Leaf-Eating Monkeys: Red Colobuses of Central Africa

Ashy red colobus

The problem with being a primate subsisting primarily on leaves is that, on their own, they aren't very nutritious. Humans on a vegan diet can hardly be expected to live on lettuce alone; they need grains, pulses, nuts and so on, and we have the advantage of cooking making much of our food easier to eat and digest. Colobus monkeys have complex digestive systems that allow them to extract more nutrition from leaves than a primate would otherwise be able to, so they have the edge over us in that respect, but they still require plenty of fresh leaves and this leads to the more exclusively folivorous ones having specific habitat requirements that leave them vulnerable to any degradation or loss of their environment.

The end result of this is that many colobine monkeys are endangered. In Africa, the most endangered group, and probably the most threatened group of primates of any kind on the continent are the red colobuses.

There are several species of these, found across the forested belts that lie on either side of the equator. The number of recognised species has, as is common for many mammal groups, changed over the years, with the recent tendency being to recognise many smaller populations as distinct species when they would previously have been considered subspecies at best. As I write this, however, I can say that there are seventeen species that are widely agreed to be distinct... and thirteen of them can reasonably be regarded as "endangered species".

In East Africa, perhaps the most widely studied of these is the ashy red colobus (Piliocolobus tephrosceles), sometimes referred to as the "Ugandan red colobus". The latter name comes from the country where it was first described as a distinct species, in 1907 by Daniel Elliott of the Field Museum of Chicago. It's also where the bulk of the population lives today, in Kibale National Park. However, they once lived along a much wider stretch of the Albertine (western) branch of the Great Rift Valley. Today, there are only three definitely known populations outside of Uganda, one on the shores of Lake Victoria and two close to Lake Tanganyika. One of the latter two populations has all but vanished in recent years, although it's possible that others live in unsurveyed regions of neighbouring countries such as Rwanda.

As the first part of the name indicates, ashy red colobuses are primarily grey in colour. They have darker fur on the back and tail (which, at roughly the same length as the body is actually fairly short for a colobus) and some black markings on the face, although the details are variable, and might possibly indicate unidentified subspecies. The only red is the patch of hair on the top of the head which has a distinctly rusty colour. The males are much larger than the females, with heavier skulls and teeth; the latter have been noted to have an unusually large clitoris.

The reason that ashy red colobuses have been faring poorly is that they prefer to live in dense primeval jungle, largely untouched by man. This is more unusual than one might think; many species prefer secondary forest with open glades, more sunlight reaching the forest floor, and so on, so properly managed forestry (rather than indiscriminate logging and encroaching farmland or highways) can actually be beneficial. Not so for the ashy red colobus, which also relies on specific climate conditions to ensure that new leaves grow when expected.

Even compared with black-and-white colobuses, it is notable that red colobuses in general, including the ashy species, eat almost nothing but leaves. They do occasionally raid cropland but they are picky about what they eat even then, apparently preferring beans. In the wild, a significant portion of their diet comes from the leaves of a leguminous tree that has high levels of phytoestrogens - plant chemicals that mimic the mammalian oestrogen hormone. Heavy consumption of this plant, for instance, after prolonged rainfall, is sufficient to cause behavioural changes in the monkeys, which become more aggressive and sexually active.

Group sizes can be large, with dozens of individuals of both sexes living together. Nonetheless, the availability of food likely puts some upper limit on the size of groups in particular locations, with larger groups being forced to travel longer distances each day to find enough to eat. Larger groups may, however, be better at responding to predators; on one occasion, ashy red colobuses were observed killing an owl shortly after they had been attacked by a raptor and apparently thought it was going to do the same. Unusually, chimpanzees have also been observed hunting red colobuses, with one group in Tanzania apparently specialising in doing so.

Tana River red colobus

For much of the 20th century, the ashy red colobus was considered a subspecies of "eastern red colobus" together with what we'd now call the Tana River red colobus (Piliocolobus rufomitratus). That was named first, and so got to keep the scientific name when the two were split apart, but it's even rarer than the ashy species, being found only along a 60 km (37 mile) stretch of the Tana River in Kenya, possibly with a separate population still clinging in the delta at the river mouth. 

Trees are only found in significant numbers close to the river itself, leaving the monkeys nowhere else to go. In 2003, only around a thousand were thought to be in existence, and four years later the primate reserve where they live was disestablished, opening the area up to exploitation and development. Since the population was already in decline before this, that isn't a positive sign and extinction within the next few decades is entirely possible.

The Tana River species looks very similar to its ashy relative, but is smaller with brighter red fur and black markings on the head. They eat mostly young leaves and, even when they do eat older ones, select for those that are lower in fibre (and thus presumably easier to digest) and higher in nitrogen, such as those legumes. This likely gives them specific requirements for habitat, explaining why they don't seem to have travelled far beyond the one river where they live. Groups are smaller than in the ashy red colobus, although that's possibly because they haven't got much choice, and contain only one or two adult males, rather than a more even mix. Uniquely among colobuses, and rarely among monkeys in general, male/female pairs have been seen sneaking away from the main group for days at a time, regularly mating with each other where their fellows can't see or interrupt them.

