The evolution of aggregation in profitable and unprofitable prey

Abstract

The relationship between gregariousness and warning signals in defended species has been the subject of much debate. While previous researchers have found that predators learn to avoid conspicuous unpalatable prey faster when they are in aggregations, they have not presented solitary and aggregated unprofitable prey to predators simultaneously to directly compare their survivorship. Furthermore, no experiment has investigated the influence of profitable prey configuration on predator response to aggregation in unprofitable prey. Here we investigated the response of human, Homo sapiens , predators to profitable and unprofitable prey in a computer version of the ‘novel world’ experiments that addressed these issues. When all prey types were cryptic and all profitable prey were solitary, then aggregated unprofitable prey had higher per capita survivorship than solitary unprofitable prey. In this case, aggregation was the sole reliable distinguishing characteristic of unprofitability. By contrast, when profitable and unprofitable prey could be distinguished in terms of their conspicuousness, then aggregation did not confer a benefit to unprofitable prey. When profitable prey were simultaneously presented in solitary and aggregated configurations, aggregated profitable prey consistently had a lower per capita survivorship than solitary profitable prey, regardless of the configuration of unprofitable prey. We conclude that while aggregation can serve as a signal of unprofitability, it may not be required for the evolution of warning signals. Instead, we suggest that the common association between aggregation and distastefulness may primarily arise for a ‘negative’ reason: the vulnerability of aggregated palatable prey.