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 concentrationsof bonobo females to be high. To test this, urinary T metabolite concentrations (measured by immunoreactive 5®androstane-17®-ol-3-one) were determined, which reliably re ect T status in both species.

According to a previously proposed coping model (De Waal [1989] Zoo Biol. 1(suppl):141-148), gregarious primates will react to crowding by adjusting their behavior in order to limit the amount of aggression displayed. Depending on the duration of the crowding, this can be achieved by the use of different strategies. Chimpanzees (Pan troglodytes) react to a medium-term duration of spatial crowding by an active tension-reduction mechanism whereby allogrooming markedly increases while aggression rises only slightly. Under comparable circumstances, a similar increase in grooming is found in bonobos (Pan paniscus). However, to test the coping model for bonobos, data on aggressive behavior during crowding are indispensable. This study provides such information by comparing the aggressive repertoire of a well-established bonobo colony during a crowded period in winter with those that occurred during an uncrowded control period in summer. Given that during winter the aggression rate differed between food and nonfood contexts, we accounted for a potential effect of food context on aggression. During the crowded period, a significantly higher total frequency of aggression was found than during the control period. However, this increase was small in relation to the reduction in space when compared to similar experiments in other species. Together with observations of increased grooming under crowded conditions, our data confirm the hypothesis that bonobos cope with the increase in tension created by crowding (medium-term duration) by applying a tensionreduction strategy. Our data further show that although not all aggressive behaviors appear to be influenced by a high-density condition, bonobos do not specifically limit their aggressive behaviors to mild, nonprovocative aggressions in a crowded environment.

The correlation between testosterone (T) and dominance rank may vary among species, and is expected to become stronger as the importance of aggressive competition for rank increases. However, it may also vary among social situations within a species, showing a stronger correlation during socially unstable periods. Knowledge on this topic in great apes, especially in females, is scant. This study presents the first data on the relationship between T and dominance rank in both sexes of the bonobo ( Pan paniscus). For each period (four socially unstable and two stable ones), linear rank orders were determined and subsequently correlated with the accompanying mean urinary T-metabolite concentrations (measured as immunoreactive 5alpha-androstan-17alpha-ol-3-one). No correlation between these two variables was found for either sex among individuals during socially unstable or stable periods. Also, within an individual over the six periods, no relationship of T with rank could be demonstrated. These results suggest that either the outcomes of aggressions have no influence on T levels, or such clear outcomes appear insufficiently frequent to affect T levels over longer periods. Even during the unstable periods, the rate of aggressions was not higher than during stable periods, suggesting that frequencies of aggression have little effect on rank. Further analyses indeed demonstrated no correlation between frequencies of overall aggressions or any type of aggressive behavior separately, or rank. Perhaps factors other than the frequency of displayed aggressions alone have a marked influence on a bonobo's rank, for example, coalition partners. Overall, in bonobos, T apparently does not form a physiological reflection of social status.