Female Sexual Signalling in Wild Tufted Capuchin Monkeys: Implications for the Graded-Signal Hypothesis in a New World Primate

Research on primate sexual signals has largely focused on testing the adaptive significance of morphological signals of fertility against two main hypotheses, the graded signal hypothesis and the reliable indicator hypothesis. However, some primates lack these morphological signals but produce behavioural displays of sexual proceptivity. Such behaviours in tufted capuchin monkey (Cebus apella nigritus) females have been proposed to be functionally equivalent to mor-phological sexual signals in catarrhines. In this study, we examined the function of these behav-iours by testing predictions derived from the two hypotheses. Specifically, we recorded proceptive behaviours and copulations on two wild groups of tufted capuchins. We observed eight cycling females over two mating seasons for a total of 15 ovulatory cycles. These behaviours were then related to the timing of ovulation as determined by analysis of faecal progesterone metabolites. Results indicate that proceptive behaviours are significantly associated with fertile phases. More importantly, within the fertile phase, proceptive behaviours varied according to the probability of ovulation. In contrast, female proceptivity did not vary with measures of female reproductive quality (e.g., fecundity and dominance rank). Finally, when considering male mating responses, copulations were synchronized in relation to female fertile phase and were intensified by domi-nant males during days with a high probability of ovulation. Therefore, proceptive behaviours in this species appear to be reliable signals of ovulation and resemble a graded signal system. Ac-cordingly, males time their mating activity around the female’s fertile phase, with dominant males gaining access to females during days in which ovulation is more likely.