# The Fauna of British India, including Ceylon and Burma/Mammalia/Class Mammalia/Subclass Eutheria/Order Primates/Suborder Anthropoidea/Family Simiidæ/Genus Hylobates/Hylobates hoolock

1. Hylobates hoolock. The Hoolock or white-browed Gibbon.
    Simia hoolock, Harlan, Trans. Am. Phil. Soc. iv, p. 52, pl. 2 (1834).
    Hylobates hoolock, Blyth, Cat. p. 4; Sclater, P. Z. S. 1870, pl. v, fig. 2; Blyth, Mam. Birds Burma, p. 1; Anderson, An. Zool. Res. p. 1; id. Cat. p. 26.
Uluk, Hindi; Myouk-lwai-gyau and Tuboung, Burmese of Arakan.
​Colour. Generally black throughout, with the exception of a white or grey band across the eyebrows. This band is usually, but not always, interrupted in the middle. Many individuals, however, both males and females, vary in colour from brownish black to light yellowish grey, the frontal band beiug always couspicuously paler. The crown, back, and outside of the limbs are often paler-coloured than the lower parts of the body, the skin of the naked part of the face below the frontal band being almost always dark-coloured. Blyth thought that the males only were black, the females always paler; but this is certainly not the case; the females, however, are more frequently pale-coloured than the males.
Dimensions. From crown to rump about 20 inches, fore limb (including hand) 23, hand alone 6, leg and foot 19, foot 6; height from crown to heel about 32 inches. An adult male skull is 4·45 inches long from the occiput to the alveolar margin of the pre-maxillaries at the base of the middle upper incisors, 3·35 in basal length from the lower or anterior edge of the foramen magnum to the same, and 2·9 in breadth across the widest part of the zygomatic arches.
Distribution. The hill-ranges south of Assam; Sylhet, Cachar, Manipiur, Irawadi valley near Bhamo, Chittagong, and Arakan in hill-forest. It is uncertain how far the species is found to the east-ward. According to Anderson it inhabits Martaban. Pemberton's assertion that this species occurs at the base of the Himalayas in the lower ranges of Bhutan is probably a mistake. The type described by Dr. Harlan came from the Garo hills.
Habits. Good accoimts of this animal are given by Burrough, in Harlan's original description, by Blyth and by Anderson. Like most other Gibbons, the Hoolock is usually found associating in flocks, often comprising fifty to a hundred individuals, or even more. An old male, however, is occasionally found solitary.
Gibbons are thoroughly arboreal, and Hoolocks are almost, if not entirely, confined to hill-forest. They move chiefly by means of their long arms, by which they swing themselves for prodigious distances from branch to branch and from tree to tree. They descend hill-sides at a surprising pace, their descent being accomplished by grasping bamboos or branches that bend beneath their weight, and allow them to drop until they can seize the ends of other bamboos or branches lower on the slope, and take another mighty swing downwards. They also ascend with great rapidity, swinging themselves from tree to tree.
When walking on the ground, the Hoolqck rests on its hind feet alone, with the sole flat on the ground and the great toe widely separated from the other digits. The arms are usually held upwards, sometimes horizontally, their great length giving the animal a very peculiar aspect. Gibbons walk rather quickly, with a waddling gait, and can easily be overtaken by men when on the ground.
The food of these Apes consists of fruit, leaves, young shoots, spiders (of which they are very fond), insects, birds' eggs, and ​almost certainly of young birds, if not of any birds they can capture. Anderson found that small birds were killed and devoured by Hoolocks in confinement with a method and eagerness that showed the prey to be the natural food of the Apes. The Hoolock drinks with its lips, putting its head down to the water as monkeys do.
All species of Hylobates have a powerful voice, and the common name of the present form is taken from its peculiar double call, which is repeated several times. At a distance, the sound much resembles a human voice; it is a peculiar wailing note, audible from afar, and in the countries inhabited by these animals is one of the most familiar forest sounds. The calls commence at day-break, and are continued till 9 or 10 a.m., several of the flock joining in the cry, like hounds giving tongue. After 9 or 10 o'clock in the morning the animals feed or rest, and remain silent throughout the middle of the day, but recommence calling towards evening, though to a less extent than in the earlier part of the day.
When captured young the Hoolock is easily tamed, and is, as a general rule, very gentle, docile, and good-tempered, exceedingly intelligent, and very cleanly in its habits. Some instances of savageness on the part of male animals have, however, been noticed. All the Gibbons are very delicate, and rarely live long in captivity.
But a single young is born at a time. Neither the period of gestation nor the age at which these animals become adult appears to have been ascertained.