a greater spread between the largest and smallest species, and body size is divergent between most sister species and more strongly so between sympatric sister species, even after correcting for phylogeny. We found evidence of rapid initial divergence in body size following speciation, which increased over time in sympatric species, but stabilized in non-sympatric species. Smallest body sizes and highest species richness are concentrated in northern hemisphere temperate latitudes. Taken together, these results suggest character displacement by body size may be a significant factor allowing coexistence of burying beetle species; however, other mechanisms of niche partitioning are likely important contributors to coexistence. High species richness in temperate, mesic areas of the northern hemisphere may be driven by habitat and climatic suitability. We encourage further experimentation to test our proposed mechanisms of body size divergence and geographic distribution in Nicrophorus.