Proboscis monkeys are among the most challenging primates to maintain in captivity, with complex dietary and social requirements that few institutions can adequately meet.
Proboscis monkeys represent the limits of conventional zoo keeping. Their specialized foregut fermentation system means that foods appropriate for most primates are toxic to them — a single piece of fruit can cause fatal acidosis. Only specific leaves, shoots, and unripe pods maintain gut health. Historical zoo attempts frequently resulted in early deaths. The handful of institutions that successfully maintain them invest heavily in sourced browse, fresh vegetation, and specialized veterinary oversight. Even with optimal diet, replicating the vast home ranges, aquatic access, and complex harem social structures of wild groups is extremely difficult. Conservation programs focused on habitat protection in Borneo are considered more welfare-appropriate than captive expansion.