They also snack on shoots, fruits and seeds, and don't eat any meat. This is because their complex stomachs don't handle proteins well. Proboscis monkeys rarely need to drink, getting all their water from their leaves. After a short bout of feeding, they rest for a long time, sometimes up to 8 hours, to allow their stomachs to digest their meal. They usually have one feeding bout in the early morning, rest the whole day, then feed again just before nightfall.
Proboscis monkeys can swing through the trees and leap from branch to branch.
But because they are so heavy, they are rather slow and ponderous, especially the bigger males. This is believed to protect them from wild cat predators like the clouded leopard . Another reason suggested is that it keeps them cooler, but this is not borne out by measurements. Yet another reason is that branches are barer over the water thus allowing females to size up the males, and the males to keep an eye on his females and prevent other males from sneaking up during the night to mate with his females. The Proboscis monkey is an excellent swimmer, some consider them the best primate swimmer.