Abstract

Sexual swellings are imprecise indicators of ovulation that allow females to attract males. The question remains why females of some species develop exaggeratingly large swellings when a less costly signal should suffice. We looked at one species, Procolobus tephrosceles, in Uganda, with small sexual swellings and a closely related species, Procolobus badius badius, in Ivory Coast, with large swellings. We investigated two possible reasons why females may require a large costly signal despite being considered the choosy sex in these primates. 1) Females could need a strong signal to outcompete other females in order to attract the right or the most males. 2) Large signals give females the opportunity to avoid male monopolisation allowing them to attract enough males to avoid infanticide risk or to achieve mate choice. To test these alternatives, we studied the length of the receptive period, operational sex ratio, female promiscuity, male interest in females and female proceptivity and choosiness in these two species with different sexual swelling sizes but similar social systems. Our preliminary results suggest that the species with the strongest female-female mating competition has the largest swellings but that the duration of the receptive period and the occurance of post-conception swellings is similar in both species. We suggest that the strength of the sexual signal (i.e. the species-specific size of the swelling) is determined by the intensity of female mating competition and male mate choice; while a long receptive period is sufficient to achieve promiscuous mating and avoidance of male monopolisation.