Testicular function, secondary sexual development, and social status in male mandrills ( Mandrillus sphinx)

Abstract

Positive correlations between dominance rank and plasma testosterone levels have been described for adult males of several primate species in captivity, but the relevance of such observations to free-ranging animals is unclear. CIRMF in Gabon maintains a breeding group of 45 mandrills in a six hectare, naturally rainforested enclosure. This study describes correlations between dominance rank (in agonistic encounters), levels of plasma testosterone, testicular volume, body weight, and development of secondary sexual characteristics (red and blue sexual skin on the muzzle and rump areas) in male mandrills under semifree ranging conditions. Two morphological and social variants of adult male mandrill were identified. Large-rumped or fatted adult males ( n = 3) remained in the social group and exhibited maximal development of sexual skin coloration as well as large testicular size and highest plasma testosterone levels. By contrast, slimmer-rumped or nonfatted males ( n = 3) lived a peripheral or solitary existence and these exhibited less development of their secondary sexual coloration and had smaller testes and lower plasma testosterone levels. Longitudinal studies of gonadal development in these six males revealed that testicular volumes and plasma testosterone levels increased most rapidly during pubertal development (4–5 years of age) in the three animals which proceeded to the fatted condition. These included the highest ranking, group-associated male which exhibited the most intense sexual skin coloration and had higher testosterone levels, although this was not correlated with testicular volume. This study shows that in the male mandrill social factors and reproductive development are interrelated. Physical features characterising dominant, group-associated males are subject to more rapid pubertal development, and circulating testosterone levels are higher in such individuals both during puberty and in adulthood.