Abstract

A high rank often confers advantages due to an increased access to resources like nutrients or females in oestrus („priority of access“ model). Sexual swellings in females may generally help males to assess the reproductive state of females and adjust their mating behaviour accordingly. However, the reliability of the signal has been found to be highly imprecise in several species. We observed red colobus monkeys (Piliocolobus tephrosceles) in Kibale National Park to investigate the role of swellings for male and female mating strategies in this species. Red colobus live in multimale-multifemale groups with male philopatry, where females copulate with several males during each cycle. Using a variety of data collection techniques during a six month study on a group of 37 individuals, we found that particularly young males tried to monopolise females with swellings. We documented several cases where pregnant females continued to display sexual swellings. It has been suggested that pregnant females exhibit sexual swellings in order to confuse paternity and thereby reduce the risk of male harassment or even infanticide. While males still copulated with these females, we found a clear reduction in sexual activities compared to females in oestrus. This result suggests that pregnant females are less attractive and that males may be able to partly assess the reproductive state of females.