Olive Colobus, Procolobus verus
OLIVE COLOBUS
Procolobus verus
Geographic Distribution and Habitat
The olive colobus monkey (Procolobus verus), also known as the green colobus and Van Beneden’s colobus, is native to the forests of West Africa. They appear sporadically from southern Sierra Leone and Guinea to southeast Nigeria, with a very patchy presence in the countries of Ghana, Togo, and Benin. The biggest gap in their range occurs in western Nigeria, between the border with Benin and the Niger River. Olive colobus groups have also been seen in the Niger Delta and in Benue, a Nigerian state east of the Niger River.
This species has a naturally low population density, but surveys show its numbers are steadily declining. Their numbers are more plentiful in a few important locations, such as the Taï National Park and Tanoé-Ehy Forest in Côte d’Ivoire.
The olive colobus primarily lives in swamp forests, dense tropical forests, and secondary growth areas, where trees have partially recovered from previous disturbances. They are especially common in forests surrounding rivers, known as riparian ecosystems.
Despite this clear preference for more humid habitats, olive colobus groups can also be found in forest corridors in drier regions, where some trees may temporarily lose their leaves for part of the season. They can live even in small, degraded forest fragments.
The olive colobus is monotypic, meaning it is the only species in its genus, Procolobus. The late primate taxonomist, Colin Groves, placed this species in its own genus in 2001. Although there are some similarities to red colobus species (Piliocolobus sp.), the skulls and teeth of the olive colobus are different enough to justify its distinct species status from both red and black-and-white colobuses (Colobus sp.). Molecular analyses later confirmed that, within the African leaf-eating monkeys, the olive colobus is most closely related to the red colobus, with both of these taxa being equally related to the black-and-white colobus species.
Size, Weight, and Lifespan
The olive colobus is the smallest of all leaf-eating monkeys, with both sexes roughly equal in size. Measured from the top of the head to the end of the body, males are between 18-20 inches (46-51 cm) long, and females are 14.9-21.7 inches (38-55cm) long. Their tails are quite long, averaging 19.7-24 inches (50-61 cm) for males and 17-27.9 inches (43-71 cm) for females.
Male olive colobus monkeys are slightly heavier than females, coming in at 9.9-15.4 pounds (4.5-7 kg) compared to 6.2-11.9 pounds (2.8-5.4 kg).
The lifespan of the olive colobus is unknown. However, it is probably similar to the closely related king colobus (Colobus polykomos), which has an overlapping geographic range and lives around 20 years.
Appearance
As the first part of their name suggests, the olive colobus has a mostly olive colored coat. Their backs, tails, and limbs are a dull, reddish-brown sort of olive shade that turns grayish white on their underside. They have slender, untufted tails and enormous feet that can be as much as one-third the length of their entire back leg.
The second part of this monkey’s name, colobus, comes from the Greek word for mutilated. This is in reference to the small, rounded, bony projections they have in place of proper thumbs. Combined with their big feet and long tails, the olive colobus is well-suited to maneuver through the trees in search of their preferred food source, young leaves. Like all African colobine, or leaf-eating, monkeys, the olive colobus is equipped with special adaptations for these fibrous foods. Beginning in the mouth, saliva packed with enzymes for breaking down the strong molecular bonds of leaves is secreted. As they move through the digestive system, the specialized, multi-chambered, or sacculated, stomach continues to break down these hearty fibers.
Their heads are small and round compared to their bodies. Their faces are mostly hairless, surrounded by an off-white fringe of hair. Like a racing stripe, a low crest of upright hairs runs from the forehead to the back of the head. This feature, offset by a pair of pale whorls, is more pronounced in males.
Diet
The olive colobus mostly eats leaves and leaf parts, accounting for 75-90% of their diet! Their favorite foods include young leaves, flowers, and unripe seeds. Leaves from lianas, which are long, woody vines that are rooted in the soil but climb up trees, are a year-round staple for the olive colobus.
They’re somewhat selective eaters, only munching from a small number of preferred trees. The trees they choose to eat from are usually deciduous, shedding their leaves annually, or have big bursts of new leaf growth in just a short period of time. The olive colobus’ diet includes 50 plant species, and their favorite leaves are usually low in fiber and tannins, making them easier to digest than leaves high in these plant compounds.
Unlike other colobus monkeys, the olive colobus treats mature leaves like a food of last resort. Although olive colobuses have multi-chambered stomachs that allow them to digest mature leaves that most other primates cannot, they ignore mature leaves for most of the year. Only in the dry season, when their preferred young leaves and seeds are scarce, will they resort to consuming a small quantity of mature leaves.
Behavior and Lifestyle
Demure and mindful is the mantra of this species. Olive colobus groups are shy and spend up to 40% of the day resting! The rest of their schedule is 27% eating, 25% moving, and 8% socializing. They are diurnal, meaning they are active during the day and asleep at night.
