Sunday 24 April 2022

Leaf-Eating Monkeys: Red and Olive Colobuses

Zanzibar red colobus
The strict food requirements of red colobus monkeys make them one of the most endangered groups of African primates but they are still found across a surprisingly wide swathe of the continent. While we used to recognise only three or four species (albeit with many subspecies), it has become clear over the last thirty years or so that there are many more, around half of which are now restricted to very small areas. In most cases, this is because of the obvious reason that their old habitat has been destroyed and that they used to range further afield. Some, however, have never strayed far from home.

The easternmost species of red colobus falls into this latter category. Named the Zanzibar red colobus (Piliocolobus kirkii) this has historically restricted to the eponymous island. Since 1964, this has been the "zan" part of Tanzania, and lies just over 20 miles off the east coast of the mainland with an area of a little under 1,500 km² (565 square miles). With a human population of around 1,500,000 it is, as one might expect, substantially given over to agriculture and other development. As a result, while they once ranged across the island, they are now primarily found in the less settled eastern half of the island, where there are some inland national parks. In the 1970s, a small population was re-introduced to a patch of land on the west coast north of the main urban areas and others were translocated to Pemba Island to the north, where the species has never existed naturally. While one of the latter groups did survive and breed, their descendants are likely quite inbred and it has been argued that we'd be better off supporting the main population in their original home.

Living on an island as they do, Zanzibar red colobuses have likely been separated from their mainland kin for some time, which may help to explain why they are one of the most distinctive examples of their genus. They are the smallest of all species of red colobus, with adults being no more than 50 cm (20 inches) long - normally around the minimum size for most other species. This is common among island species, which are adapted to living in restricted areas, and it's notable that females are not much smaller than males, unlike the pattern elsewhere among colobuses. Their legs, underparts, and the hair around their faces are all creamy-white, with distinct black fur on the arms, shoulders, and feet. The back, however, is a bright, brick-red as is the back of the head, giving them a distinct tricolour pattern. Another distinction is that the skin around the nose and mouth is pink, something normally only seen in infant colobuses, but here retained throughout life. The young are born white, not black, as is the case for related species.

While it likely originally lived in relatively dense jungle, there really isn't any of that left on Zanzibar, and the monkey has instead adapted to secondary forests with plentiful groundwater. They are also found in dry scrubland growing on old, inland, coral formations where leaf monkeys would not normally be expected to thrive. They are common in forests close to agricultural land, although such populations tend to be unstable, and mangrove swamps are also a suitable, if not especially favoured, habitat. Like most colobuses, they mostly eat young leaves and buds, although they do have a significant proportion of fruit in their diet at certain times of the year. Left to their own devices, they form large mixed-sex groups of 30 or so individuals, although restricted habitat and range can restrict or alter these patterns.

A 2017 study revealed that the total population of Zanzibar red colobuses was around 5,800, which was higher than previously thought. This does not necessarily mean that their population is not in decline, just that earlier population estimates may not have been accurate and, since they live in such a small area of land, with all the risks that that entails, they are currently listed as an endangered species. Nonetheless, there is some positive evidence that the monkeys are doing well in park areas benefiting from ecotourism and that the installation of speedbumps at key areas has helped reduce a previously high rate of deaths as roadkill.

Udzungwa red colobus

The closest living relative of the Zanzibar red colobus is the Udzungwa red colobus (Piliocolobus gordonorum) which lives in the foothills and valleys of the Udzungwa Mountains in central mainland Tanzania, only occasionally venturing onto the forested upper slopes. Although they also have a tri-coloured pattern, it is less extreme than in the island species, with more black and less white on the body and red hair restricted to a "toupee" on the head and sometimes a patch on the rump. They live in groups of highly variable size, ranging from just 7 to 83 individuals, which seem to be affected more by the degree of local human disturbance than by the details of the habitat. Although their population is declining, due to a mixture of illegal hunting and encroaching agriculture (especially after the construction of the main railway line to Zambia in the 1970s which necessarily runs through the mountains), it remains over 50,000, which is just enough for it avoid a formal "endangered" status.

