Gibbons are relatively small, slender, and agile. They have fluffy, dense hair. They are not sexually dimorphic in size. Mature females usually weigh more than mature males. They have very long arms, which they use in a spectacular arm-swinging locomotion called brachiation. Their hands and fingers are also very long. The relatively short thumb is set well down on the palm, and their fingers form a hook, which is used during brachiation. Gibbons have very good bipedal locomotion, which they use on stable surfaces too large to grasp. When walking bipedally, arms are held up to keep from dragging and to assist with balance. Gibbons are sometimes observed putting their weight on their hands and swinging their legs through as if using crutches.
Gibbons do not build nests like the great apes. They sleep sitting up with their arms wrapped around their knees and their head tucked into their lap. They have ischial callosities (fleshy, nerveless pads attached to the hip bones, a characteristic otherwise found only in Old World monkeys).
Gibbons are one of the few apes where the adult female is the dominant animal in the group. The hierarchy places her female offspring next followed by the male offspring and finally by the adult male.
Gibbons are physically independent at about three, mature
at about six, and usually leave the family group at about
eight, though they may spend up to ten years in their family
Siamangs have a louder call than white-cheeked gibbons, amplified by a throat sac. Their call can be heard up to two miles away. Also used to defend territory, it includes more of a boom, bark, and a loud call increasing in speed as the call goes on, as compared to the chatter and calling of the other gibbon families.