Impacts of Intergroup Interactions on Intragroup Behavioral Changes in Javan Gibbons (Hylobates moloch)

Agonistic intergroup interactions can cause individual costs such as physical injuries, increased physiological stress, and disrupted intragroup social relationships. Therefore, individuals should employ behavioral strategies to minimize the cost associated with aggressive intergroup encounters (IGEs). We investigated the behavioral strategies of territorial, pair-living Javan gibbons ( Hylobates moloch ) in response to intergroup aggression, including 1) changes in activity budgets, 2) affiliative behaviors within pairs, and 3) potential intergroup avoidance strategies, such as sleeping tree selection. We observed 129 encounters in three habituated gibbon groups surrounded by four unhabituated groups in Gunung Halimun-Salak National Park, Java, from 2014 to 2016. Overall, gibbons significantly altered their activity budgets during IGEs: they foraged less and were less active than when there was no encounter. We also found a decrease in grooming within pairs following IGEs compared to a matched-control period. However, we did not find any effects of the intensity of aggression, outcome, length of, or female participation in IGEs on grooming within pairs in the hour after an encounter. Male gibbons slept farther away from the IGE location after aggressive IGEs than after neutral encounters. Our results suggest that not only primates living in large groups but also primates living in small groups, such as Javan gibbons, may develop behavioral strategies to deal with direct (during encounters), immediate (within 1 h after encounters), and possibly longer (at the end of a day with encounters) effects of intergroup interactions.