If the Zanzibar red colobus is the most easterly species of its genus, the most westerly is, appropriately enough, the western red colobus (Piliocolobus badius). This lives in two distinct areas in the far west of Africa, with a more northerly population in Senegal, Gambia, and Guinea-Bissau and a second one from southern Guinae through Sierra Leone and Liberia into western Cote d'Ivoire. The two populations are traditionally regarded as distinct subspecies, although some primatologists consider them to be entirely separate species, with the northern one being named Temminck's red colobus (P. temmincki).

Western red colobus (northern subspecies)

Western red colobuses are typically darker than other species of the genus, being black over the back and limbs, with deep red fur on the undersides and cheeks. Those living further north become noticeably paler, although the transition seems to be more gradual than one would expect if they were really separate species. They mostly live in groups of a couple or dozen or individuals, although larger bands of up to 60 individuals have been seen in some restricted patches of forest.

Historically, they lived, as is typical of colobus monkeys, in dense forest, whether it be the damp forests of the southern part of their range, or the drier ones of Senegal. Since at least the 1990s, however, some of the northern population have left the forests altogether and headed into more open savannah woodland, where they seem to be able to survive despite being forced to travel across the ground far more frequently than they do elsewhere. This, of course, has been forced on them by loss of their original habitat which has been considerable across much of their range. It also seems to have compelled them to change their diet, with them now eating more fruit than before, and even some grasses and other ground vegetation which they would normally avoid.

While western red colobuses are found across a much wider area than many other red colobus species, their remaining populations are scattered and isolated from one another. While they may be able to survive in savannah better than one might expect, it clearly isn't optimal for them, and, together with widespread hunting, this has led to a dramatic decline in their population sufficient to place them, too, on the endangered species list.

Similarly endangered is Bouvier's red colobus (Piliocolobus bouvieri) which was only raised to full species status in 2007, partly on the grounds of an unusually pale face compared with its relatives. It was originally thought to have lived only along a short stretch of the Congo River, where it apparently went extinct in the 1970s. Fortunately, some individuals were spotted alive in 2007, much further north than they were previously known along the west bank of the Sangha River, a major tributary of the Congo. Further sightings in the area followed, and, in 2016, some were also seen closer to their original assumed home in the south. Although this does mean that they are more widespread than previously thought, they remain endangered, and sightings are relatively rare. 

Pennant's red colobus

Its closest relative is thought to be Pennant's red colobus (Piliocolobus pennantii) which looks rather similar, but is darker in colour, with red-speckled black fur across much of the body and the only truly red patches on the flanks and thighs. This is another island species, being found only on Bioko Island in Equatorial Guinea, and even then restricted only to a patch of hilly forest on the southwestern coast that may cover no more than 160 km² (60 square miles). The construction of a major new road in the area has naturally disrupted the forest and also likely made it easier for hunters to access. Since red colobus is a particularly popular form of bushmeat in the area this does not bode well, and whatever population there may have been is now so small that its future survival remains in doubt.

Preuss's red colobus (Piliocolobus preussi) lives in and around two national parks in the south of Cameroon, with one population likely extending just over the border into Nigeria. It is said to have thicker hair than other red colobuses, and the females have bright pink sexual swellings that dramatically enlarge during the mating season - this is generally true of other red colobuses, too, but is apparently much more extreme in this species. It is likely all but gone from the easternmost of the two parks, with the population at the other thought to be less than 4,500

Further west we find the Niger Delta red colobus (Piliocolobus epieni) living only in the eponymous patch of swampland in Nigeria. This wasn't even known to western science prior to 1993, and was raised to subspecies status (of western red colobus) in 1999 then to full species in 2007 at the same time as Bouvier's species. It looks very similar to Preuss's species but with white arms and a slightly paler body, and relies heavily on a particular type of swamp tree for its food. The tree is itself listed as a threatened species, and an assessment in 2015 suggested that only a few hundred of the monkeys were likely alive at that time... and that they might disappear entirely by 2020. It's currently too early to know whether or not this actually happened.

