Risk of predation affects aggregation size:: a study with tadpoles of Phrynomantis microps (Anura: Microhylidae)

Tadpoles of the microhylid frog Phrynomantis microps form swarms near the water surface of savannah ponds during the day. Observations of tadpole distribution at night showed that these aggregations disappear and that tadpoles instead swim in a random distribution near the water surface. The clarity of pond water had significant effects on the distribution of tadpoles during the day. The size of aggregations, and the densities of tadpoles within them, both increased with increasing water clarity. Since tadpoles of this species are known to aggregate in response to visually hunting aquatic predators, these results suggest that tadpoles respond to high water clarity as an indicator of increased predation risk. This supports the hypothesis that, under conditions of increasing predation risk, animals should form larger and more dense groups up to a critical group size. (C) 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.