The proboscis monkey is a large member of the Old World monkey family endemic to the mangrove and swamp forests of Borneo. Males possess a distinctive long, large nose that is thought to amplify calls to attract females. They are diurnal and primarily arboreal, feeding on young leaves, fruits, seeds, and flowers. Their stomach is complex and nearly four-chambered, adapted for digesting leaves, so they rest for long periods and have relatively short travel distances. Social structure is layered; single-male, multi-female groups gather into larger bands and exhibit a matrilineal social organization. Classified as Endangered (IUCN Red List category I B), their numbers are declining due to habitat destruction and hunting.