The Relationship Between Predator Activity State and Sensitivity to Prey Odor

Predators searching for prey commonly alternate periods of endogenous locomotory activity with rest. We examined the effects of activity state on behavioral responses to prey odor by predatory blue crabs (Callinectes sapidus) and spiny lobsters (Panulirus interruptus). All animals to be tested were placed individually in large seawater tanks (1.5-m diameter) outdoors, where they were assayed for their responses. Initial experiments were conducted at night, the period of greatest normal endogenous activity, in either moonlight or dim, far-red illumination. Prey odor was presented to crabs and lobsters as they either spontaneously walked or rested (for 15-30 min) between locomotory bouts. Only walking animals significantly responded; they either turned towards the site of odor delivery or probed the substratum with chelae and legs. An additional experiment was conducted by presenting a purified prey attractant (ATP) to spiny lobsters. Remarkably, ATP was 498 times more potent in evoking response from walking animals than from resting ones. Neither the duration (15-30 min compared with 4-8 h) nor the timing (night compared with day) of the rest period influenced the sensitivity of the behavioral response. On the basis of our current results, chemical stimuli appear principally to modulate searching by predators already aroused rather than to initiate foraging or feeding from the quiescent state. Significantly, whenever experiments restrict the activity of freely ranging animals, determinations of chemosensory-stimulated behavior may substantially overestimate the concentrations needed to evoke responses.