The roles of food quality and sex in chimpanzee sharing behavior (Pan troglodytes)

Both wild and captive chimpanzees (Pan troglodytes) share food with non-relatives. Researchers have proposed several hypotheses to explain this behavior, including ‘food for sex’, ‘food for grooming or agonistic support’, and ‘sharing under pressure’. We examined food sharing in two captive groups of socially-housed chimpanzees. In contrast to previous captive studies, which only examined transfers of low-quality foods, we conducted seven trials with high-quality food and seven with low-quality food for each group to directly compare transfers of different food qualities. We recorded how male chimpanzees shared food, including active transfers, passive transfers, and co-feeding. We also noted all instances of copulations, female estrous states, benign attempts to access food (termed ‘perseverance’), and aggressive attempts (termed ‘harassment’) to examine whether any of these factors influenced food sharing. Male food possessors shared at the same rate in both food quality conditions, but seemingly for different reasons, indicating that food quality may affect the exchange of social benefits in chimpanzees. In the low-quality condition, there was an interaction with rank and perseverance: while low- and middle-ranking females received more food the more they persevered, high-ranking females received more food without perseverance and gained relatively little benefit from persevering. In the high-quality condition, there was an interaction between copulations and perseverance: females who copulated with the male food possessor received more food during that trial with less perseverance. Non-copulating females received more transfers the more they persevered. This result was only observed in the short-term — copulations over the previous year were not correlated with food transfers. Further, the copulations observed here were unusual for these chimpanzees in that they were not confined to peak fertility, suggesting a non-conceptive function for copulations in chimpanzees. Copulations in this study may have functioned to reduce tension and increase short-term tolerance, allowing females better access to food.