Shell wars II: the influence of relative size on decisions made during hermit crab shell fights

The value of contested resources (shells) in hermit crab fights depends on the sizes of the crabs relative to the sizes of the resources. Thus when relative contestant size is the main experimental variable, motivational factors associated with shell size will also be an experimental problem. Two experiments are described that together overcome this problem. Relative crab size influences all stages of shell fights including pre-fight display, escalation, eviction and examination of the opponent's shell by the victor both before and after eviction of the loser. Shell fights occur more often between disparately-sized animals than between those similar in size. This apparent contradiction of recent theory (Maynard-Smith & Parker 1976) is probably due to the high cost of being without a shell and the small chance that an escalated fight will result. Relative crab size influences the time taken in resource assessments and thus the effectiveness of these assessments is also probably influenced. Causal factors influencing each of the major decisions in shell fights are described and although these fights are more complex than most they are in general agreement with theory on animal contests.