Hand modulation of vocalization in Siamangs Hylobates syndactylus

Abstract

Although hand modulation of vocalization in Siamangs Hjfohates syndactylus in captivity has been reported in the literature its functional significance within the Hylobatidae has not been thoroughly investigated. One‐hundred‐and‐ten sequences of vocalizations were observed in a pair of Siamangs maintained at the International Center for Gibbon Studies, Santa Clarita. The ♀ used both the palmar aspect of the hand and the distal part of the forelimb to modify vocalizations during diphasic barking in the ‘great‐call’ duet. Analysis of this behaviour indicates that it occurs independently of intragroup social context. Data collected for 150 gibbons, representing all four subgenera and 11 species, indicate that hand modulation of vocalization is unique to Siamang. Hand‐rearing did not effect the development of the behaviour in ♂♂ but did appear to be conducive to its development in ♀♀.