Should vigilance always decrease with group size?

Abstract

Levels of anti-predator vigilance often decrease with group size in birds and mammals. Vigilance may also serve other purposes such as scrounging. While scrounging, individual adopt the vigilant posture to locate and eventually exploit the food discoveries of their companions. Models that combine anti-predator alertness and scrounging were developed to examine how changes in scrounger use with group size could alter the shape of the function relating vigilance to group size and whether the additional investment in vigilance while searching could be used to reduce the allocation of time to vigilance while in the food patch. Results indicate that increased allocation of time to scrounging during search can add to the overall level of vigilance and even counteract the expected decrease in vigilance with group size. However, the addition of scrounging was found to have little impact on vigilance while in the food patch. Behavioral effects that are dependent on group size, such as scrounging, may lead to changes in the shape of the vigilance function and may mitigate against any gain garnered from a reduction in individual levels of vigilance in larger groups.