Who cares who calls?: Selective responses to the lost calls of socially dominant group members in the white-faced capuchin (Cebus capucinus)

In many social mammals and birds, soft vocalizations are habitually produced during dispersed moving and foraging, the function being to maintain contact and regulate spacing between group members. In some species, much louder calls are given sporadically by specific individuals when they become separated from the group, or 'lost'. The function of these calls has seldom been specifically tested, particularly among social primates, but is often assumed to involve regaining contact with the group based on a combination of individually distinctive calls and antiphonal responses to them from within the group. To test these assumptions, we conducted research on two groups of white-faced capuchins (Cebus capucinus) in Costa Rica. We analyzed 82 bouts of 'lost' calls given by 13 different adult individuals when separated from the group and the antiphonal responses they elicited. Lost calls were individually distinctive and were answered in 35% of calling episodes. Answers were selective: dominant males and females were answered more than were subordinate callers of either sex. As a result, dominant callers relocated and returned to the group more quickly than did subordinate callers. We discuss the potential proximate motivations for, and ultimate benefits of, such selective answering of dominant group members.