Leaf-eating Monkeys: Black-and-White Colobuses

King colobus

The Old World leaf-monkeys are formally known as "colobine" monkeys, with the name of their subfamily coming from what British zoologist Thomas Jerdon considered to be the most typical member of their subfamily when he first named it in 1867. These, of course, are the colobus monkeys, of which there are a great number of species. Jerdon, however, was thinking in particular of what we would now call the black-and-white colobus monkeys, so it's these against which, taxonomically speaking, all other leaf monkeys are compared.

The first colobus monkey of any kind to be scientifically described was the animal now known as the king colobus (Colobus polykomos). This was in 1780, with Eberhard von Zimmerman basing his description on a specimen collected from an island off Sierra Leone. His original name meant something like "many-singing monkey", and it was only in 1811 that the word "colobus" was coined. This comes from the Greek word for "maimed" and refers to the fact that colobus monkeys have no visible thumbs, just a small lump on the side of the palm, making it look as if they have been cut off.

(As an aside, since "colobus" is an adjective in both Latin and Greek, the plural "colobi", while often used, is technically inaccurate for the same reason that "boni" isn't the plural of "bonus". There is a more legitimate debate about whether the plural should be "colobuses" or the uninflected "colobus", but I'll be using the former).

During the mid-20th century, the king colobus was considered to be the only black-and-white colobus monkey species - and is therefore often referred to solely by that name in older publications. Since at least the 1980s, however, it has become apparent that there are at least five, and possibly six, different species that fit this description. With the others having been split off, therefore, the term "king colobus" now refers only to that species found along the coastal regions of West Africa, from Guinea-Bissau in the north to Cote d'Ivoire in the south and reaching about 200km (125 miles) inland.

As it happens, the king colobus is not as obviously black-and-white in colouration as some of the other species in the genus. Most of the body is black, but the hair around the face and on the shoulders is more accurately described as grey or grizzled. The tail, however, is almost pure white, and is remarkably long - a typical adult measures about 60 cm (2 feet) in length, but has a tail around 85 cm (3 feet) long. And, since it's an African monkey, the tail isn't prehensile, and just drapes.

King colobuses live among a wide variety of forests, at least as they are found in that part of the world, although they avoid those that have been disturbed by the advance of agriculture and other human activities. Their leafy diet would suggest that they ought to be fairly inactive animals, since there isn't much nutrition in the leaves that they eat but, in fact, they spend less of their time resting than we'd expect, at least during the dry season. 

This is probably because, at that time of year, certain trees are in fruit, and the monkeys eat a significant proportion of hard seeds, with one of their favourites being African oil beans (a leguminous tree). These provide them with plenty of calories and compensate for the lack of soft fruit in their diet compared with other types of monkeys. Other than that, they prefer fresh, young leaves over older ones, and consume a fair proportion of lianas, rather than the leaves of trees.

King colobus live in groups of up to 19 individuals, roughly half of which will be infants, with adult females outnumbering the males about two to one. The females are apparently more sociable than the males, and assist one another with child-rearing. The males are mainly just competing to impress potential mates and launch occasional forays to attack their neighbours. Despite this, it seems to be the female that selects her sexual partners, rather than the other way around.

In 2020, the king colobus was formally listed as an endangered species by the IUCN, since its population is thought to have halved over the last 30 years. It's probably no coincidence that the human population of their area has doubled over the same time span which will inevitably have reduced the amount of available forest to live in. The monkeys are also regularly hunted for food and this may be an even larger threat than the loss of habitat.

Angola colobus

As a result, most research on black-and-white colobuses has instead been conducted on two of their more common relatives. These were first identified as separate species in the 19th century, before being merged into the king colobus in 1929, and split off again more recently. One of these is the Angola colobus (Colobus angolensis) which, although first identified from that country, is much more common in the Democratic Republic of the Congo to the north and in Tanzania to the east.

The Angola colobus is slightly larger than the king colobus, and has pure white fur around the face and on the shoulders, the former flaring out into distinctive crescent-shaped mutton chops. The tail has variable combinations of black and white, which can help to distinguish between the many different subspecies.

They live in dense equatorial jungles and, while they do sometimes come down to the ground to feed in lush herbs by river banks, they spend most of their time in high branches up in the forest canopy. They eat a wide range of different plants, with about a third of their diet consisting of high energy fruit rather than young leaves; they can also rely on older leaves and even lichen when their preferred foods are in short supply.

Short-term observations suggest that Angolan colobuses live in small family groups of three or four individuals, led by a single male. But this, it turns out, masks a more complex reality. Instead, the individual families gather into multi-male troops of up to 23 individuals that spend so much of their time together that the "family" is arguably not a very meaningful social group. At least some of these troops regularly meet up and mingle with other neighbouring troops, even swapping members. The resulting "clans" can contain dozens of individuals and, in at least some areas, are themselves grouped into larger bands that meet up less frequently, forming a three-tier society. 

In Rwanda, where suitable food is apparently plentiful, the largest social groups can contain over 300 individuals without significant intragroup competition. The existence of such large groups and the social interactions they open up may have some connection to the fact that males spend more time looking after infants than they do in related species, and have preferred grooming partners of the opposite sex. In one recorded instance (albeit not in the Rwanda subspecies) a mother was recorded as adopting an infant from outside her group and continuing to raise it after her own child had died.