Although the ashy colobus is thought to be the closest relative of the Tana River species, over the years a number of other populations, now thought to be separate species, were grouped together with them. The most widespread of these is Oustalet's red colobus (Piliocolobus oustaleti) named for a prominent French zoologist of the early 20th century. This was originally discovered along the Ubangi River, which forms part of the border between the two African countries both known as "Congo". 

It is found right across the north of both countries, and just across the borders into some of their northern neighbours. This covers such a wide area that the species was formerly considered to be the least threatened of all red colobus species. Rapid declines in its population over the last ten to twenty years mean that it is no longer thought to be particularly safe, although it has managed to remain one of the few species of its kind not to be internationally listed as endangered.

It's one of the larger red colobuses, with males weighing around 12 kg (26 lbs), and has a mostly greyish-brown coat. Even the hair on the head is more brownish than anything else, making it perhaps the least red "red colobus". 

The same cannot be said of Foa's red colobus (Piliocolobus foai) which has red limbs and red fur on the top of the head offset by a black band. Much of the back is also reddish, although fading to black in places, while the underparts are a paler grey. Like Oustalet's species, it has a distinct tuft of hair at the base of the tail, as well as the one on the head seen in almost all red colobuses.

"Elliot's red colobus"

Although, like other species, it was once found across a wider area, today it lives in only two small patches of forest on the eastern slopes of the Itombwe Mountains west of Lake Tanganyika. Further west, where the lowlands stretch down towards the Congo River, lives the very similar, but slightly darker, Ulindi River red colobus (Piliocolobus lulindicus) which was only split off into a separate species in 2016 and even then largely on the grounds that it couldn't be listed as an "endangered species" if it had remained as a subspecies. 

Moving downriver we come to Lang's red colobus (Piliocolobus langi), east of a stretch of the Congo between the Maiko and Aruwimi Rivers. This, too, inhabits dense lowland jungle. It's notable for being the darkest coloured of the red colobuses, with near-black fur across the chest and belly, and deep reddish fur on the back, limbs, and crown of the head.

Moving back east, towards the Rift Valley, we find the Semliki red colobus (Piliocolobus semlikiensis), another lowland forest species that, along with the others in the region was previously thought to be united as a "Central African red colobus" species. With a darkish grey colour over most of the body, black shoulders, hands, and tail, and red arms and crown, it was only identified as a subspecies in 1991, and raised to full species status in 2007. Although its population is declining, it is not so small, nor so rapidly diminishing as some of the other species and so, while undeniably threatened, it is not formally listed as endangered.

The Lomami red colobus (Piliocolobus parmentieri) is yet another newly identified species from in the area, living between the Lomami and Congo Rivers although the exact extent of its range is unclear. It is red and black on the back and limbs, with an almost pure white belly and chest. The speckled reddish hair on the head is much shorter than those of its relatives and lacks the distinctive red colobus  "panache", a sort of crest named for the plumes on some 19th-century military helmets. From the few specimens that have been closely examined, it's also probably the smallest such species.

Although all the above species are recognised by most of the usual sources for such things, there is some confusion about them, and they have often been considered as synonyms or subspecies of each other and their current recognition is recent enough that that may yet change again. Part of the reason for the confusion is the existence of "P. ellioti", a purported species (or subspecies) that lives in the area between Oustalet's, Lang's, and the Semliki red colobus and that has a rather variable appearance. This is now generally thought to be a "hybrid swarm" - a self-perpetuating population consisting of hybrids of all three species breeding with each other and with their ancestral populations. Which, naturally, raises the question of how distinct they all really are.

This is a very rough map, and shouldn't be
taken too literally. Click to enlarge.

Before we leave the Congo, however, we should also mention the Tshuapa red colobus (Piliocolobus tholloni). This is another comparatively widespread species, being found across the Congolese lowlands east of the Congo River and south of the more westerly populations of Oustalet's species. Its distinctive appearance led to it first being identified in the 19th century, but its current status as a species dates only from 2001, and hasn't been consistently recognised since then. It has red fur on the back and arms, fading to a paler orange on the legs and silvery-grey on the rest of the body. The crown of the head is dark brown, rather than red, as it is in most other red colobuses, but is distinct from the black fur around the face.

Despite the fact that it lives over a moderately large area, this is deep in some of the forested and inaccessible parts of the Congo jungle and there is some evidence it may prefer swampy terrain, which would make it even harder to get to. As a result, we know very little about it beyond the fact that it seems to live in large groups and has a similar, strongly leaf-based, diet to other red colobuses. It does, however, seem to be quite numerous when it finds the right sort of habitat and isn't subject to hunting for bushmeat and so, again, is another one not considered to be truly endangered.

That's a lot of species, many of which do seem very similar to each other and few of which have been studied enough to determine how distinct their habits are. But even so, it's only half way through the many different kinds of red colobus, with others known to both the west and east of those named above. Next time, I will be turning to those.

[Photos by Charles Sharp and Michal Sloviak, from Wikimedia Commons. Painting by Daniel Elliot, in the public domain.]