Description
Northern white-cheeked gibbons are sexually dimorphic, with males and females having different colourations and the former also being slightly larger. Males have black hair over their entire bodies, except for distinct white patches on their cheeks, as well as a prominent tuft of hair on the crown of head, and a gular sac. Females are reddish-tan in colour, lack a cranial tuft, and have a crest of black or dark brown fur running from the crown to the nape of the neck. They are reported to have an average weight of 7.5 kilograms (17 lb), although this is based on only a small number of wild individuals, and those in captivity appear to be larger.
Like other members of their genus, both males and females have unusually long arms, even for gibbons, with the arms being 1.2 to 1.4 times as long as the legs. They are also more muscular, with heavier thighs and shoulders that suggest a greater bodily strength. Adults have been shown to demonstrate a hand preference while swinging through the trees, with individuals being equally likely to be right or left handed.
The species closely resembles the southern white-cheeked gibbon, but has slightly longer body hair and subtly different vocalisations. The males can also be distinguished by the shape of the white patches on their cheeks; in the northern species these reach the upper borders of the ears, and do not touch the corners of the mouth, whereas in the southern species, they reach only half way to the ears and entirely surround the lips.
Both males and females have been reported to produce reddish-brown secretions from glands around their upper chest, hips, and ankles. However, samples of sweat taken from the axillae and chest possess lower levels of steroids in white-cheeked gibbons than in many other species of ape, suggesting that olfactory signals may be less important in these animals than in their relatives.
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