The burying beetle (Nicrophorus vespilloides) is thought to be predominantly sexually monomorphic. Males carry an additional abdominal segment and produce pheromones, but otherwise the sexes are alike. Both sexes bear bright orange bands on their black elytra, which probably function as part of a warning display rather than in mate choice. In wild populations, the sexes do not differ in mean body size, nor in its variation.

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 pronotum width (body size), and that the scaling relationship between head width and pronotum width is hyperallometric in males, but isometric in females. We also show how absolute head width, as well as the extent of sexual dimorphism in head width, differs among seven wild populations inhabiting different woodlands that are within c.10 km of each other. We suggest that head size dimorphism is functionally related to bite force. We hypothesise that dimorphism could be due to divergent selection arising from task specialisation during biparental care, as the duties of care favoured by males are likely to require a greater bite force.