Rhesus monkey copulation calls: honest signals for female choice?

In a wide variety of mating systems, female choice is based on the assessment of male signals, both morphological and behavioural, presumed to correlate with fitness. A crucial problem, therefore, is for females to determine whether the signal represents an 'honest' reflection of male fitness. A dominant theoretical perspective in evolutionary biology suggests that signals are honest if and only if they are costly to produce. At present, there are relatively few empirical studies of the costs and benefits of signalling in the mating context, and this is especially the case for Primates. In this paper, I examine the possibility that copulations calls--vocalizations that often elicit aggressive competition within the mating arena--are honest signals of male quality. Observations of rhesus monkey (Macaca mulatta) mating behaviour reveal that the proportion of copulating males who call decreases as competition for oestrous females increases. Males who call during copulation are more likely to be aggressively attacked than males who are silent during copulation. However, males who give copulation calls obtain more matings than males who do not, and this is true for high- and low-ranking males. Because of the cost-benefit tradeoffs, copulation calls are interpreted as honest indicators of quality that may serve an important function in female mate choice.