In a wide variety of mating systems, female choice is based on the ass
essment of male signals, both morphological and behavioural, presumed
to correlate with fitness. A crucial problem, therefore, is for female
s to determine whether the signal represents an 'honest' reflection of
male fitness. A dominant theoretical perspective in evolutionary biol
ogy suggests that signals are honest if and only if they are costly to
produce. At present, there are relatively few empirical studies of th
e costs and benefits of signalling in the mating context, and this is
especially the case for Primates. In this paper, I examine the possibi
lity that copulations calls - vocalizations that often elicit aggressi
ve competition within the mating arena - are honest signals of male qu
ality. 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 copu
lation are more likely to be aggressively attacked than males who are
silent during copulation. However, males who give copulation calls obt
ain 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 a
n important function in female mate choice.