Gibbons, or lesser apes, belong to the family Hylobatidae.
The great apes, which include orangutans, gorillas, bonobos, chimpanzees and humans, are the closest living relatives of gibbons.
The common ancestor of gibbons and great apes lived until between about 15 and 18 million years ago.
Gibbons live in rainforests in southeast Asia and Indonesia.
They are smaller than great apes and weigh between about 11 and 26 pounds (5 and 12 kilograms). They are usually between 2 and 3 feet (0.6 and 0.9 meters) tall. Males and females are about the same size. Their bodies are slim, with very long arms and legs. They have thick coats that come in a variety of colors.
Like great apes, gibbons are tailless.
Gibbons spend most of their time in trees. They use their arms to swing from branch to branch and from tree to tree, a form of locomotion known as brachiation.
Like orangutans, gibbons have ball and socket joints in their wrists; this makes it easy for them to change direction as they move will hanging onto branches.
In addition to brachiating, gibbons walk upright on wide branches. They also walk upright when they are on the ground.
Like old world monkeys, gibbons have thick pads of skin on their buttocks. These pads are known as ischial callosities. They allow gibbons to sit comfortably and to sleep while sitting. Unlike great apes, gibbons do not sleep in nests.
A gibbon's diet mostly consists of fruit. Gibbons also eat leaves, flowers, insects, eggs and small birds.
Gibbons live in family groups and are monogamous. Female gibbons give birth to one child at a time; and adult female gives birth about every three years. Gibbons reach sexual maturity and leave their parents when they are between seven and nine years old.
Most gibbon species are endangered due to habitat loss, hunting and the sale of gibbons as pets. Sometimes gibbons' body parts are used in traditional medicine.
Gibbons are known for their singing. They have loud voices that can be heard from as far as a mile away. Gibbon songs are very complex. Families sing together; couples sing duets, with children often joining in. Singing encourages bonding between couples and families. It is also used to mark territories and to attract mates.
A 2012 study, in which gibbons were given helium, showed that when they sing, gibbons use the same vocal techniques as human opera singers.
Some gibbons have very large throat sacs, which enable their voices to resonate loudly.