_Hylobates_, Illiger, Prodr. Syst. Mamm., p. 67 (1811).
The group of Tree-walkers, as the term _Hylobates_ signifies, {149}embraces
the smallest-sized, the slenderest-bodied, the longest-limbed, and the most
perfectly arboreal of all the Man-like Apes. All are covered with thick
woolly hair, which, on the arms and fore-arms, converges (except in _H.
agilis_) towards the elbow.
Their head is small and round, and the face compressed. Except the Orangs,
the Gibbons have the longest arms of all the Apes, so long that when they
stand erect the points of their fingers can touch the ground. Compared with
the spinal column, their arms are as 19 to 11, while the legs are one-third
longer than it. The fore-arm is much longer than the arm itself; the hand
is longer than the foot, and the thumb is very long in proportion to the
hand. The knee is free from the side of the body, and the great-toe is well
developed and nearly one-half the length of the foot. The nails of both the
thumb and the great-toe are flat. Callosities, which are wanting in all the
other genera, are present in _Hylobates_, but are very small.
In the skull the occiput is convex; the orbits are very large and deep, and
the supra-orbital ridges prominent. The canine teeth are much larger than
the others, and equally large in both sexes. They are generally the last of
the permanent teeth to come in, but in the Gibbons they generally precede,
or are developed along with, the last molar.
The vertebral column is nearly straight, presenting but little of the
spinal curvature seen in Man; it has also in the dorso-lumbar region one
vertebra more than in the human skeleton. The articulating head of the
arm-bone (_humerus_) loses the direction it had among the Monkeys, and
looks upward and forward as in Man. The wrist (_carpus_) has nine bones, as
in the lower _Anthropoidea_. The skeleton of the hand is more {150}than
half the length of the spine, and the foot is slightly under half its
length. The Gibbons have two pairs of ribs more than Man. The ends of the
ischial bones are much everted to support the callosities.
With regard to the brain, this genus is remarkable for the great reduction
of the occipital lobes of the cerebrum.
The tongue is very similar to that in Man, but it is furnished with a
sub-lingual process like that already described among some of the
Lemuroids. The Gibbons (except the Siamang) have no laryngeal sacs. The
stomach closely resembles the human organ.
The Gibbons are very delicate, and rarely live long in confinement, even in
their own country. They are in general highly intelligent, very gentle, and
become most affectionate and engaging animals if kindly treated. They are,
however, occasionally irascible and ill-tempered, especially when adult.
Their feats of climbing and leaping are almost proverbial. It would be
impossible to excel them as acrobats. When walking on the ground they
assume the erect posture, putting the soles of their feet to the ground,
separating the thumb and the great-toe widely from the neighbouring digits.
"They walk erect, with a waddling or unsteady gait, but at a quick pace;
the equilibrium of the body requiring to be kept up, either by touching the
ground with the knuckles, first on one side then on the other, or by
uplifting the arm so as to poise it. As with the Chimpanzee, the whole of
the narrow, long sole of the foot is placed upon the ground at once and
raised at once, without any elasticity of step." (_Martin._)
Their voice is very powerful and can be heard at a great distance,
especially when they are howling in chorus. The {151}Wau-Wau and the
Siamang, the one without, and the other with, a laryngeal sac, are equally
vigorous in this respect.
The female produces but a single young one at a birth, of which she takes
the greatest care. She carries it about, clinging to the under side of her
body, for many months. It is said that she even takes it to the waterside
from time to time, and with much solicitude, and in spite of its cries and
resistance, washes its face.
The Gibbons frequent the great upland forests; but the Siamang (_H.
syndactylus_) may be met with at quite low levels and close to the coast.
Their food consists of fruit, leaves, and insects, eggs of birds, and
apparently birds and lizards, and especially spiders. They drink either by
putting the mouth down to the water, or by dipping in their hands and thus
carrying it to their mouths.
The Gibbons are confined to two Sub regions of the Indian Region. With the
exception of the Siamang, all the so-called species of _Hylobates_ are so
closely allied to each other, and differ by characters of such slight
importance, that they seem to be hardly worthy of specific distinction.
(_Thomas._)