If it did, it may not be alone. The most recent species to be split off from the western red colobus is Miss Waldron's red colobus (Pilicolobus waldronae) which used to live in the coastal regions on either side of the Ghana-Cote d'Ivoire border. It looked very similar to the western species, although with slightly brighter fur, but genetic analysis seems to show that it is more distinct than physical appearance alone would indicate. It was last seen alive by naturalists in 1978 and an attempt to prove its continued existence in the 1990s failed, leading to it being declared "probably extinct". Since then a recently dead specimen turned up in the hands of a hunter in 2001, and some researchers claimed to have heard one calling in 2008, although they couldn't see it. While some analyses suggest that they may still be alive somewhere in the forest, their future is undoubtedly bleak.

The red colobus genus, Piliocolobus, was first proposed by Alphonse de Rochebrune in 1877 but was largely ignored through the 20th century, with its members instead being placed in the genus Procolobus. More recently, the two have been separated again, so that only one species still remains in the genus that once also contained all of the red ones: the olive colobus (Procolobus verus). 

This lives in West Africa, from Sierra Leone to Ghana, with isolated populations further east in Benin and Nigeria. This overlaps, not only with many of the species listed above, but also with the king and ursine colobuses. Physically, it is the smallest of all the colobuses, although only slightly more so than the Zanzibar red. It is also perhaps the drabbest species, having a relatively uniform olive-brown coat that becomes paler on the underparts and a white ruff of hair on either cheek. Its feet are unusually large, at around the third the length of the entire leg, but perhaps its most distinctive feature is a narrow crest of vertical hair sticking upwards from the top of its head like a mohawk haircut.

They inhabit a wide range of forest types from the region and may survive alongside other colobus monkeys by feeding from different trees, since they seem to be unusually picky eaters, ignoring the more mature leaves that others often consume. Instead, they largely feed at lower levels within the forest, often as far down as the understory and focussing on a small number of shorter and relatively uncommon trees as well as on creepers and lianas.

Olive colobus

Olive colobuses live in much smaller groups than red colobuses do, with most groups having fewer than ten members, less than half of which are adults. Unusually, however, olive colobuses are almost always seen close by to Diana monkeys (Cercopithecus diana) a predominantly fruit-eating species that rarely comes down from the trees and generally forages up above the colobuses. They don't seem to move more than 50 metres (150 feet) away from the Diana monkeys if they can help it, and do so without any apparent encouragement from the other species. As a result, they travel wherever the Diana monkeys do, and end up sticking with the same group for protracted periods, since they don't have much opportunity to find a new one. While it isn't absolutely clear why they do this, it seems likely that they benefit from increased protection, since the Diana monkeys are slightly larger than they are, and the greater numbers can better keep an eye out for potential predators.

It's also possible that the presence of the larger, darker-coloured monkeys makes its easier for other olive colobuses to find those already following around a group, giving males more opportunity to meet up with new females. (The females seem to choose who they mate with, rather than being dominated by the males, so hanging around where females are likely to look is probably a sensible strategy for the latter). Breeding in olive colobuses is weakly seasonal, rather than year-round and, in another difference from other colobus species, mothers carry their young around in their mouths for the first few weeks of life rather than in the more obvious manner.

The various kinds of colobus monkey appear to have diverged from one another by the end of the Miocene, 5 million years ago. This was likely after they had lost their thumbs, since 9 million-year-old fossils, such as Microcolobus still had theirs, while even extinct genera from later times, such as the 2 million-year-old Cercopithecoides, did not. At some point before this strange adaptation arose, however, a group of leaf monkeys must have left Africa and travelled into Asia, where they gave rise to several species that still have the full complement of five digits on their hands. Next time, I will be looking at some of those Asian species...

[Photos by Harvey Barrison, "Haplochromis", and "Kreuzberger" from Wikimedia Commons. Drawings by Daniel Giraud Elliot and Jean Charles Chenu, in the public domain. Cladogram adapted from Springer et al. 2012.]

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