The theory of sexual selection suggests that certain traits may evolve as a result of "sperm competition," or competition between males vicariously through their bountiful production of sperm. Although there is evidence of this in many animal groups, one of the clearest documented cases in primates is the result of a study by Earthwatch-supported scientists working in Awash National Park, Ethiopia.
Drs. Jane Phillips-Conroy (Washington University, St. Louis) and Clifford Jolly (New York University) reported on the findings in a recent article in International Journal of Primatology. Reporting on the body mass and testicular size of 317 baboons, the authors found that adult male hamadryas (Papio hamadryas) baboons had significantly smaller testes than anubis baboons (P. anubis) (averaging 46 mm and 57 mm long, respectively).
"Our findings support theoretical predictions about the relationship between relative testis size and mating systems," said Phillips-Conroy, who with Jolly has been studying the baboons of Awash National Park for over 25 years. Hamadryas society is structured around permanent one-male harems, while in anubis baboon males compete for access to estrous females. The inherent differences in overt male competition is indicated biologically by the larger testes of anubis males
Other scientists have reported similar findings in primates, such as the relatively small testes of gorillas and large testes of chimpanzees. However this is the most complete data set for any primate species by far, representing years of measurements taken from tranquilized individual animals. It is also the first primate study to investigate the growth of testes during the male's development, requiring the accurate measurement of testes across the male lifespan.
"No-one has looked at the ontogeny of testis development," said Phillips-Conroy. "This is a totally unique component of the paper with quite unique findings. Although anubis baboons reach a larger final testis size, hamadryas baboons reach their full testis size much earlier in their development, reflecting on the divergent social systems of the two species."
The authors suggest that hamadryas baboons, which stay with their natal group as adults, will ultimately compete with males from their own social group. This within-group competition might explain the more rapid onset of testicular growth and its earlier completion than in anubis males, which by contrast emigrate from their natal group when nearly fully adult.
These results were supported by Earthwatch volunteers who assisted Phillips-Conroy and Jolly from 1983 to 1990, in their capture studies investigating the hybrid zone between hamadryas and anubis baboons. Their studies have yielded other important findings, most recently the documentation of "couch baboons," baboons that are relatively inactive and experience symptoms reminiscent of human obesity, with implications for the understanding of diabetes.
For more information, see Jolly, Clifford J., and Jane E. Phillips-Conroy, "Testicular Size, Mating System, and Maturation Schedules in Wild Anubis and Hamadryas Baboons." International Journal of Primatology, Vol 24 (1), February 2003, pp. 125-142.
For more information about "couch baboons," see http://www.newsday.com/mynews/ny-lsfit3268882may05.story
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