Long-Distance Vocal Signaling in White-Handed Gibbons (Hylobates lar)

Few primates are characterized by strict territoriality, pair-bonding, and loud, complex vocalizations. Gibbons (Hylobatidae) are amongst them, with mated pairs of most species performing duet songs that may strengthen pair-bonds and/or defend a territory. Gibbon duets often are audible well beyond a pair’s territory boundaries, suggesting that they function in intergroup signaling not just with close neighbors, but also with distant neighbors, yet no clear evidence for communication between distant neighbors has been found and direct tests of long-distance signaling are lacking. We studied vocal interactions in wild, white-handed gibbons (Hylobates lar) over two 10-day collection periods, from 340 recording hours on passive acoustic monitors at Khao Yai National Park, Thailand. Singing animals exhibited rapid control over vocal output (call inhibition) in response to short climax-coda components of distant pairs’ songs, supporting the intergroup long-distance signaling hypothesis. These climax-coda sequences are the only occasions in the species’ repertoire when phrases from both members of a pair regularly and unambiguously occur in rapid succession, suggesting a signaling of pair-bonded status to other groups, because only pair-bonded individuals sing in this way together. The call inhibition may allow distant receivers to estimate the location of a singing group’s territory, to identify potential mates or rivals, and may provide information that facilitates natal dispersal of mature offspring. We also provide evidence to support the hypothesis that the great call may have evolved a lengthy onset to allow distant receivers to detect the great call sequence before its climax-coda components occur.