Asymmetric effects of contaminant exposure during asymmetric contests in the hermit crab Pagurus bernhardus

Contaminant exposure is known to alter behaviour in a wide range of contexts, including animal contests. We exposed hermit crabs to sublethal concentrations of copper, a common contaminant in coastal ecosystems. When hermit crabs engage in shell fights over empty gastropod shells they adopt two distinct roles. Attackers initiate the fight and perform bouts of vigorous shell fighting, while defenders remain withdrawn into their shells until the encounter is resolved. This role asymmetry means that the two opponents make different strategic decisions during the contest and we utilized an experimental design that allowed us to assess the effect of copper exposure independently on each role. We found that exposure of attackers to copper reduced the number of fights that were initiated, leading to fewer fights in total where the defender was evicted. Of attackers that did fight, those that were exposed to copper were also less likely to evict the defender, but this effect varied with the size difference between the crabs. During the fight, copper exposure caused attackers to rap less vigorously. When defenders were exposed to copper, however, this had no effect on the defender's chance of resisting eviction. Thus, we show for the first time that exposure to a contaminant can alter the outcome of asymmetric contests via differential effects on each of the two roles.