Collective detection in escape responses of temporary groups of Iberian green frogs

When confronted with a predator, prey are often in close proximity to conspecifics. This situation has generated several hypotheses regarding antipredator strategies adopted by individuals within groups of gregarious species, such as the "risk dilution," "early detection," or "collective detection" effects. However, whether short-term temporary aggregations of nongregarious animals are also influenced in their escape decisions by nearby conspecifics remains little explored. We simulated predator approaches to green frogs (Rana perezi) in the field while they were foraging at the edge of water, either alone or spatially aggregated in temporary clusters. "Flight initiation distances" of frogs (i.e., the distance between the simulated predator and the frog at the time it jumped) that escaped by jumping into the water were influenced by microhabitat variables (vegetation at the edge and in water and the initial distance of the frog to the closest water edge) and also by the responses of nearby individuals. In clusters, risk dilution did not influence the first individual to respond to the predator simulation or the average response of all frogs in the cluster as the frog's responses were independent of group size. Also, flight initiation distances of individuals that first responded to the predator within clusters did not differ from those of solitary individuals, which is contrary to the predictions of the early detection hypothesis. However, the remaining frogs in the cluster had longer flight initiation distances than expected from the comparison with solitary individuals. We suggest that this pattern originated because the response of the first frog within a cluster triggered the sequential response of the remaining frogs in the cluster, which agrees with the expectations from the collective detection hypothesis. Our findings give insight into an early stage in the evolution of grouping as they suggest that individual frogs may benefit from being part of a cluster, even for short periods of time. Copyright 2006.