I am perpetually interested in monkeys. One of the most remarkable and interesting of them all has to be the uniquely Bornean Proboscis monkey Nasalis larvatus, also sometimes called the Long-nosed monkey or Bekantan.

Proboscis monkeys are famously named for the enormous, pendant, tongue-shaped noses of adult males; those of juveniles and females are shorter and upturned. While there have been suggestions that the nose plays a role in heat dissipation or in improving the loud, resonant ‘honk’ calls made by males, it seems most likely that it’s a sexually selected visual signal, its size presumably conveying information on a male’s maturity and genetic quality. It’s that enormous nose, and apparently the monkey’s pink face and rotund belly, that led people in the Indonesian half of Borneo to call them ‘Dutchman monkeys’. If you're wondering, the adjacent illustration is explained below.

The penis is permanently erect (why, and how, this permanent tumescence is present remains unanswered, so far as I know. I assume it’s an aspect of sexual display). The pelt is greyish-white ventrally and reddish dorsally. Sexual dimorphism is pronounced in body size as well as nose shape. Males can have a head and body length of 76 cm and weigh as much as 22 kg while females rarely exceed 60 cm and 11 kg. Babies are dark-furred with a bluish face. Social groups tend to be male-led harems of 6-16 individuals, though there are all-male groups as well. Neighbouring groups sometimes meet and feed together, and some researchers have drawn parallels between Proboscis monkey society and that of Geladas Theropithecus gelada.

As you’ll know if you’ve ever seen the species in books, on TV, or in life, the Proboscis monkey is associated with waterside forests, including coastal mangroves and palm swamps [adjacent photo by David Dennis]. However, the idea that the species is limited to coastal regions (as thought or stated by many primate workers during the 1980s and 90s) is inaccurate: they were reported from inland, ‘upstream’ areas between the 1920s and 50s, and in fact still occur in some of these places today, in some areas being as far as 750 km inland (Meijaard & Nijman 2000).

Proboscis monkeys are strongly arboreal, typically clambering about branches when foraging for leaves. As seems sensible for an animal that spends a lot of time over water, Proboscis monkeys are good divers and swimmers: they both leap into water when threatened, and swim across channels and rivers when needing to move to new areas. Their fingers and toes are partially webbed. More on the swimming in a moment.

Proboscis monkeys in trouble

A 2008 study found that Proboscis monkeys were more widespread and more abundant than previously thought, with a minimum population estimate of about 5900 (Sha et al. 2008). Previous estimates were around the 2000-3000 mark. However, the majority of the regions the monkeys inhabit are endangered by encroachment and destruction, and hunting is also a major problem. Stone-like bezoars are sometimes found in the guts of these monkeys (this isn’t unique to Proboscis monkeys: bezoars are found in other Asian colobines as well) and are highly prized in (surprise surprise) traditional Chinese medicine.

A significant amount of Bornean waterside forest used by Proboscis monkeys was lost during the El Niño event of 1997-1998, and loss of habitat due to burning continues to represent a major cause of habitat loss, as does the conversion of forest to oil palm (and other crop) plantations. These and other causes of habitat loss mean that populations are becoming increasingly fragmented; the extinction of some populations has been documented (e.g., Meijaard & Nijman 1999) but it’s thought that others have gone without their loss being officially recognised (Sha et al. 2008).

It is hoped that ecotourism of the sort that has aided orangutan conservation on Borneo might assist in the preservation of the Proboscis monkey. The conversion of tropical Asian forest to oil palm plantation is a terrifying, escalating problem. We can help to do something about this by choosing not to buy products that contain palm oil, and by spreading awareness of its origin and environmental impact.

Proboscis monkeys in the water

Proboscis monkeys are amazing divers and swimmers. An entire group was once seen to leap into the water from a height of 16 m (Nowak 1999, p. 595). If you’re wondering what they need to leap from, clouded leopards are documented arboreal predators of Proboscis monkeys, even attacking and killing them during daylight hours (Matsuda et al. 2008).

Once in the water, Proboscis monkeys swim with a powerful and confident dog-paddle, but they can also dive and propel themselves for distance beneath the surface: underwater swims of up to 20 m have been recorded (Redmond 2008, p. 142). A lone male was once captured while swimming across the mouth of the Sabagaya River (where the river is about 400 m wide). The animal dived to avoid the boat that drew up alongside “and remained submerged so long that the occupants of the boat began to fear for its welfare” (Brandon-Jones 1996, p. 329).

