Motor pattern during fights in the hermit crab Pagurus bernhardus: evidence for the role of skill in animal contests

Fighting involves the repeated performance of demanding agonistic behaviours and winners usually fight more vigorously than losers. While 'vigour' describes the rate and duration of a behaviour, 'skill' refers to well-coordinated motor movements. We investigated the role of skill in animal contests for the first time, focusing on the shell-rapping behaviour of hermit crabs during contests over the ownership of gastropod shells. We quantified vigour by recording the total number of raps and the mean number of raps per bout, and we quantified skill by measuring the distances that attackers displaced their shell during each rap. Winners displaced their shells through shorter distances than losers, indicating that motor pattern, as well as vigour, differed between contest outcomes. Both vigour and skill improved as fights progressed for eventual winners, but worsened for losers. We suggest that in a contest, skilful motor movements allow vigorous fighting, and both aspects deteriorate with fatigue. Skill may be important in the wide range of contests where outcomes are driven by energetic constraints. Understanding the links between skill, vigour and energy could provide new insights into strategic decision making during animal contests.