Changes in vigilance with group size under scramble competition.

Abstract

Animals often interrupt foraging to scan the surrounding environment. In several taxa, such vigilance decreases with group size (Elgar 1989). The decline in vigilance is usually interpreted as a decrease in the predation risk to an individual in large groups. Mechanisms such as increased detection ability and numerical dilution of risk may provide more protection to foragers in large groups, thus leading to lower vigilance (Pulliam 1973; Roberts 1996; Bednekoff and Lima 1998). Nevertheless, recent work indicates that the decrease in vigilance with group size may be mitigated against if individuals in large groups scan more to avoid displacements by aggressive companions (Slotow and Coumi 2000; Treves 2000) or to locate the food discoveries of other group members (Beauchamp 2001). We explore the role of resource availability on vigilance patterns. In earlier vigilance models, a decrease in vigilance would allow foragers either to decrease the total amount of time needed to accumulate a fixed amount of resources (Pulliam et al. 1982) or to increase feeding rate over a given time horizon (McNamara and Houston 1992). In either case, no competition for resources is assumed to take place. However, when resources are limited, a reduction in vigilance may allow individuals to allocate more time to foraging and so obtain a greater share of the re-