Copulation calls in male formosan macaques: honest signals of male quality?

Male copulation calls are found in a number of polygynous primate species, but their functions still represent a puzzle [1]. It has been proposed that among rhesus macaques, male copulation calls are sexually selected handicap signals that females can use to evaluate male quality on account of the cost to males of calling (specifically increased aggression shown to callers) [2]. Evidence in favor of this hypothesis is inconclusive, however, and an alternative explanation of their occurrence is that the calls simply reflect overall levels of male courtship intensity [3]. This report describes for the first time the acoustic structure and pattern of occurrence of male copulation calls among the little-studied Formosan macaques. This study also evaluates whether calling is related to male rank and leads to increased levels of aggressive harassment received by mating males, as predicted by the handicap signal hypothesis.