Orangutans are currently only found in the rainforests of Borneo and Sumatra. Classified in the genus Pongo, orangutans were originally considered to be one species. From 1996, they were divided into two species: the Bornean orangutan (P. pygmaeus, with three subspecies) and the Sumatran orangutan (P. abelii). In November 2017 it was reported that a third species had been identified, the Tapanuli orangutan (P. tapanuliensis).
Genomic comparisons show that the Tapanuli orangutan separated from the Sumatran orangutan about 3.4 million years ago. The Tapanuli orangutan separated from the Bornean orangutan much later, about 670,000 years ago. The orangutans are the only surviving species of the subfamily Ponginae, which also included several other species, such as the three extinct species of the genus Gigantopithecus, including the largest known primate, Gigantopithecus blacki. The ancestors of the Ponginae split from the main ape line in Africa 16 to 19 million years ago (Mya) and spread into Asia.
Orangutans are the most arboreal of the great apes and spend most of their time in trees. Their hair is reddish-brown, instead of the brown or black hair typical of chimpanzees and gorillas. Males and females differ in size and appearance. Dominant adult males have distinctive cheek pads and produce long calls that attract females and intimidate rivals. Younger males do not have these characteristics and resemble adult females. Orangutans are the most solitary of the great apes, with social bonds occurring primarily between mothers and their dependent offspring, who stay together for the first two years. Fruit is the most important component of an orangutan's diet; however, the apes will also eat vegetation, bark, honey, insects and even bird eggs. They can live over 30 years in both the wild and captivity.
Orangutans are among the most intelligent primates; they use a variety of sophisticated tools and construct elaborate sleeping nests each night from branches and foliage. The apes have been extensively studied for their learning abilities. There may even be distinctive cultures within populations. Field studies of the apes were pioneered by primatologist Birutė Galdikas. All three orangutan species are considered to be critically endangered. Human activities have caused severe declines in populations and ranges. Threats to wild orangutan populations include poaching, habitat destruction as a result of palm oil cultivation, and the illegal pet trade. Several conservation and rehabilitation organisations are dedicated to the survival of orangutans in the wild.