Shell Wars: Assessment Strategies and the Timing of Decisions in Hermit Crab Shell Fights

Abstract

The shell fighting behaviour of the hermit crab Pagurus bernhardus was observed under experimental conditions. The relative sizes of the crabs and shell quality of the larger crab influenced the probability of occurrence of a shell fight. These two factors along with the quality of the smaller crab's shell influenced the probability of an escalated fight occurring. During a shell fight, the attacker was able to assimilate information concerning the defender's shell and to compare it with the shell in possession. On the basis of this comparison the attacker decided whether or not to evict the defender and effect a shell change. The time that the attacker took in assessment of the opponent's shell was influenced by a) the quality of the attacker's own shell and b) the ease of discrimination between the qualities of the two shells. Thus decisions were easier in some instances than in others. The defending crab did not have access to information about the attacker's shell and therefore could only estimate the quality of its own shell. This asymmetry of information experienced by the crabs ensured that the attacker decided the outcome of a shell fight.