A comparative study of predator-induced social aggregation of tadpoles in two anuran species from western Madagascar

In the dry forest of western Madagascar, mixed-species social aggregations of tadpoles are frequent. Two species are often found in one aggregate. We explored the proximate mechanisms leading to the formation of tadpole aggregations that include the two species Aglyptodactylus securifer (Mantellidae) and Dyscophus insularis (Microhylidae). We show that aggregations are induced by the direct presence of predators, or by indirect chemical cues indicating a predation risk. However, the specific cues that initiated the formation of aggregations differed between the two species. Aglyptodactylus securifer reacted to con- and heterospecific tadpole homogenate ("Schreckstoff"). Dyscophus insularis' reaction was predator-specific to fish, i.e. directly to fish and indirectly to chemical cues released by fish. Although the ultimate benefit of this behaviour is thought to be to reduce predation, it also has costs. Tadpoles of A. securifer in the presence of predatory cues showed reduced growth and retarded development compared to tadpoles in control treatments.