Loud calling, also known as howling, is the single most distinctive behavioral attribute of the social system of howler monkeys, yet no concensus has been established regarding its function. At a proximate level, I examine the spatial and temporal patterns of loud calling across different contexts to assess how howling functions in intergroup spacing. At an ultimate level, I investigate whether howling mainly functions in the defense of food resources, mates, or infants by examining how loud calling is influenced by female reproductive states, resource availability, and the presence of infants vulnerable to infanticide.
Collective Group Defense
Rates of intergroup encounters vary greatly among primate species and populations, and are influenced by many factors, including population density, home range sizes and overlap, daily path lengths, and intergroup communication and spacing systems. I examine rates of intergroup encounters in black howler monkeys and whether groups actively seeks or avoid their neighbors. Additionally, I examine the ultimate functions of intergroup conflict; whether opponents compete over group membership, space, food resources, or mates.
Behaviors associated with group defense, such as fighting, vocalizing, and vigilance, may provide benefits to both the actor and other group members, when the benefits can not be monopolized. Cooperation might thus be threatened by the presence of 'free-riders', group members who reap the benefits of successful group defense but incur no costs by not joining in these actions. Despite this collective action problem, cooperation during intergroup conflict is commonly observed in group-living animals. Howler monkeys mainly defend their group and home ranges through loud calling. Both males and females participate in howling bouts, yet not all group members participate equally. I examine whether males' age, reproductive success, or intragroup kinship influence male participation in naturally occurring loud calls. Equally, I examine whether female participation is mediated by reproductive state, proximity to food resources and context of howling bouts.