The Northern Sumatran leaf monkey feeds mainly on young leaves, but may also take fruits, flowers, toadstools and stalks of coconuts (1), using its sharp teeth to pierce through the outside of hard fruits or through thick handfuls of leaves (4). Sometimes it will even eat gastropods such as ground snails (1). The stomach of this monkey is specially adapted to enable it to digest cellulose in the leaves that it eats; microbes in the forestomach break down the cellulose into nutrients that can be utilised by the monkey (4).
This species is semi-arboreal, meaning it often frequents the trees, but will also spend time on the ground, typically coming down to the ground once a day to feed (4). The Northern Sumatran leaf monkey moves through the horizontal branches of trees on all fours, but has also been known to leap through the forest (4). Female Northern Sumatran leaf monkeys give birth to single offspring, which may be born at any time of the year (1).
The Northern Sumatran leaf monkey is typically found in uni-male groups (one male with lots of females) (4), although all-male groups also occur. These all-male groups receive a lot of aggression from males in uni-male groups, and so tend to live lower in the forest (4). Aggression can also occur between females when food is scarce, and it has been suggested that a hierarchy exists within the group according to the individual’s age (4). The reticulated python, the clouded leopard and the tiger are all predators of the Northern Sumatran leaf monkey (4).