Male Pagurus minutus hermit crabs use multiple types of information in decisions to give up male-male contests.

Organisms use information to make adaptive decisions in various contexts, including aggression. Potentially weaker, but better-informed, contestants should give up earlier to reduce fighting costs by using information related to their own lower success such as their size relative to their opponent and past contest outcomes to make this choice. Here, we examined whether intruders of the hermit crab Pagurus minutus could use information about their (1) smaller size, (2) past contest defeats, (3) opponent's past wins, or (4) relationship in the dominance hierarchy to their opponent when making a decision to give up during male-male contests for a female. In all trials, we randomly matched a smaller intruder with a larger opponent that was guarding a female. Our analyses suggest that P. minutus intruders can use all four types of information to decide whether to give up a contest without escalation or decrease its duration after escalation; it is the first species of Pagurus reported to do so, and the second reported to be able to distinguish familiar opponents from others in the context of male-male contests. These findings demonstrate the importance of cognitive abilities in minimizing costs when competing for vital resources.

Do weaponless males of the hermit crab Pagurus minutus give up contests without escalation? Behavior of intruders that lack their major cheliped in male-male contests.

In dyadic contests, theoretical studies have predicted that weaker contestants are less likely to engage in fights to minimize the cost of aggression. Since the major cheliped of decapod crustaceans is critically important as a weapon, contestants without a major cheliped should be more likely to give up the contests. We therefore examined whether loss of the major cheliped by the hermit crab Pagurus minutus would affect their decision to escalate male-male contests over guarded females. Intruders without a major cheliped showed no difference in the frequency of escalation compared with intact intruders, and the decision to give up was affected by the body size difference between the contestants. After escalation, compared with intact intruders, intruders without a major cheliped had significantly decreased success of takeover of a female from opponents, suggesting a strong disadvantage of losing their major cheliped. Although the decision of weaponless intruders to escalate seems irrational, several factors, such as poor accuracy of resource holding potential assessment, the influence of body size, and a high benefit to cost ratio of male-male contests, may have affected their behavior.