The gibbons are sixteen species, which make up the whole of the lesser apes, and are found in Asia. They are distinct from the great apes - humans, chimpanzees, orangutans, and gorillas, being smaller. In addition, while the four gibbon genera share a common ancestor, their common ancestor with great apes existed longer ago (16.8 million years ago) than the common ancestor between any of the great ape species (from which orangutans diverged around 14 million years ago). Like other apes, gibbons do not have tails.

The gibbon family was originally split into two groups, consisting of the siamang, and then all the other gibbon species. However, today the gibbons are simply divided into their four genera - namely the Hoolock (hoolock gibbons) found in Bangladesh, India and China, the Hylobates (dwarf gibbons) found in most of South East Asia, Nomascus (the crested gibbons) found in China and Vietnam, and Symphalangus (the siamang) found in Sumatra and peninsular Malaysia.

The four species of gibbon in Sumatra, are the endemic Kloss's gibbon (the Mentawai gibbon), Hylobates klossii; the agile or black-handed gibbon, Hylobates agilis (found south of Lake Toba/Singkil); the Sumatran lar gibbon (white-handed gibbon) Hylobates lar vestitus; and the siamang, Symphalangus syndactylus.

Siamang 

The siamang is the largest gibbon/lesser ape, at up to 34kg, and 1 metre tall. They are distinguished by their throat sac, which they can puff to the size of their head to make loud vocalizations, and their long black fur. They are classified as endangerd.

There are many siamang found in the national park near Bukit Lawang.

Their diet consists of 60% fruit, mainly figs. It also eats flowers and leaves. It will peel larger fruit discarding the seeds.

Generally, but not exclusively siamangs form monogamous pairs, caring for their children until the age of six to eight years, when they will leave the group. Male siamangs assist in carrying and caring for their babies.

The siamangs loud vocalizations are mainly directed at rival groups, in order to define their territory to rivals. Most calls take places in early to mid morning. Siamang are arboreal, staying in the trees. They use high, non-fruit-bearing trees as points to better project their calls to other apes.

Lar gibbon

The lar gibbon, or white-handed gibbon is also found at Bukit Lawang. The subspecies in Sumatra is the Sumatran lar gibbon, Hylobates lar vestitus.

They have white hands, and a white ring around their faces.They mostly eat fruit, but also a large proportion of leaves, as well as insects and flowers. Life expectancy is around twenty five years, with females giving birth after around ten  years of age.

As with siamang, they vocalize, making hooting calls in the morning. These calls are between the chiefly monogamous pair of the family group, as a warning to other pairs. 

The lar gibbon occupies the same ecological niche as the agile gibbon, and therefore the two species do not share a range.