Early sexual maturity in male hamadryas baboons ( Papio hamadryas hamadryas ) and its reproductive implications

We present data on sexual maturity in young hamadryas baboon males ( Papio hamadryas hamadryas ) and its reproductive consequences in a large captive baboon colony. Hamadryas baboons live in a multilevel social system, with one‐male units (OMUs) as the smallest social entity. Male leaders of OMUs are believed to monopolize matings within their OMUs; hence mating is believed to be polygynous and monandrous. In a captive colony of hamadryas baboons, we found evidence that young males less than 4 years old fathered at least 2.5% of 121 offspring born subsequent to vasectomy of all adult males, and males aged 4–5 years fathered at least 16.5% of the offspring. Additional evidence that these young males are able to sire offspring came from a morphological comparison of sperm from hamadryas males of different ages. The sperm of a 48‐month‐old hamadryas baboon were morphologically indistinguishable from viable sperm from adult males, whereas sperm from a 45‐month‐old male showed some aberrations. If successful copulations by adolescent males constitute a regular pattern even in free‐ranging hamadryas baboons, a hamadryas male's chances to reproduce would not be limited to his role as an OMU leader as previously assumed, and a male's reproductive career would consist of two phases: the adolescent phase, and the OMU leader male phase. Am J Phys Anthropol, 2006. © 2005 Wiley‐Liss, Inc.