Evolutionarily stable levels of vigilance as a function of group size

Abstract

An animal's level of vigilance is usually interpreted as a trade-off between gaining food and reducing the danger of predation. In the context of a group of animals, vigilance has been analysed using the evolutionarily stable strategy (ESS) concept. In this paper a general ESS model of vigilance as a function of group size is developed. The model is based on an explicit foraging process that may be terminated prematurely by events such as bad weather that are outside the animal's control. It can be used to investigate the effect of both environmental parameter, such as the rate of attack by predators and the rate of food intake, and the animal's internal state on evolutionarily stable levels of vigilance. The biological assumptions underlying other models of vigilance are explored, demonstrating why some factors influence the level of vigilance in some models but not in others. Pulliam et al. (J. theor. Biol., 1982, 95, 89-103) presented data on vigilance as a function of group size that they found hard to explain in terms of an ESS model. This paper introduces various modifications to their model and shows that a reasonable fit to the data can be obtained. © 1992 The Association for the Study of Animal Behaviour.