Presentation Information
[P2-40]Female gibbons’ great calls change tempo in the presence of their offspring
*Yoichi Inoue¹, Waidi Sinun², Kazuo Okanoya¹ (¹Teikyo University, ²Research and Development Division, Yayasan Sabah Group)
Gibbons are small apes distributed throughout Southeast Asia known for their loud vocalizations (songs). Female gibbons produce a loud, elaborate, and stereotyped song known as the great call (GC). From 2009 to 2025, we observed four females in three groups of Northern Gray gibbons (Hylobates funereus) in northern Borneo and recorded their songs to investigate whether female GCs change before and after giving birth. Observations suggest that infant gibbons begin producing high-pitched vocalizations synchronized with their mother's GC from birth. By approximately 26 months of age, offspring begin to sing the initial portion of the GC in synchrony with their mother, gradually lengthening their contribution. We hypothesized that females may adapt their vocalizations to match their offspring’s developing song—similar to human motherese—by producing slower, shorter calls with higher pitch. To test this hypothesis, we analyzed the GC’s duration, number of notes, maximum and minimum frequencies, and note speed. The GC consists of a series of notes: the first half begins slowly and accelerates, while the second half maintains a steady tempo. Therefore, note speed was measured separately for the first 20 notes and for the 21st note onward. We compared these acoustic features before and after the birth of offspring. Although our overall findings did not support the hypothesis, we found that note speed in the first half of the GC was significantly faster after birth. This change may reflect hormonal effects related to lactation or heightened maternal arousal for offspring protection, but the underlying cause remains unclear. Further studies are needed to clarify the function and mechanisms of this vocal change (Work supported by JSPS 25H21984).