Urinary Testosterone Metabolite Levels in Bonobos: A Comparison with Chimpanzees in Relation to Social System

Abstract

[Bonobo ( Pan paniscus ) social structure is characterized by partial female dominance, in contrast to the male dominated chimpanzee ( Pan troglodytes ) society. Furthermore, female bonobos exhibit more overt aggressiveness and a prolonged period of proceptivity during menstrual cycle compared to chimpanzees. Since dominance, aggressiveness and proceptivity are suggested to relate to high T levels, we expected T concentrations of bonobo females to be high. To test this, urinary T metabolite concentrations (measured by immunoreactive 5αandrostane-17α-ol-3-one) were determined, which reliably reflect T status in both species. We predicted a larger overlap between T metabolite concentrations of both sexes in bonobos compared to chimpanzees. Our results support this hypothesis. However, interspecific comparison for each sex revealed substantially lower T metabolite concentrations in bonobo males than in chimpanzee males, while female bonobos displayed levels about equal to those of chimpanzee females. It thus appears that low T metabolite levels in bonobo males are responsible for the large overlap instead of high T metabolite levels in bonobo females. In conclusion, this study suggests that quantitative sex differences in T levels may be predictive of social system. However, partial female dominance in bonobos is probably not mediated by high female T levels. Rather it appears that T levels of mainly male bonobos are adapted to social organization. We hope that these preliminary results ignite further research in this novel area., Bonobo ( Pan paniscus ) social structure is characterized by partial female dominance, in contrast to the male dominated chimpanzee ( Pan troglodytes ) society. Furthermore, female bonobos exhibit more overt aggressiveness and a prolonged period of proceptivity during menstrual cycle compared to chimpanzees. Since dominance, aggressiveness and proceptivity are suggested to relate to high T levels, we expected T concentrations of bonobo females to be high. To test this, urinary T metabolite concentrations (measured by immunoreactive 5αandrostane-17α-ol-3-one) were determined, which reliably reflect T status in both species. We predicted a larger overlap between T metabolite concentrations of both sexes in bonobos compared to chimpanzees. Our results support this hypothesis. However, interspecific comparison for each sex revealed substantially lower T metabolite concentrations in bonobo males than in chimpanzee males, while female bonobos displayed levels about equal to those of chimpanzee females. It thus appears that low T metabolite levels in bonobo males are responsible for the large overlap instead of high T metabolite levels in bonobo females. In conclusion, this study suggests that quantitative sex differences in T levels may be predictive of social system. However, partial female dominance in bonobos is probably not mediated by high female T levels. Rather it appears that T levels of mainly male bonobos are adapted to social organization. We hope that these preliminary results ignite further research in this novel area.]