A male's enlarged mandibles are used to grip and lever rivals off a branch, tipping them over the edge. That function needs reach and grip rather than crushing power, and the long jaws work as a third-class lever in which force falls as length increases. Comparative measurements have shown male stag beetles bite with considerably less force than the smaller-jawed females.
Females, whose mandibles are short and stout, use them for boring into decaying wood and can deliver a bite that draws blood. The sexual dimorphism therefore runs opposite to intuition: the sex with the dramatic weapons has the weaker jaws.
The adults themselves eat little, taking sap and other fluids during a few weeks of summer flight. Nearly the whole life cycle happens out of sight as a larva in rotting hardwood, typically over three to six years, which is why the species declines when dead wood is tidied away.