Only a few studies on this intriguing primate have been carried out and little is known about their ecology and behaviour (4). Groups consist of single mature males and around 6 females and their young; adolescent males form bachelor groups until they can take over their own harem (4). Groups join together in larger more fluid troops to rest at dusk (5); these encounters may be noisy with rival males displaying to each other and often crashing through the branches (4). Unusually, females may switch harems several times in their lives (4), and they compete between each other to mate with the male of their group. When a female is ready to mate she will perform a head shaking and presenting display (5). A single offspring is born after a gestation period of nearly 6 months, remaining with their mother for the first few years (4); males will then leave to join bachelor groups (5).
Young leaves make up the majority of the proboscis monkey diet between June and December, and fruit from January to May (2), although seeds and flowers are also consumed (7). These monkeys are excellent swimmers and have partially webbed feet; they can be seen readily leaping into the water with a dramatic belly flop in order to cross rainforest rivers (4).