The Sumatran orangutan (Pongo abelii) is one of the three orangutan species and the rarest of the three — fewer than 14,000 are left in the wild, almost all in the forests of Gunung Leuser. They are smaller, leaner and lighter-coloured than their Bornean cousins, and noticeably more arboreal: a fully grown male will rarely touch the ground, preferring to move between trees along the canopy roads of vines and branches. Their long, careful arms and prehensile feet are perfectly adapted for the task.
Diet is mostly fruit, with figs a particular favourite, and the orangutans' movements through the forest follow the fruiting cycles. A female with a young infant — and you may see one of these on a trek out of Bukit Lawang or Ketambe — is usually accompanied by an older juvenile as well, since orangutans nurse their young for up to seven years. The babies cling to their mothers continuously and learn the forest by watching her every move. Watch for as long as the animals allow but keep your distance and your voice down; they react badly to crowding.