Endocrine Responses to Two Distinct Foraging Contexts in the Bonobo (Pan paniscus)

In gregarious animals, a social structure characterized by female dominance is relatively rare. However, bonobos (Pan paniscus), one of the two species most closely related to humans, are characterized by partial female dominance, with females forming close social bonds. In contrast, chimpanzees (Pan troglodytes), a close relative of the bonobo, are characterized by strong male-male bonding and social dominance with female chimpanzees lacking close social bonds. There is a lack of consensus concerning the reasons for these behavioral differences in Pan, specifically, the physiological processes responsible for female dominance in bonobos. The role of hormones and their effects on behavior has been widely studied across species. The primary male hormone testosterone (T) has been studied in relation to male dominance and its correlation with aggression. Past studies have found a positive correlation with T and male dominance. Adversely, studies looking at the stress hormone cortisol have found that subordinates tend to have higher cortisol levels, while other studies have found dominant individuals in social animals have higher cortisol levels. The purpose of this study is to further understand the roles that hormones play in the physiology and behaviors of a species with a unique social structure. Therefore, to measure the physiological responses of competition, and to determine if there were any differences between the sexes, a foraging experiment was conducted in which testosterone and cortisol were measured before and after individual’s were given the opportunity to forage with conspecifics. In one condition, food was scattered throughout the enclosure, whereas in a second condition, the food was placed in a single pile in the center of the enclosure. In addition, social behaviors and social proximity were quantified to complement endocrine results. We found a significant difference in social proximity scores in the pile vs the scatter condition. We did not find a significant difference based on sex in proximity scores. Additionally, we found no difference in cortisol or testosterone shifts in either condition. Although this study did not find a connection between foraging conditions and hormone levels, it contributes to the growing body of research on endocrine responses in captive-living animals and highlights the need for further investigation into the hormonal influences on sociality in species like the bonobo. Further studies investigating this connection are therefore warranted.