Non-aggressive inter-group interactions in wild Northern Gray gibbons (Hylobates funereus)

Gibbons exhibit territorial behavior, and mated pairs defend exclusive territories. However, reports on contacts with neighbors near or outside the respective range boundaries are scarce because observing unhabituated neighboring groups is difficult. Studies of encounters have yielded a wide range of observations (aggressive, neutral, communicative, or affiliative) across gibbon species and geographic regions. As it is important to understand behavioral ecology and social relationships among gibbon groups, we studied three Northern Gray gibbon groups (Hylobates funereus) in the Danum Valley Conservation Area (DVCA), Malaysia and observed their interactions with neighboring groups. From 2001 to 2020, three groups were followed for 162 days (2236.4 h), and we observed 30 instances of encounters at the range boundary or intrusions between a study group and a respective neighboring group. Neutral or communicative encounters were observed in most cases, with the exception of one instance of aggressive encounter (chasing). Gibbons interacted through particular vocalizations (termed songs) in many cases. We report details of some characteristic interactions among gibbon groups during encounters at the range boundary or intrusions. Regarding intrusions, we observed an unusual instance where a neighboring group intruded deep into the study group’s territory, and the neighboring group did not respond to the songs of the resident group. Considering that no physical fighting occurred during encounters or intrusions, gibbon groups in the DVCA seem to be non-aggressive toward each other. We briefly discuss possible explanations for such non-aggressive relationships among gibbon groups.