Javan Gibbons (Hylobates moloch) Vary Gesture Use by Recipient’s Attentional State at the Gibbon Conservation Center, California

Gestural communication of large-bodied apes has been extensively studied, however data on small-bodied ape communication are less common. We explored gestures used in commun,ication by captive Javan gibbons (Hylobates moloch) housed at the Gibbon Conservation Center in Santa Clarita, California. We hypothesized that a sender gibbon’s gesture modality would vary with the recipient gibbon’s attentional state. We predicted that senders would be equally likely to use all gesture modalities (i.e., tactile, visual, actions, and facial expressions) when the recipient was attending or facing the sender, but would be biased toward tactile gestures and actions when the recipient was non-attending or oriented away from the sender. We collected data from three gibbon groups, n=10 individuals, using all-occurrences sampling and an ethogram to score behaviors from digital video recordings. Occurrences of gestures through the four modalities were recorded during a gibbon’s attempt to interact with another group member. We observed 1,143 interactions over 20 days. When all data were aggregated, gibbons used visual gestures, t(18)=2.79, p=0.01, and facial expressions, t(18)=2.60, p=0.02, significantly more when the recipient was attending and used tactile gestures, t(17)=2.47, p=0.02, significantly more when the recipient was non-attending. There was no significant difference in the actions modality, t(18)=0.82, p= 0.43. These data show that Javan gibbons used gestures that are appropriate to the recipient’s attentional state in three out of the four modalities