Metamorphic synchrony and aggregation as antipredator responses in American toads

Safety in numbers serves as an antipredator defense strategy in many organisms, as aggregation can reduce the probability of predation for individual group members. It has long been suggested that some organisms are under selective pressure to synchronize vulnerable life stages with conspecifics, but there have been very few experimental tests of this hypothesis. Furthermore, the role of aggregation of conspecific animals within cohorts during vulnerable life stages is poorly understood. Previous studies indicate that predation pressure may select for synchronous metamorphosis and the subsequent formation of metamorphic aggregations in North American toad species. In a series of laboratory experiments, I demonstrated that 1) Bufo americanus tadpoles in the presence of a predator exhibit higher levels of aggregation than tadpoles maintained in the absence of predator stimuli, and 2) B. americanus exposed to predator chemical cues metamorphose more synchronously than control tadpoles. I also found that toads at the climax of metamorphosis exhibit higher levels of aggregation than pre-metamorphic individuals. These results support the hypothesis that predation pressure has played a role in selection for life stage synchrony, and that aggregation serves as an antipredator defense in animals with synchronous transitions.