Subordinate male black howler monkey (Alouatta pigra) responses to loud calls: Experimental evidence for the effects of intra-group male relationships and age

When assessing a group's overall fighting ability, functional group size (i.e. the number of cooperative members willing to confront opponents) may be more important than actual group size. Despite obvious benefits, group members do not always act in a collective manner. For example, participation by subordinate male black howler monkeys (Alouatta pigra) in natural group-level contests is highly variable. Using playback experiments, we examined whether alpha-subordinate relationships, number of intruders, or age affected subordinate contest participation. Long-term census data were used to estimate the nature and duration of intra-group male associations. Some subordinate mates had long-term relationships with the alpha that existed prior to residence in the current group or they were the alpha male's grown sons. Other subordinate males were in more recent associations with the alpha that formed under seemingly antagonistic circumstances (e.g. the deposed alpha or his grown sons). We found that subordinate males in long-term alpha-subordinate associations had stronger howling and approach responses than males in short-term associations. Younger long-term associates had the strongest reactions to simulated intruders, similarly aged short-term associates rarely responded, and older males in both association categories responded at intermediate levels. We discuss the variable strategies of subordinate male black howlers and suggest that males who were more likely to participate in group-defence might be gaining direct or indirect fitness benefits by group living.