Disentangling the various mechanisms that account for the decline in vigilance with group size

Foraging in groups is widespread in animals. In birds and mammals, reduced predation risk represents one of the main benefits of group foraging. In a group, more eyes and ears can monitor the surroundings for signs of danger, and the presence of alternative targets for a predator dilutes individual risk. Enhanced detection and risk dilution provide extra safety; this allows foragers to reduce their own investment in antipredator vigilance in a group at no increased risk to themselves. The prediction that vigilance decreases with group size has been tested in hundreds of studies, but disentangling the relative contribution of detection and dilution to this decline has proved difficult. Using models, I determined how vigilance is expected to decline with group size assuming that detection or dilution acted alone or in combination. I tested the fit of these predictions using 53 published relationships between vigilance and group size in birds and mammals. I found statistical evidence in support of detection or dilution acting alone or in combination in 24 of these cases with no obvious differences between birds and mammals. Detection and dilution acted together more often in species that form larger groups. This more quantitative approach holds much promise to understand the contribution of various mechanisms to the group-size effect on vigilance in group-foraging species.