Sexual dimorphism in head size in wild burying beetles (Nicrophorus vespilloides)

Male burying beetles ( Nicrophorus vespilloides ) carry an additional abdominal segment and produce pheromones but otherwise the species is thought to be sexually monomorphic. Both sexes bear bright orange bands on their black elytra, which probably function as part of a warning display rather than in mate choice. Although larger individuals are more likely to win contests to secure the small carrion required for reproduction, both sexes compete for this vital breeding resource. In wild populations, the sexes are alike in the mean and distribution in their body size. Here we describe a form of sexual size dimorphism in wild populations that has previously been overlooked. We show that males have wider heads than females, for any given body size, and that the scaling relationship with body size is hyperallometric in males, but isometric in females. We also show how absolute head width, and the extent of sexual dimorphism in head width differs among wild populations which are within c.10km of each other. We suggest that head size dimorphism could be adaptive, and could be due to divergent selection arising from task specialisation during biparental care, since the duties of care favoured by males are likely to require a greater bite force.