Social monitoring and vigilance behavior in brown capuchin monkeys (Cebus apella)

I investigated the vigilance behavior of brown capuchin monkeys (Cebus apella) in Iguazu, Argentina to determine the relative importance of anti-predatory vigilance versus social monitoring. Predator detection has been found to be the major factor driving vigilance patterns in other populations of Cebus, and a positive relationship between vigilance and the number of nearby neighbors has been found. In the Iguazu population, social monitoring is the primary function of vigilance, as indicated by a negative relationship between vigilance and neighbor density. Even if social monitoring and predator detection are totally compatible, the time tradeoffs associated with vigilance behavior are mostly driven by social monitoring in this population. In addition to recording vigilance data in relation to the number of neighbors, several other variables were recorded in conjunction with vigilance behavior in order to control for possible confounding factors. After controlling for these factors, it was found that the number of neighbors within 10 m was still highly correlated with the percent of time spent vigilant. The average percent of time spent vigilant was found to be much lower in Iguazu, compared to another population of C. apella with higher predation pressure. The average vigilance bout was much closer in length to known social monitoring bouts than to known anti-predatory vigilance bouts. These patterns indicate that the predominate function of vigilance in this population of C. apella is social monitoring. In populations with low predation pressure, the time costs associated with social monitoring should increase with group size, and increasing neighbor density. The fitness costs of social monitoring have not been incorporated into current social models, and these results indicate that these models may not be appropriate for all study populations.