Vigilance and aggregation in black howler monkeys (Alouatta pigra)

Animals are expected to reduce per capita vigilance with increasing group s ize. Widespread support for this expectation has been found in studies of b irds and ungulates. Primate vigilance patterns appear to differ, but studie s of primates may have sampled groups with too many members to detect inter group differences in vigilance. We tested this idea by measuring individual vigilance rates in wild black howler monkeys (Alouatta pigra) living in st able groups of two to ten individuals. No group size effect on vigilance em erged. As in previous studies, inter-individual proximity explained variati on in individual vigilance rates. Focal animals with associates within 2 m and with multiple associates in the same tree were less vigilant than isola ted animals. Activity and class of focal animals as well as the presence of neonates influenced the average vigilance of adult and subadult black howl er monkeys. We conclude that functional explanations must be considered for the general lack of a group size effect on primate vigilance.