Simias is found mainly in hilly primary forests of the interior, not in swamp or mangrove. It is totally arboreal, descending only when disturbed. The diet consists predominantly of leaves, but fruits and berries are also taken. Group foraging area is about 25-30 ha. The social unit is an adult pair with one to three immature offspring.
Noticeable vocalizations are rare.
Newborn young were observed only in June and July. It is probably among the more seriously endangered mammals of the world. It is intensively hunted for food, and its conspicuous size makes it especially vulnerable in this regard. More significantly, it is restricted to a few islands and is dependent on primary forests, which are now being logged. No specimens are known to be in captivity.