In a frequently mentioned case from 1950, another lone male was seen far out in the South China Sea. He was misidentified by cruise ship passengers as a human, and a boat was lowered for assistance. The monkey climbed aboard the boat, rested for a while, but then jumped back into the sea and carried on its journey, destination unknown. This individual was photographed (two of those photos are shown above, from Michell & Rickard (1983)), and artistic reconstructions of the incident have occasionally featured in children’s books. The illustration at the top of this article is from the 1983 Mysteries & Marvels of the Animal World (Goaman & Amery 1983). In the illustration below from Maurice Burton’s Animal Oddities (Burton 1971), the animal in the water was clearly copied from one of those photos from 1950.

I know that the Proboscis monkey is hardly an ‘ordinary’ primate when we come to aquatic abilities, but it’s worth noting that the old idea that non-human primates are poor or incapable swimmers is proving increasingly inaccurate the more we learn. Swimming has now been reported in lemurs, macaques, mangabeys, guenons, baboons, swamp monkeys, gibbons, and orangutans. Chimps, bonobos and gorillas have all been photographed or filmed while wading. Feel free to add to this list if you know of other groups I’ve missed.

Proboscis monkeys will also wade when in shallow water, holding their arms up and above the water, out to the sides. Slow bipedal walking on land has also been observed. If you know the literature on the Aquatic Ape Hypothesis you may recall the suggestion that hominin bipedality may have originated via wading, a hypothesis resurrected by Niemitz (2010) within the context of the 'Amphibian Generalist Theory'. It has also been intimated that the similarity between the projecting hominin nose and that of the Proboscis monkey may not be coincidental. It probably is coincidental. Firstly, it doesn’t seem that the Proboscis monkey’s nose is anything specifically to do with its use of mangroves and other waterside habitats – rather, the nose is a secondary sexual characteristic, and merely one among several unusual 'display' noses that evolved (in a terrestrial context) within the clade that the Proboscis monkey belongs to. We'll come back to that issue in the next article. Secondly, we don’t have good evidence showing that early hominins went through the sort of ‘aquarboreal’ lifestyle that would be needed to explain convergent evolution with Proboscis monkeys.

More next – this time focusing on evolutionary history. You have been warned.

For previous Tet Zoo articles on primates, do check out...

Refs - -

Brandon-Jones, D. 1996. The Asian Colobinae (Mammalia: Cercopithecidae) as indicators of Quaternary climatic change. Biological Journal of the Linnean Society 59, 327-350.

Burton, M. 1971. Animal Oddities: the Strangest Living Creatures. Odhams Books, London.

Goaman, K. & Amery, H. 1983. Mysteries & Marvels of the Animal World. Usborne, London.

Matsuda, I., Tuuga, A. & Higashi, S. 2008. Clouded leopard (Neofelis diardi) predation on proboscis monkeys (Nasalis larvatus) in Sabah, Malaysia. Primates 49, 227-231.

Meijaard, E. & Nijman, V. 1999. The local extinction of the proboscis monkey Nasalis larvatus in Pulau Kaget Nature Reserve, Indonesia. Oryx 34, 66-70.

- . & Nijman, V. 2000. Distribution and conservation of the proboscis monkey (Nasalis larvatus) in Kalimantan, Indonesia. Biological Conservation 92, 15-24.

Michell, J. & Rickard, R. J. M. 1983. Living Wonders: Mysteries and Curiosities of the Animal World. Thames & Hudson, London.

Niemitz, C. 2010. The evolution of the upright posture and gait—a review and a new synthesis. Naturwissenschaften 97, 241-263.

Nowak, R. M. 1999. Walker’s Mammals of the World, Sixth Edition. The Johns Hopkins University Press, Baltimore and London.

Redmond, I. 2008. The Primate Family Tree. Firefly Books, Buffalo.

Sha, J. C. M., Bernard, H. & Nathan, S. 2008. Status and conservation of Proboscis monkeys (Nasalis larvatus) in Sabah, East Malaysia. Primate Conservation 23, 107-120.