Body and mandible size as key factors in male contest outcomes, but no evidence of assessment models in Helm’s stag beetles

ABSTRACT Males of many species compete with one another, allowing access to mating opportunities. During combat, contestants are constantly making decisions, e.g. to continue competing or retreat, based on the relative difference in resource holding potential (RHP) or some internal threshold. The outcome of these interactions is often based on the competitors RHP, which can be measured as the size of exaggerated traits (e.g. horns, mandibles, etc.) or overall body size. Competitive assessment models have been used to describe these behaviours and determine how animals assess their RHP and their opponent’s RHP. This study looked at the contests of the Helm’s stag beetle, Geodorcus helmsi, in a controlled laboratory environment. We found that larger males were more likely to win in an agonistic interaction and assumed that mandible length is a suitable proxy for RHP in this species. However, we did not find support for any assessment model being used in these contests. We also detail competitive behaviours of G. helmsi in a wild population for the first time. This study contributes to the research about the fighting behaviour in stag beetles and adds to the body of knowledge on New Zealand stag beetle behaviour, which has not been well studied.