The form and function of a multi-functional weapon system in male and female burying beetles ( Nicrophorus vespilloides )

Animal weapon systems are used for attack and defence during competition for resources including, though not confined to, competition for mates. They comprise the weapon itself and associated morphological structures – or ‘weapon-supportive traits’ – that are essential for the deployment of the weapon in combat. We investigate the form and function of a weapon system in burying beetles Nicrophorus vespilloides , to better understand why it differs between the sexes. Both males and females engage in contests with members of their own sex to monopolise a scarce carrion breeding resource. We show that mandibles (weapon during biting) and head width (weapon-supportive trait) are larger in males, and that males exhibit a disproportionately larger increase in bite force with head width than females. However, in staged contests with size-matched rivals of the same sex, the weapon system functioned in the same way for males and females: for each sex, the combined effects of head width and maximum bite force best predicted contest outcome. We suggest that the component parts of the weapon system each serve multiple additional functions, including tasks associated with parental care, which contribute differently to fitness in each sex. The resulting divergent selection pressures may explain why sexual dimorphism persists.