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The Influence of Group Size and Neighbors on Vigilance in Two Species of Arboreal Monkeys

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In theory, one of the main benefits of group-living is the sharing of vigilance among group-mates. However, data on scanning in redtail and red colobus monkeys indicate that only one class of individuals in each species derives clear benefits from shared vigilance. Moreover, the expected negative relationship between individual scanning and social group size was not met in these monkeys. Nor was time spent scanning influenced by the sex or species composition of groups. Shared vigilance was observed only among red colobus adult males and redtail adult females and only when they had neighbors within 2 m. Red colobus adult males saved 10% of their scanning time when they had one neighbor within 2 m, while redtail adult females saved 16% of their time under the same conditions. No other age-sex class demonstrated a significant decrease.

The role of near neighbors has been underemphasized in previous work on grouping and vigilance, an oversight made more serious because of the often confounded relationship between spatial cohesion and group size. In redtails, but not in red colobus, the number of neighbors within 2 m was significantly positively correlated with group size. This prompts the hypothesis that the inconsistency and poor explanatory power of group size in studies of vigilance may be due to the underlying and undetected role of near neighbors. The paramount role of overall group size in optimality models is therefore questioned.

Affiliations: 1: Dept. of Anthropology, Harvard University, Cambridge, MA 02138, USA, Department of Psychology, University of Wisconsin, 1202 West Johnson Street, Madison, WI 53706-1696, USA;, Email:


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