Within-group vigilance in red colobus and redtail monkeys.

In theory, animals are expected to relax vigilance in the safety of large groups. Four controlled studies of primates have failed to detect relaxed vigilance as group size increases. The counter-intuitive behavior of primates might arise if another component of vigilance increases with group size, masking any decrease in outward, anti-predator vigilance. Surveillance of associates is a major component of primate vigilance and might be expected to increase with group size due to an increase in competitive interactions. The present analysis uses data on glances toward associates to determine if within-group surveillance is related to group size in wild red colobus and redtail monkeys of Uganda. Although males glance at associates more frequently during mating periods and females glance at associates more often when infants are young or out of contact, there was no evidence of a group-size effect on within-group surveillance. As with previous studies, primate vigilance patterns reflect reproductive strategies such as infant protection and mate defense. This study joins several others that indicate that primate vigilance is unresponsive to group size. New models that take into account association patterns below the level of the group may be needed.