Guereza

The other common species is the guereza (Colobus guereza). This is physically the largest of the colobus monkeys, with the largest males weighing in at over 13 kg (29 lbs). They are also among the distinctive, with clear white fur around their faces and a mantle of long white fur running down each flank and over the rump. The tail is also white and has unusually long fur, making for an impressive tuft. 

Although there is some overlap in their ranges, they mostly live further north than the Angolan colobus, across central Africa from Cameroon and Gabon in the west to Ethiopia and Kenya in the east. The guerezas living on the slopes of Mount Kilimanjaro in Tanzania (and nowhere else) may be a separate species (Colobus caudata) although this is not yet universally accepted. Whether one species or two, guerezas live in a wide variety of local forest types and seem adaptable enough to cope with at least some human disturbance to their environment - a few even live in agricultural plantations.

They are more highly folivorous than Angolan colobuses, with some estimates suggesting that 87% of their diet consists of leaves. When they do eat fruit - often when young leaves are not available in large quantities - they seem to prefer it unripe. This may be due to a desire to avoid competition with the local species of fruit-eating monkey, which naturally prefer to eat it when it is ripe. They will apparently travel long distances to get at sodium-rich food absent in their normal place of residence.

Guereza groups usually number from six to ten individuals although they can be larger in areas with wide patches of unbroken forest, so that there are more trees within easy reach. Each group typically has a single adult male, with the few multi-male groups being restricted to larger forests and tending to break apart before long. When different groups meet up, the result is often aggression mostly, but not entirely, spurred on by the males, who probably want to protect "their" females from rivals.

The scientific name of the king colobus reflects the fact that it is a vocal monkey, and this is no less true among their close relatives. Guerezas have alarm calls that signal the presence of predators, with some ability to distinguish between ground-based (leopard) and aerial (eagle) predators, although not to the highly specialised extent seen in some other monkeys. Males are also known to roar loudly into the forest, often accompanied by wildly jumping about, especially when the females in their group are sexually fertile. This likely serves as a warning to other males, since it is possible to gauge the size of a roaring male by the pitch it can produce. The volume is enhanced by an air sac in the throat, making it louder for a given body size than it would otherwise be.

Black colobus

Of the two more restricted species, the black colobus (Colobus satanas) is unique among colobuses in being entirely black. Even the young, which are born white in the other species of the genus, are brown in black colobuses. They have the same sort of long-haired mantle as the guerezas do but, since it's the same colour as the rest of the animal, it is much less noticeable.

Black colobuses live to the west of the guerezas and Angolan colobuses, along the Atlantic coast from southern Cameroon to central Gabon, with a separate subspecies living on Bioko Island in Equatorial Guinea. Even more so than the king colobus, they supplement their leafy diet with a large number of seeds and their digestive tract is highly effective at breaking down plant toxins. This not only allows them to eat a wider range of seeds than they might otherwise, but also to feed on leaves that would normally be inedible.

They live in moderately sized multi-male groups of up to 30 individuals and, on the mainland, are seminomadic in order to make the most of seasonally unreliable food sources. On Bioko Island (which is roughly the same size as Maui in Hawaii and smaller than Lewis & Harris in the UK) their groups are smaller and are more likely to travel into the high altitude heathland of the central volcanos doubtless due to the lack of other options.

The remaining species is most commonly referred to as the ursine colobus (Colobus vellerosus) although a number of sources prefer the alternative name "white-thighed colobus" which does at least have the advantage of being more descriptive. It appears similar to the king colobus, and lives just east of it, from Cote d'Ivoire to the western tip of Nigeria. Unlike the king colobus, however, it has a shorter tail with an even brighter white colouring and has clearly white fur around the face, forming a visible ruff. 

They live in lowland forests, and even some sparse woodlands in the north, where they may cross open ground to move between trees. Their diet consists largely of young leaves, especially from lianas where those are available, although they do also eat seeds and other plant parts. They live in groups of up to 33 individuals with multiple males and relatively tight bonding between females. There does seem to be some variation in this general pattern, however, with some females going as far to leave the group of their birth to find a new one (normally only the males do this, keeping the gene pool viable), others having close matrilineal kinship bonding, and still others somewhere in between.

The species faces the same threats as the king colobus, but has fared even less well against them. Populations have declined dramatically in the last few decades, with them vanishing entirely from a number of areas and less than a thousand adults estimated to remain in the wild. The largest remaining population is in the Boabeng-Fiema Monkey Sanctuary in Ghana, an area where they have the good fortune to be considered sacred animals. Although that population is thought to be fairly stable, overall the species is rated as critically endangered, the highest available risk category.

At one point, an additional population, Dollman's colobus, was thought to live in central Cote d'Ivoire between the ranges of the ursine and king colobuses. It was sometimes regarded as a full species in its own right, but more commonly as a subspecies of one or the other of its neighbours. It now seems to be more commonly regarded as a hybrid between the two, and may, in any case, already be extinct.

However, not all colobus monkeys have the black and white colouring of those described above. Indeed, the majority do not and next time I will be looking at some of those...

[Photos by Paul Korecky, Ryan E. Poplin, and "Cacophony" from Wikimedia Commons. Illustration from Brehm's Tierleben, in the public domain. Cladogram adapted from Springer et al. 2012.]