Martin N Muller

Dominance, aggression and testosterone in wild chimpanzees: a test of the ‘challenge hypothesis’

The ‘challenge hypothesis’ posits that variation in male testosterone levels is more closely associated with aggression in reproductive contexts than it is with changes in reproductive physiology. Numerous bird studies support this idea, but few tests have been conducted with primates. We conducted behavioural observations and noninvasive hormone sampling of 11 male chimpanzees, Pan troglodytes schweinfurthii, in the Kanyawara study site, Kibale National Park, to test predictions of the challenge hypothesis. Results indicated that adult male chimpanzees showed significant testosterone increases during periods when parous females showed maximally tumescent sexual swellings. These periods were also marked by increased rates of male aggression. Male testosterone levels did not increase in the presence of maximally tumescent nulliparous females. Such females are less attractive to males: they are not mate-guarded, nor do rates of male aggression increase when they are swelling. Male chimpanzees copulate with parous and nulliparous females at similar rates, however, suggesting that testosterone increases in the presence of cycling parous females are associated with aggression rather than sexual behaviour. High-ranking chimpanzees were more aggressive than low-ranking males and produced higher levels of urinary testosterone. Thus, the predictions of the challenge hypothesis were generally upheld. This suggests that the hypothesis may have wider applicability among primates, including humans.

Comparisons of Limb Structural Properties in Free-ranging Chimpanzees from Kibale, Gombe, Mahale, and Taï Communities

Structural characteristics of limb bones provide insight into how an animal dynamically loads its limbs during life. Cause-and-effect relationships between loading and the osteogenic response it elicits are complex. In spite of such complexities, cross-sectional geometric properties can be useful indicators of locomotor repertoires. Typical comparisons use primates that are distinguished by broad habitual locomotor differences, usually with samples garnered from several museum collections. Intraspecific variability is difficult to investigate in such samples because knowledge of their behavior or life histories, which are tools for interpreting intraspecific variability, is limited. Clearly, intraspecific variation both in morphology and behavior/life history exists. Here we expand an ongoing effort toward understanding intraspecific variation in limb structural properties by comparing free-ranging chimpanzees that have associated behavioral and life history data. Humeral and femoral data from 11 adult chimpanzees (Pan troglodytes) of Kibale National Park (Uganda) are compared to 29 adult chimpanzees from Gombe (Tanzania), Mahale Mountains (Tanzania), and Tai Forest (Cote d’Ivoire) National Park communities. Overall, limb structural morphology of Kibale chimpanzees most resembles limb structural morphology of Mahale chimpanzees. Shape ratios and percentage cortical areas of Kibale chimpanzees are most similar to non-Gombe chimpanzees, while Kibale structural properties, e.g., maximum rigidity, are most similar to non-Tai structural properties. Even after adding Kibale females, Tai females continue to stand out from females in other communities.