The white-handed gibbon
This short field note by C. Boden Kloss records the occurrence of the white-handed gibbon (Hylobates lar) in East Sumatra and offers observations on its colour variation, social structure, and family group dynamics.
Summary
Kloss reports that the species, previously regarded as restricted to the Tenasserim Province and the Malay Peninsula, was encountered in large numbers in the swampy regions of the Siak and Indragiri districts of East Sumatra, where he and Dr. W. L. Abbott observed it in biscuit-coloured, brown, and sooty pelage. He argues that these colour forms are not distinct races, noting pie-bald individuals and dark females with pale infants, and infers that the pale form is the rarer, innate condition (p. 79).
On social behaviour, Kloss describes gibbon family groups of two parents and offspring of different births, typically numbering four or fewer, with the eldest leaving to form a new pair when a third infant appears. He contrasts this family-based social unit with the banding habits of lower monkeys and links it to theories of primitive human social organisation, while noting that the gibbon’s agility distinguishes it from the more solitary gorilla and orang (p. 80).