Alpha male black howler monkey responses to loud calls: effect of numeric odds, male companion behaviour and reproductive investment

Abstract

When fighting is costly, avoiding contests with superior opponents should confer fitness advantages. Black howler monkeys, Alouatta pigra, produce loud calls that reliably indicate the minimum number of male group members. Using playback recordings, I manipulated ‘numeric odds’ (number of defending to intruding males) to examine whether alpha males use loud calls to assess relative fighting ability, and whether they modify responses to intruders based on relationships with male companions, level of subordinate male cooperation and/or presence of vulnerable offspring in the group. Results indicated that alpha males assess relative fighting ability; howling and approach responses were weakest when subjects faced unfavourable odds. Subordinate male contest participation was inconsistent and alpha males relied less on these unpredictable companions during risky approaches; whereas intruder number affected most howling and approach responses, companion number only influenced howling and orienting duration. Post hoc analyses suggested no effect of alpha–subordinate relationships on alpha reactions. However, preliminary evidence suggested that if subordinates participated, alpha males escalated most when they outnumbered intruders. Alpha males also had shorter howling delays as numeric odds (based on the presence of all subordinates) were increasingly in their favour. Thus, alpha males seemed to escalate in order to signal group-level fighting superiority rather than as a means to gather information on closely matched opponents. Finally, alpha males howled more quickly and for longer at potentially infanticidal intruders if small offspring were present in their group and never called when odds were against their group unless offspring were present, suggesting that contest value was influential.