Vocal and movement responses of mantled howler monkeys (Alouatta palliata) to natural loud calls from neighbors.

Loud calls (i.e., long-range acoustic signals) regulate resource competition among neighboring groups of conspecifics in several nonhuman primate species. Ultimate explanations for primate loud calls include mate, offspring, and food defense. Additionally, loud calls may provide valuable information pertaining to the identity and health status of callers, their competitive abilities, and their spatial location. The loud calls of howler monkeys (Alouatta) have been thoroughly studied and seem to play an important function in the defense of valuable resources in a variety of socioecological contexts. Here, we examined whether the behavioral responses of mantled howler monkeys (Alouatta palliata) to natural loud calls from neighbors are linked to three factors: food availability, familiarity, and distance between groups. We studied three groups of mantled howler monkeys at La Flor de Catemaco (Los Tuxtlas, Mexico) for 15 months (1817 observation hours), during which we recorded 236 neighbor loud calls. Food availability per se did not influence the behavior of groups receiving loud calls, although males produced longer vocal responses toward unfamiliar neighbors when food availability decreased. Groups vocalized quicker and both vocalized and moved for longer after loud calls from unfamiliar neighbors. Additionally, groups vocalized and moved for longer at shorter distances from unfamiliar neighbors compared with familiar neighbors. Finally, groups usually moved away from calling neighbors that were closer. These results indicate that the behavioral responses of mantled howler monkeys to loud calls from neighbor groups are associated with the integration of information pertaining to caller identity as well as to their ecological and spatial context.