Odd prey in mixed species groups suffer fewer attacks than lone individuals

Abstract

Mixed-species groups are common in nature. Such groups are characterised by the presence of one or more majority species, and smaller numbers of minority species. Minority individuals are expected to be subject to oddity effects; by looking or behaving differently to majority members they should be disproportionately targeted by predators. Given this, why might minority species remain in mixed-species groups? To address this question, we used threespine sticklebacks (Gasterosteus aculeatus) as predators and two species of virtual prey presented via videos. We compared predator attacks on solitary prey, and odd and majority grouped prey individuals in groups of different sizes. We found that solitary prey were attacked significantly more than odd and majority grouped prey, while, in fact, odd and majority grouped prey did not differ from each other in terms of attacks received. We also found that prey in smaller groups suffered significantly more attacks than prey in larger groups. These findings provide no evidence for oddity effects but suggest evidence of a confusion effect. Natural mixed-species groups persist for various reasons, for example as foraging guilds, or because some members take advantage of more effective vigilance or alarm calls of others. We suggest, based on these findings, an additional non-mutually exclusive reason; under some circumstances, odd individuals might join larger heterospecific groups because any costs of being odd are greatly outweighed by the predation risk costs of remaining alone.