Leaf-Eating Monkeys: Leading by a Nose

Proboscis monkey (male)

Perhaps the most distinctive and well-known of all the colobine monkeys is the proboscis monkey (Nasalis larvatus). Sufficiently distinctive that it's hard to confuse it with anything else, it was first described as a species all the way back in 1787 by botanist Friedrich von Wurmb, then working for the Dutch East India Company, and given its own genus in 1812.

Indeed, it is strange enough that, during the 20th century, it was assumed to represent a very early side-branch in colobine evolution, existing outside all the other groups in the subfamily. That wasn't just because of its odd appearance, but also because it had two extra pairs of chromosomes to every other colobine monkey. But it turns out that that's a false signal and that, not only are proboscis monkeys a relatively recent branch within the subfamily, their closest relatives include the snub-nosed monkeys whose noses are noted for being extraordinarily short.

This fact may be less surprising when we consider that the huge, dangling, nose of the proboscis monkey is only present in adult males. The nose of females does look fairly strange, a narrow projection sticking straight out like that of Pinocchio in the Disney film, but it is much smaller than that of the males, and does not droop. Infants not only have even smaller, upturned noses, but the skin of their faces is also blue, and remains so for up to a year. This, it's worth noting, mirrors the adult faces of the other, closely related, "odd-nosed" monkeys, such as snub-nosed monkeys and some of the doucs. 

The nose isn't the only thing that's unusual about proboscis monkeys compared with their kin. For one thing, they are larger, and the difference in size between males and females (often significant in colobines anyway) is even greater than in other species. The females weigh 7 to 12 kg (15 to 26 lbs), half-again heavier than most langurs or colobus monkeys, but on a par with a male douc. The males, however, weigh more than twice as much as their sisters, typically between 20 and 24 kg (44 to 53 lbs). 

Furthermore, the multi-chambered stomach of a proboscis monkey is larger, in proportion to their body, than that of any other colobine - so much so that they have a visible paunch. As with other colobine monkeys, the multi-chambered stomach does not allow them to ruminate or "chew the cud" in the way that, say, a cow would but, apparently uniquely among colobines, they do practice "merycism". This involves partially regurgitating food into the mouth to chew it up again into smaller chunks before re-swallowing it, differing from true rumination only in that the food particles aren't sorted out in the stomach.

Proboscis monkeys are native to Borneo, being found across most of the low-lying parts of the island. Supposedly, there are two subspecies, with one restricted to the northeast, but there doesn't seem to have been a truly convincing description of the differences between them, so they may not really be distinct. Many of them inhabit swampland, including coastal mangrove forests, but others live in lowland forests close to rivers and other plentiful supplies of water. Indeed, they prefer to sleep in trees less than 35 metres (115 feet) from a body of water and are remarkably efficient swimmers, with partial webbing on their toes. This does mean that crocodiles are a major predator of the species, and they seem to be very careful when crossing rivers, picking suitable spots to minimise the risk. They also prefer large trees to sleep in which, again, is likely an anti-predator strategy, and may also reduce the number of mosquitos that bite them, since those tend to fly lower down.

Their diet is about evenly divided between leaves and fruit. In some respects, they are picky eaters, choosing only the freshest leaves and ripest fruit but they seem to feed on a remarkable array of different riverside or swamp plants, with one study identifying 188 different plants in their diet, although they do prefer some over others.

Female proboscis monkey and infant

The basic social unit is, like that of other colobines, a single adult male and his harem, which typically contains about eight females and their young. However, as is also the case with snub-nosed monkeys, these groups are not territorial and gather together into larger bands of up to fifty individuals, especially when sleeping together near a river - perhaps providing safety in numbers when they need to cross. Females, especially younger ones, do sometimes move from one harem to another, but otherwise the families - but not the larger bands - are generally stable.

Different bands may avoid one another by making loud calls when they wake up in the morning, but there is little aggresion between different families within the band, implying a true two-tier social structure. When females or young within a harem do fight amongst one another, the male has been observed to use a special "braying" call that essentially tells them to pack it in - an unusual case of non-physical third-party intervention in an animal conflict. 

In most colobine species, we would expect to see that the dominant males have larger canine teeth, since these enable them to fight better, but the reverse seems to be true in proboscis monkeys, possibly because the large nose gets in the way of them biting. The purpose of that nose is obviously a sexual signal, and, if you're a male proboscis monkey, the larger your nose, the larger your testicles. In addition to looking sexy to the females, an especially large nose also enables the male to make more resonant calls, which is presumably also attractive.

