Ecological and social factors affect the occurrence and outcomes of intergroup encounters in chimpanzees

Male chimpanzees, Pan troglodytes, defend group territories, and sometimes injure or kill members of other groups. To test which factors best predict the occurrence and outcomes of intergroup encounters, we analysed 15 years of data on intergroup encounters, party composition, ranging and feeding behaviour in the Kanyawara community of chimpanzees, Kibale National Park, Uganda. During this time, researchers observed 120 intergroup encounters, most of which (85%) involved only acoustic contact. The majority of encounters (63%) occurred in the southeast quadrant of the range. Multiple logistic regression found that intergroup encounters occurred more often when chimpanzees were far from the centre of their range, especially in the south, and when eating foods that were most abundant in the south. Multiple linear regression found that chimpanzees travelled further from the centre of their range, and further south, when eating a few key foods, especially fruits of Uvariopsis congensis, and when in parties with more males. Upon hearing calls from foreign chimpanzees, Kanyawara chimpanzees were more likely to vocalize in response, and to travel towards the foreign chimpanzees, when they had more males in their party. Measures of two resource values, food and mates, were negatively correlated with the probability of vocalizing and approaching intruders, respectively. These findings indicate that, in this population, resources affect the timing of intergroup encounters, but the decision to escalate a contest depends mainly on numerical strength, rather than the value of resources being contested.