UC Santa Barbara

Long-distance vocalizations mediate spacing patterns by allowing groups to choose whether to draw close enough to a neighbor to initiate a short-range interaction. It is unclear, however, whether the patterning of calls and the resulting movements are invariant or change in response to social and ecological variables. In this study, we compare the impact of long-distance calls on neighboring groups' movements in two populations of grey-cheeked mangabeys (Lophocebus albigena) in Uganda, which differed widely in group density and food availability. We monitored movement responses of target groups to both naturally occurring and experimentally simulated "whoop-gobble" vocalizations of neighboring adult males to test whether responses indicated mate and food defense. When defending access to fertile females, males might coerce group members to move away from neighboring males; when defending fitness-limiting food resources, however, groups might approach and attempt to evict neighbors. Controlling for pre-call movement prior to the vocalization, we found no difference in group responses between the low- and high-density populations, and little support for either form of defense. Mangabey groups generally continued moving in their original direction, albeit at a slower pace, and deviations from this pattern were predictable: they avoided nearby callers but approached distant callers; they approached if the caller was in the area where home ranges overlapped; and approached when their own group contained multiple peak-estrous females. The effect of multiple fertile females potentially indicates a breakdown of male control as a result of within-group intrasexual mating competition, and greater-than-expected female control over travel movements. As suspected, grey-cheeked mangabeys use neighbors' long-distance calls to maintain a spatial buffer between groups but changes in group movements indicate a conflict between male and female interests.