Mating takes place throughout the year, and results in the birth of a single young 166 days later. Females initiate mating by making head movements and presenting their hindquarters, and, if they are rebuffed, have been often seen to work off their frustration by having sex with another female instead. If the male does respond, the mating pair are often harassed by their infants, which have been seen to jump up and yank on the male's nose while he's in the act - presumably because it's the only time they can get away with it. It's also not unusual for males to "mate" with other males... to the bemusement of at least one researcher who couldn't imagine why they might want to do this.

The swamps and lowland valleys where proboscis monkeys live are particularly vulnerable to being turned into farmland, with one estimate being that half of the total available habitat was destroyed just in the two decades between 1990 and 2009. Conservation efforts have not proven very effective so far, and while some new protected areas were recently established in Sabah in Malaysia, at least one nature reserve in Indonesia saw its local population entirely wiped out in the late 1990s. The increasingly fragmented swamps and forests are pushing the surviving populations together to the point that the  remaining plantlife can no longer support them and the species has been formally listed as endangered since 2000.

Despite their rarity, proboscis monkeys are large, distinctive, and charismatic animals, and have frequently been the subject of study. We know rather less about their closest relative, and the last remaining colobine species on my list: the pig-tailed langur (Simias concolor), sometimes referred to by the local name of "simakobu".

The pig-tailed langur was not identified as a species until 1903 and is found only on the Mentawai island chain off the west coast of Sumatra - also home to some other unique colobine species and, for that matter, a gibbon. While they are similar in size to typical langurs, they also have the narrow skulls and elongated nasal bones of proboscis monkeys, although their nose is narrow with an upturned tip and doesn't project outwards, lacking the extreme form of their closest kin. Their limbs seem to be less suited to climbing than other colobines, although they are perfectly happy doing so, so it can't be that great of a problem. 

Most are a dark, almost black colour, with paler streaks on their chest, but some are tan or buff, a difference that's seemingly irrelevant to the monkeys. Their most distinctive feature, however, is their tail. Every other colobine has a long well-furred tail, but that of the pig-tailed langur is, as one might expect from the common name, short, somewhat curled, and almost entirely lacking in hair. This doesn't give them any obvious advantage, and may be a product of the genetic isolation that gave rise to the species in the first place.

Like proboscis monkeys, pig-tailed langurs live in lowland forests and mangrove swamps, although, perhaps because there are few other places to go on the islands, they do venture further into the hills than their kin, perhaps reaching forests up to 500 metres (1,600 feet) above sea level. They also have a similar diet, feeding mainly on leaves, but also with plenty of seeds, and taken from a wide variety of the local plantlife. They manage to live alongside the local true langur species by foraging lower in the tree canopy and having slightly different food preferences, including a higher proportion of leaves than the others prefer.

Their social structure is based on a harem, with up to eight females being present in a typical group on the island of Siberut. However, the monkeys found on the smaller islands to the south (which are regarded as belonging to separate subspecies) live in much smaller groups of two to four adults, so that some are effectively monogamous. A few larger multi-male groups have been reported, although it's possible that these are aggregated bands, similar to those of proboscis monkeys, rather than family units. Unlike other Asian colobines, the females exhibit engorged sexual swellings when they are ready to mate, perhaps to compete with their fellow harem members for the male's attention.

The monkeys are rarely seen, partly because their response to the presence of a human is to freeze and remain deathly silent, hoping that the leaves of their tree will keep them hidden. Nonetheless, males make loud calls throughout the day, but especially as a dawn chorus in the early morning. Such calls are often in response to another they have just heard, but as many as a third are spontaneous, and they will also call if they hear a loud sound in the forest, such as a falling tree or a clap of thunder. The calls can be heard up to 500 metres away, and are probably used to indicate physical fitness and alertness rather than as a direct aggressive challenge to possible competitors.

Although there are still thought to be a few thousand pig-tailed langurs spread across the Mentawai Islands, their population has declined so rapidly in recent - at least 80% of the last three decades, and probably more - that they are listed as a "critically endangered" species, at high risk of extinction. This is primarily due to the loss of their habitat, especially on the smaller southern islands, but they are also heavily hunted and most of the monkeys do not live in legally protected areas.

This, sadly, has been a theme of this particular series. I have covered 82 different species of colobine monkey, a full 50 of which are considered endangered species. Of those, 21 are "critically" endangered, at high risk of extinction, in some cases possibly in the very near future. That's over a quarter of the total, and a reflection of these particular species' reliance on relatively untouched forest and, in particular, the danger that hunting can have on them. Some, such as the widespread grey langurs of India, are doing well, but for many the future is not so rosy.

Next year, I will be looking at a different group of animals and it's one that should hopefully be rather less depressing...

[Photos by Malik Teringin and Charles J. Sharp, from Wikimedia Commons. Cladogram adapted from Leidigk et al. 2012.]