Watch out! Insecure relationships affect vigilance in wild spider monkeys (Ateles geoffroyi)

Abstract

Vigilance is used to monitor extra-group threats as well as risky group members. We examined whether relationship quality affects vigilance patterns of spider monkeys. We used focal animal sampling to collect data on social interactions and individual vigilance of all adults and subadults (N = 22) in a community of well-habituated Geoffroy’s spider monkeys living in the protected area of Otoch Ma’ax Yetel Kooh, Yucatan, Mexico. Through a principal component analysis of seven indices of social interactions, we previously obtained three components of relationship quality, reflecting the levels of compatibility, value, and security. Such components could differentially affect vigilance depending on whether vigilance is directed to extra-group threats or risky group members. We tested whether an individual’s vigilance was affected by (1) the mean level of compatibility, the mean level of value and the mean level of security across subgroup members; (2) the lowest level of compatibility, the lowest level of value, and the lowest level of security with any subgroup member; and (3) the mean level of compatibility, the mean level of value, and the mean level of security with neighbors (i.e., subgroup members within 5 m). We did not find evidence for any effect of compatibility and value; however, security did affect vigilance, as individuals were more vigilant when they had a less secure relationship with the subgroup member with the lowest level of security or with the average neighbor. Vigilance for monitoring group members is common in primate species. We examined whether the quality of social relationships with subgroup members and neighbors modulates vigilance in wild spider monkeys. We used three components of relationship quality (reflecting the levels of compatibility, value, and security) and predicted each component would affect vigilance depending on whether vigilance was directed to extra-group threats or risky group members. We found no evidence that compatibility and value affected vigilance. However, an increase in vigilance occurred when spider monkeys had a less secure relationship with (1) the subgroup member with the lowest level of security and (2) the average neighbor. Our results show monitoring risky group members is an important component of vigilance, especially in species facing low predation pressure.