Although we read “The Other Closest Living Relative” by Parish and de Waal (2000), I want to broaden the discussion to consider some recent research on bonobos. Parish and de Waal (2000) outline some of the major differences between bonobos and chimpanzees, and their implications for understanding human evolution. One of the differences they highlight is female dominance in bonobos. Another difference is in female bonding and sexuality. I wanted to share some recent research on these topics that might change some of the ways we understand these differences.
1) Dominance in males and females
Female dominance does occur in other mammals, such as lemurs and hyenas. However, compared to male dominance, it’s comparatively rare. When Alison Jolly drew attention to female dominance in lemurs, it was a challenge to the assumption that male dominance was “natural” among primates. Similarly, when Amy Parish emphasized female dominance in bonobos, it challenged our chimp-centric assumptions about human ancestors. Because female dominance is comparatively rare, it seems likely to assume that the major differences are due to changes in female physiology However, recent research on testosterone in bonobos and chimpanzees indicates greater differences in male physiology.
Although testosterone is a primary male reproductive hormone, it is associated with dominance and competition in both sexes (Wingfield et al., 1990; Sannen et al., 2005). Sannen and colleagues (2003) predicted that female dominance in bonobos was related to higher testosterone levels than female chimpanzees. To test this, they examined urinary metabolites in chimpanzees and bonobos. However, they found that female bonobos and chimpanzees comparable amounts of testosterone metabolites. Instead, the major differences they found were in the male bonobos testosterone metabolites. These were substantially lower than male chimpanzees. Although male bonobos did have significantly higher levels than females, this difference was relatively small. Male chimpanzees, on the other hand, had very high levels of testosterone metabolites. Sannen and colleagues (2003) hypothesize that low male testosterone might be an adaptation to female dominance. Further differences between male chimpanzees and bonobos were found by Wobber and colleagues (2010). They examined differences in competitive strategies by placing male chimpanzees and bonobos in a dominance contest and measuring urinary testosterone. While male bonobos paired with dominant partners exhibited a rise in cortisol concentrations, male chimpanzees place in the same condition exhibited a rise in testosterone. They suggest that males in the two species perceive the identical situation differently; while male bonobos perceive feeding competition as a stressor, chimpanzees perceive it as a contest for dominance.
2) Sex and Social Bonds in Females
Bonobos engage in high rates of sexual behavior in a variety of different pairings, and one of the most predominant behaviors is gentital-genital (GG) rubbing between females. Given this obvious difference between chimpanzees and bonobos, it’s easy to link GG rubbing to the strong female social bonds found in bonobos. However, Hohmann and Fruth (2000) examined hypotheses regarding the functions of G-G rubbing, and found little support for the social bonding hypothesis. They proposed five hypotheses for the function of GG rubbing in female social relationships: 1) reconciliation, 2) mate attraction, 3) tension regulation, 4) expression of social status, and 5) social bonding. However, in testing these hypotheses in a field study of wild bonobos, they found some support for every hypothesis except social bonding. GG rubbing increased following conflicts, supporting the reconciliation hypothesis. It also occurred at higher rates in mixed-sex groups, but did not affect copulation rates, providing mixed evidence for the mate attraction hypothesis. GG rubbing increased when food availability was limited, supporting the tension regulation hypothesis. However, GG rubbing did not occur at higher rates between close spatial associates, and it did not correlate with grooming relationship. In another study, Hohmann and colleagues (2009) tested the tension regulation hypothesis in captive female bonobos by comparing rates of genital contacts to salivary cortisol concentrations. They found that cortisol concentrations rose in anticipation of food competition, but genital contacts did not have a consistent relationship with cortisol concentrations.
What do these findings tell us? Unfortunately, many of them bring up more questions than they answer. However, they suggest caution in making assumptions about human evolution. Parish and de Waal (2000) raise important points to consider in re-evaluating some of our previous models for understanding human evolution. However, it appears that some of the most notable characteristics of bonobos, such as female dominance, sexuality, and bonding, are more complicated than we realize. Furthermore, as Parish and de Waal (2000) mention, chimpanzees and bonobos have many differences that arose within the past million years. Some of the unique features we seen in bonobos may be recently derived traits. We need to be careful about assuming that all of these behaviors are directly linked, or in proposing a bonobo-centric model of human evolution. I think that bonobos provide a great comparative perspective for considering the role of non-reproductive sexual behaviors, including homosexual behavior, in human evolution. However, bonobos do not form the pair-bonds that are characteristic in human societies. Studying the underlying mechanisms between these differences are what is likely to lead to new insights to help us better understand our own evolution.
Hohmann G, Fruth B. 2000. Use and function of genital contacts among female bonobos. Anim Behav 60:107–120.
Hohmann G, Mundry R, Deschner T. 2009. The relationship between socio-sexual behavior and salivary cortisol in bonobos: tests of the tension regulation hypothesis. Am J Primatol 71:223–32.
Lancaster JB, Kaplan HS. 2009. The Endocrinology of the Human Adaptive Complex. In: Ellison PT GP, editor. The Endocrinology of Social Relationships. Cambridge, MA: Harvard University Press. p 95–118.
Sannen A, Van Elsacker L, Heistermann M, Eens M. 2005. Certain aspects of bonobo female sexual repertoire are related to urinary testosterone metabolite levels. Folia Primatol 76:21–32.
Wingfield J, Hegner R, Dufty Jr A, Ball G. 1990. The “Challenge Hypothesis”: Theoretical Implications for Testosterone Secretion, Mating Systems and Breeding Sytrategies. Am Anthropol 136:829–846.