Modeling vigilance in mixed-species groups

Abstract

Mixed-species groups are usually explained by foraging advantages and reduced predation risk for at least one of the participating species. Given that animals trade-off foraging and vigilance, the optimal level of vigilance of individuals in mixed-species groups depends partly on the vigilance levels of both conspecifics and heterospecifics. However, the benefits and costs of being part of a mixed-species group do not need to be evenly distributed between the species in a group. In this paper, we modeled the evolutionary stable strategy (ESS) for the optimal level of vigilance of an individual in a mixed-species group influenced by the effects of “many eyes,” “dilution” and “attraction,” and unequal costs and benefits between the species. Our model illustrates under what conditions associations with other species may facilitate reduced predation risk for at least one of the participating species. We show that vigilance of individuals in mixed-species groups becomes a social game, and that the ESS of these vigilance games may predict the individual's adaptive level of vigilance. This paper provides the first step in the development of a predictive theory for the numerous empirical studies on mixed-species groups.