Prey responses to pulses of risk and safety: testing the risk allocation hypothesis

The risk allocation hypothesis predicts that prey responses to predation risk should depend on the temporal pattern of risk. In systems where activity is dangerous, predicted activity levels should be ranked as follows: activity during a pulse of safety>activity during continual safety>activity during continual risk>activity during a pulse of risk. We conducted the first experimental test of the basic predictions of the risk allocation hypothesis by examining responses of freshwater physid snails, Physa gyrina, to chemical cues associated with predation on snails by predatory crayfish, Orconectes juvenilis. As predicted, the snails' pattern of activity, microhabitat use and response to risk depended on the temporal pattern of risk. Snails held in continual risk had very low activity levels, but showed an immediate, large increase in activity during a brief period of safety. In contrast, snails held in continual safety showed moderate levels of activity, but surprisingly, only a weak reduction in activity when exposed to a pulse of danger. Further studies are needed to identify general patterns for how temporal variation in risk influences antipredator behaviour. 2002 The Association for the Study of Animal Behaviour.