Chimpanzees (Pan troglodytes) modify grouping and vocal behaviour in response to location-specific risk

Chimpanzees have hostile intergroup relations and are reported to use two strategies to reduce risk in the territory periphery: travelling in larger subgroups ('parties'), and travelling silently. We examined data from the Kanyawara chimpanzee community, Kibale National Park, Uganda to test for evidence of these strategies. We compared behaviour in the territory core with two potentially dangerous contexts: the periphery and croplands. Parties that visited the periphery had over twice as many adult males as parties that remained in the core. Analysis of vocal production rate of 249 parties revealed that, controlling for time of day and party composition, chimpanzees produced fewer pant-hoot calls in croplands than in the core. Pant-hoot production varied in different sectors of the periphery, being reduced in three sectors, unchanged in one, and increased in one. Focal follows of 12 males found results similar to party follows, but with rank-related individual variation. Overall, these results indicate that chimpanzees have the ability to modify grouping and vocal behaviour to reduce risk in areas with a high risk of detection. However, rather than consistently remaining silent in the periphery, chimpanzees in this population sometimes increased their vocalization rate, perhaps to advertise territory ownership and coalition strength.