Morphological traits determine the winner of "symmetric" fights in hermit crabs

This study aimed at investigating in the hermit crab Pagurus longicarpus whether some morphological traits and their slight variations might determine the winner of a contest in symmetric pairs, i.e. pairs composed of individuals matched by their overall size. In the pre-experimental phase, 400 crabs were individually kept in isolation for a week to eliminate the effects of their previous social experiences and were allowed to enter adequate shells to equalize their motivation to fight. Then, we formed 200 pairs matched for shell and body size and observed their agonistic behaviour for 15 min. Alphas (and betas) were deemed as those individuals that won more than half of the recorded fights. Finally, crabs were sacrificed and sexed, and 6 and 8 measures were taken of their shells and bodies, respectively. Raw data were converted to compositional data and subjected to an isometric log-ratio transformation prior to statistical analysis. Alphas were found to occupy heavier and wider shells than betas, both characteristics that, together with the correlated high level of encrustation, make the apparent dimension of the crab larger and thus favour that individual during agonistic encounters. As expected, hermit crabs were sexually dimorphic in their chelae, but sexes differed also in the length of the dactylus of their third walking leg and in their body weight. Alphas were heavier than betas and had longer propodi and wider dactyli on the third walking legs than betas. The latter character might be advantageous during shell fights when the walking legs of the attacker are used to hold the defender's shell in the spasmodic shaking and their wider dactyli might assure a firmer seizing of it. Indeed, a correct execution of this pattern allows for the eviction of the defender. Our results suggest that more attention towards variations in morphological traits other than body size might improve our understanding of the factors that affect individual Resource Holding Power and fight dynamics.