Know your enemy and know yourself: mechanism of contest resolution in the European stag beetle (Lucanus cervus)

Abstract Intraspecific competition rely on the assessment of the best strategies to resolve conflicts among individuals. Assessing their own and/or the opponent’s fighting ability, referred to as the resource holding potential (RHP), is crucial to minimise energetic costs. A multiple analysis approach was used to test the main assessment models and describe contest resolution in the European stag beetles ( Lucanus cervus ), according to game theory. L. cervus males possess enlarged mandibles, which exhibit great morphological and size variation, used as weapons to dislodge their opponent from branches. We measured RHP associated with offensive (mandibles) and defensive weapons (strength in seizing the substrate) and staged dyadic contests between size-matched and size-unmatched males. To describe the mechanism of contest resolution we analysed the effect of RHP on contest duration, contest dynamic (sequential analysis) and behavioural matching between opponents. Relationships between contest duration and RHP cannot reject partial mutual assessment, but sequential analysis, behavioural mismatch and absence of injuries supported the mutual assessment mechanism. Both small and large L. cervus males assess both self and opponent RHP, performing high-energy costly behaviours only when deemed necessary. This limited war strategy may have evolved to minimise the costs and prevent injuries associated with aggressive interactions.