Responses to predation risk: alternative strategies in the crab Heterozius rotundifrons

The New Zealand endemic crab Heterozius rotundifrons displays an effective antipredator behaviour of fully extending all limbs for a number of seconds or even several minutes if presented with single sensory inputs (alarm odour of crushed conspecifics or an overhead shadow presentation) but fully extends all limbs for a shorter period when chemical and visual cues are presented simultaneously (Hazlett & McLay 2000, Animal Behaviour, 59, 965-974). Because the diversity of the sensory channels experienced and the total magnitude of cues indicating elevated risk were confounded in our earlier study, we separated the diversity and magnitude of cues to study the conditions that result in a switch in antipredator strategies. Crabs showed similar extended durations of the limb-extended state for all magnitudes of single modality treatments that were above threshold. Similarly, for all above-threshold combinations of modalities, the crabs showed similarly reduced durations of the limb-extended state compared with controls. It appears that the switch in strategies is induced just by the diversity of categories of risk cues, not their magnitude.