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 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 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.