Changes in the secondary sexual adornments of male mandrills (Mandrillus sphinx) are associated with gain and loss of alpha status

Two semifree-ranging mandrill groups, inhabiting large, naturally rainfores ted enclosures in Gabon, were studied to measure morphological, endocrine, and behavioral changes that occurred when adult males rose, or fell, in dom inance rank. Gaining alpha rank (N = 4 males) resulted in increased testicu lar size and circulating testosterone, reddening of the sexual skin on the face and genitalia, and heightened secretion from the sternal cutaneous gla nd. Blue sexual skin coloration was unaffected. New alpha males increased i n rump "fattedness," but not in body mass, and spent more time associated w ith other group members, rather than ranging alone. Loss of alpha position (N = 4 males) resulted in less pronounced effects than those that occurred after males had risen to alpha positions. Deposed alpha males showed decrea sed testicular volume, decreased body mass, a reduction in the extent of re d (but not blue) sexual skin coloration, and decreased sternal gland activi ty. Deposed males did not decrease in the brightness of sex skin coloration . These results demonstrate that male-male competition and rank reversals h ave remarkable effects upon testicular function, secondary sexual traits, a nd behavior in the adult male mandrill. Secondary sexual traits respond to changes in male social status and therefore may be important as intrasexual signals of dominance rank. (C) 2001 Academic Press.