Circannual changes in the secondary sexual adornments of semifree-ranging male and female mandrills (Mandrillus sphinx).

Abstract

Male mandrills (Mandrillus sphinx) have spectacular secondary sexual adornments. These include red and blue sexual skin on the face, rump, and genitalia; a sternal scent-marking gland; and a "fatted" rump. Mandrills are seasonal breeders, and in other seasonally-breeding primate species members of both sexes may show increased expression of secondary sexual characteristics during the mating season. We examined changes in male secondary sexual adornments and testosterone levels, in relation to seasonal changes in the female reproductive cycle and sexual skin morphology, in two semifree-ranging mandrill groups. Females showed circannual changes in sexual skin tumescence, and periods of tumescence peaked from May-July in a long-established group. However, formation of a second, smaller group, two years previous to commencement of the study, disrupted the seasonal pattern of sexual skin tumescence and births. As the groups occupied adjacent enclosures, it appears that social factors, as well as physical environment, affected the seasonal patterning of reproduction in females. Male mandrills, by contrast, did not exhibit marked circannual changes in secondary sexual traits. Although adult male testicular volume and circulating testosterone levels increased significantly during the mating season, sexual skin coloration and rump "fattedness" showed no consistent changes with season. There was some evidence to suggest that maturing males (ages 5-8 yr) showed increased development of red sexual skin during mating periods, but once males had fully developed secondary sexual adornments, they remained stable throughout the year. The possible reasons for this are discussed in relation to intermale competition and social organization in mandrills.