Terleph, T. A., Saralamba, C., & Reichard, U. H. (2025). Long-term vocal phrase stability in the white-handed gibbon (Hylobates lar). Animal Behavior and Cognition, 12(2), 235-242.  https://doi.org/10.26451/abc.12.02.03.2025

Gibbons are Asian apes that live in stable territories, often for decades. They produce loud, elaborate vocalizations (songs) that include well-coordinated male/female duets that are detectable by immediate neighbors and more distant groups. The female great call, the longest and most conspicuous phrase of the gibbon vocal repertoire, has been hypothesized to serve in territorial defense. Great calls of different individuals are statistically distinguishable, and a structurally stable great call should benefit a caller as it would reliably transmit caller identity over the long-term, to distant animals. The loud climax notes of this vocalization, however, are difficult to produce. We therefore predicted that physical decline with aging should reduce the frequency of high pitch climax notes within the great call. To determine the degree to which great call performance is stable over an extended time frame, we analyzed great calls from a wild population of white-handed gibbons at Khao Yai National Park, Thailand, each summer between 2013 and 2019 (except for 2018). We obtained multiple great calls per year from 9 subjects (5-6 years per animal) and compared calls within each animal between years, using a spectrogram cross-correlation procedure, and by comparing peak fundamental frequencies from call climaxes. We found within-subject stability in the great call and its climax over the 6-year period. This supports the territorial function hypothesis for great calls, as the calls can serve as a reliable indication of caller identity to distant animals. Our data do not support the hypothesis that climax notes change, at least over the time frame analyzed.

Gibbons, Bioacoustics, Vocal Communication, Primate vocalizations