Why do Warning-Coloured Prey Live Gregariously?

The striking gregarious behaviour of many conspicuously coloured unpalatable prey has puzzled evolutionary biologists ever since the kin selection hypothesis was formulated. Nevertheless, no convincing evidence has yet been provided in support of any hypothesis so far proposed to explain the gregariousness of aposematic prey. We here report experimental results with domestic chicks (Gallus gallus domesticus) demonstrating that prey aggregation may be an aposematic signalling strategy that enhances predator discriminative aversion learning. In addition, our results show that aggregation works by enhancing the effectiveness of the visual, not the chemical, defence, illustrating the crucial influence of receiver psychology in the design of an animal signalling system.