Evidence for a "monitoring call" in red-bellied tamarins

In addition to a variety of calls whose functions are to increase or decrease space among individuals, many species of primates use one or more vocalizations that are emitted in no one particular context and elicit no one particular response. Although investigators have given this type of vocalization a number of different names (e.g., contact call, cohesion call), what these calls seem to have in common is that 1) they allow individuals to monitor the general whereabouts of groupmates and 2) they somehow contribute to group cohesion. Neither of these assumptions has been tested, however. In the red-bellied tamarin (Saguinus labiatus), a vocalization we call the "slide" appears to be a monitoring call. In the current study we tested three hypotheses about the slide call: 1) If slide calls are not direct requests for action, there should be no reliable correlation between slide calls and movement toward or away from the slide caller; 2) if slide calls are emitted to monitor groupmates' whereabouts, they should occur at comparatively low rates when the presence of groupmates can easily be affirmed by visual means (e.g., when the animals are confined in a small space); 3) if slide calls contribute to intragroup cohesion, they should be produced at comparatively high rates during activity that requires coordinated behavior. Each of these predictions was supported. Although variants of the call and/or additional functions of the call(s) may emerge from further testing, social monitoring appears to be at least one important function of the slide calls of red-bellied tamarins, especially in circumstances where coordinated and/or cohesive behavior is required of the group.