Aggregation Behaviour of Bufo maculatus Tadpoles as an Antipredator Mechanism

Most anurans of the species‐rich community of the Comoé National Park (Ivory Coast) use ephemeral savannah ponds to spawn. Owing to the great risk of desiccation and the large number of predators, the mortality for tadpoles is very high in these ponds. Therefore, colonization of other spawning habitats might be highly advantageous. Such spawning sites are presented by the Comoé river, which is characterized by frequent and unpredictable changes of the water level. Only Bufo maculatus (Anura: Bufonidae) and one other bufonid species breed in small and shallow inlets and puddles created by the rising and falling of the predator‐rich Comoé river. We observed that predatory fish advanced to the spawning sites of the toads, when the water level rose. If the toad larvae were attacked by these predators, they formed dense aggregations of up to several thousand individuals. These aggregations were maintained for longer periods on one place at the river’s edge where they are inaccessible to the larger predators. In field experiments we ascertained that this aggregation behaviour in tadpoles was caused by a combination of two stimuli: first, a chemical cue from injured tadpoles and second, a mechanical stimulus caused by rapid movements of aquatic predators. Initial trials indicated that tadpoles aggregating at the bank were, however, disadvantaged compared with free‐swimming larvae in having slower growth and an increased risk of desiccation. This is presumably why aggregations broke up within 24 h after the predators had left these puddles, as the water level fell. At this point these tadpoles either spread out or formed loose swarms near the bottom of the puddles. This swarming behaviour differed considerably from that induced by aquatic predators.