Arrested Development of Secondary Sexual Adornments in Subordinate Adult Male Mandrills (Mandrillus sphinx) Joanna M. Setchell 1 * and Alan F. Dixson 2 1 Subdepartment of Animal Behavior, University of Cambridge, Madingley, Cambridge CB3 8AA, UK 2 Center for the Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, California 92112 KEY WORDS socioendocrinology; dominance rank; testosterone; sexual skin; alternative strategies ABSTRACT Previous studies of semifree-ranging mandrills identified two morphological and social variants of the adult male, based on behavioral and secondary sexual characteristics. “Fatted” males are social, with highly developed sex skin coloration, large testes, high plasma testosterone levels, and fat rumps; while “nonfat- ted” males are peripheral or solitary, with paler sex skin, smaller testes, lower plasma testosterone, and slimmer rumps. We present a detailed study of morphology and group association for 10 adult male mandrills, living in two semifree-ranging groups in Gabon, in order to relate differences between males to dominance rank. The results show that rather than existing as two distinct morpho- types, male mandrills represent a continuous spectrum of possibilities between social males with fully developed secondary sexual characteristics, and solitary males with muted secondary sexual characteristics. Alpha males (N = 2) had the highest testosterone levels, the most colorful sex skin, and the most active sternal glands, and were the only males to spend 100% of their time with the social group. Rank relationships between nonalpha males (N = 8) were not always clear, but all subordinate males had lower testosterone levels and less development of the secondary sexual adornments, and were less group-asso- ciated than alpha males. These findings suggest that only alpha males have sufficient testosterone to develop full secondary sexual characteristics, and we propose possible socioendocrine mechanisms underlying the suppression of testosterone and secondary sexual development in subor- dinate adults. We discuss differences in secondary sexual development in relation to reproductive strategies, and discuss the evolution of alternative reproductive morphs in primates. Am J Phys Anthropol 115:245–252, 2001. © 2001 Wiley-Liss, Inc. The mandrill (Mandrillus sphinx) provides an ex- treme example, among the mammals, of sexual di- morphism in body weight and secondary sexual traits (Darwin, 1871). Arguably the most dimorphic primate, adult males are massive (30 –35 kg body mass) and more than twice as heavy as females (Setchell, 1999). Adult males have long canines; pro- nounced, bony, supramaxillary swellings; brilliant red and blue sexual skin on the face, rump, and genitalia; a yellow beard; a cape; and a white epi- gastric fringe (Hill, 1970). Unusually for Old World monkeys, mandrills have a sternal cutaneous gland, and this is also larger in males (Hill, 1970). Previous studies on a small number of males showed that the degree of development of secondary sexual traits is subject to a large measure of indi- vidual variability in adult males. The secondary sex- ual development of 6 adult males appeared to be related to the extent to which they associated with the social group. Social, “fatted” males were brightly colored with large testes and a stocky appearance, while solitary or peripheral “nonfatted” males were paler, with smaller testes and generally less devel- oped secondary sexual adornments (Wickings and Dixson, 1992b). This raises the question of whether some male mandrills have suppressed secondary sexual adornments in order to reduce intermale competition as part of an alternative reproductive strategy. Here we investigate in detail, for a larger number of males living in the same semifree-rang- ing colony as that studied previously (Wickings and Dixson, 1992a,b; Dixson et al., 1993), the question of male dominance rank in male mandrills, how this relates to other behavior such as degree of group association, and how behavior is related to male morphology and endocrinology. METHODS Subjects The mandrill colony at the International Medical Research Center in Franceville, Gabon, was estab- lished in 1983, when 14 animals (6 males, 8 females) were released into a 6.5-hectare naturally rain- Grant sponsor: Medical Research Council of Great Britain. *Correspondence to: Dr. Joanna M. Setchell, now at the School of Life Sciences, University of Surrey Roehampton, West Hill, London SW15 35N, UK. E-mail: j.setchell@roehampton.ac.uk Received 13 June 2000; accepted 5 March 2001. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 115:245–252 (2001) © 2001 WILEY-LISS, INC.