Why individual vigilance declines as group size increases

A reduction in individual vigilance with an increase in group size is one of the most frequently reported relationships in the study of animal behaviour. It has been argued that this phenomenon may not be a direct consequence of an increase in group size but may be due to other factors relating to increased group size, such as increased foraging competition. However, there is evidence for a direct relationship between group size and vigilance where other variables have been controlled. The aim of this review is to highlight the fact that the functional explanation of the group size effect remains unclear. Some authors have considered just one hypothesis, the group vigilance or 'many eyes' hypothesis. This states that, by taking advantage of the vigilance of other group members, individuals can reduce their own vigilance. However, there is an alternative, or additional, possibility that if individual vigilance declines with a reduction in individual predation risk, the group size effect could be accounted for by a reduction in individual risk at higher group sizes, as is widely thought to occur through encounter, dilution and confusion effects. In this review, it is shown that evidence previously interpreted in terms of one hypothesis may also be interpreted in terms of the other. Future research should be directed towards explicit consideration of the two effects and empirical tests to distinguish their relative importance. It is proposed that the individual risk hypothesis, with group vigilance as one element, provides a more general framework for understanding variation in vigilance behaviour with group size and with other factors. (C) 1996 The Association for the Study of Animal Behaviour.