Testicular Size, Developmental Trajectories, and Male Life History Strategies in Four Baboon Taxa

Chapter Summary

Sociobiological theory predicts that natural selection via sperm competition will favor greater relative testicular size in adults of polyandrous species than in their monandrous relatives. We have previously shown that, among baboons of the Awash National Park, Ethiopia, “multimale” olive baboons have testes larger relative to total body mass than “one-male unit (OMU)” hamadryas, with most of the difference attributable to a late growth spurt in olives. In this chapter, we use a sample of yellow baboons captured in the Mikumi National Park, Tanzania, and Guinea baboons living in a captive colony to test the prediction that they will resemble olive and hamadryas baboons respectively, in relative, adult testicular size. Like olives, yellow baboons develop large testes in a late growth spurt, while Guineas, like hamadryas, do not. Yellow baboons apparently have relatively smaller testes than olives at all ages, but this effect is probably an artifact of their long-limbed body build and disappears if a measure of trunk volume (rather than total body mass) is used as a proxy of functional body size. Previous work also showed a difference between olive and hamadryas baboons in juvenile testicular ontogeny, explicable in terms of dispersal rather than adult testicular size. Male hamadryas, which breed in their natal group, undergo testicular enlargement earlier than olives, perhaps reflecting general sexual precocity and/or opportunities to sneak copulations while “following” an OMU.

Olive and yellow baboons, which have few mating opportunities before dispersal, have less developed testes as juveniles. Yellow baboons seem to be more extreme than olives in this respect, perhaps reflecting a lower propensity to disperse as juveniles, and thus fewer preadult mating opportunities. The few available data suggest that Guineas tend to resemble hamadryas in testicular ontogeny.