This tree-dwelling, or arboreal, species mostly spends time in the lower and middle strata of the forest, but will occasionally descend to the ground. They prefer dense tangled growth in the lower levels of the forest, less than 65.6 feet (20 m) off the ground, but sometimes ascend higher to eat leaves or follow a monkey friend.
Olive colobus monkeys are rarely found by themselves and are almost always observed in groups of up to five different monkey species. They have a permanent, evolutionarily-rooted association with Diana monkeys (Cercopithecus diana) for predator avoidance. Because of their cryptic behavior, proclivity for dense foliage, and associations with guenons, olive colobus groups are quite successful at evading predators. When an alarm call is sounded, they have been observed quickly moving to the center of the multi-species group and silently tucking their heads into their knees, while flattening their bodies against branches.
Olive colobus monkeys are rarely seen by themselves. Instead, they prefer to be in large groups with many other monkey species. They primarily associate with Diana monkeys (Cercopithecus diana), but also interact with western lesser spot-nosed guenons (Cercopithecus petaurista buettikoferi) and western red colobus monkeys (Piliocolobus badius). Although these are the olive colobus’ “best friends”, these massive polyspecific groups, meaning containing multiple species, have also been observed containing Campbell’s mona monkeys (Cercopithecus campbelli), Lowe’s mona monkey (Cercopithecus lowei), king colobus (Colobus polykomos), white-thighed colobus (Colobus vellerosus), and sooty mangabeys (Cercocebus atys).
Olive colobus moms are the only Afro-Eurasian monkeys known to hold their babies in their mouths for the first weeks of their lives.
Olive colobus groups generally keep to home ranges spanning 0.1-0.2 square miles (28-58 hectares) and move an average of 3,937 feet (1,200 m) on any given day.
This species lives in small social groups of 5-20 individuals, though most will include fewer than 10. Typically, there will be 1-2 dominant adult males with a matching number of adult females. Membership is ever-changing, with both young males and females leaving their birth groups. Most of this change is driven by adult females, who will change their affiliation more frequently than the adult males.
This shy and cryptic species likes to keep the company of other forest monkeys, especially guenons, which belong to the Cercopithecus genus. These relationships are rather one-sided, with the olive colobus groups being the ones who put in the work to stay together. Similar to schooling fish or a herd of zebras, this association likely offers protection against predators by diluting the risk to any individuals and providing more surveillance for threats. The olive colobus responds to the alarm calls of guenons.
This species’ absolute besties, however, are Diana monkeys. Although they eat from different strata of the forest and have no dietary overlap, each olive colobus group associates with one Diana monkey group that they follow around. As a result, territorial disputes between Diana monkeys also become impromptu meetings of associated olive colobus, which is when membership transfers between olive colobus groups normally happen. Scientists believe olive colobus monkeys might be using Diana monkeys both for safety and as a way to meet other olive colobus monkeys to mate with.
Hear ye! Hear ye! Olive colobus monkeys use vocalizations as their primary method of communication. They are surprisingly loud for their size and make calls that carry over a considerable distance. They usually don’t have a lot to say, though, since they live in such small groups, have simple social structures, and mostly follow other monkeys around instead of making their own travel decisions.
When olive colobus monkeys are vocalizing, however, they almost always combine individual sounds into structured sequences that communicate detailed information. They have a deceptively simple repertoire of two sounds — a short, low-frequency call and a long, high-frequency call — that form this syntax. Instead of making a generic alarm call, for example, olive colobus monkeys can specify whether the threat is a falling tree, a leopard, or an eagle. 97% of their vocals are combinations, rather than single calls.
This ability to combine calls is especially important for males; they are responsible for up to 90% of call sequences. These have not been observed in newborn or juvenile olive colobus monkeys and are likely behaviors learned from hearing calls of older individuals.
But why would a species with such a simple social structure put all this effort into loud, detailed communication? Scientists theorize that because olive colobus monkeys switch groups so frequently, members of neighboring groups likely know each other and may even be related. This could motivate olive colobus monkeys to ensure their messages are heard even by distant listeners from other groups. This theory is supported by the fact that olive colobus monkeys systematically wait until calls from other species have quieted down before beginning their own vocalizations.
Scent marking has also been observed in olive colobus monkeys. When leaving a sleeping site, adult males urinate, defecate, and rub what are thought to be two scent glands against the nearby branches. Though vocalizations are quite prominent, this may be another subtle way of communicating between olive colobus monkeys.
Among olive colobus monkeys, sexual encounters are usually initiated by the female, and she overwhelmingly chooses the dominant male in a group. Mating can happen within a group or between individuals of different groups. Liaisons with outsiders usually happen while an olive colobus group is following their Diana monkey group.
When she’s about to ovulate and at her most fertile, a female olive colobus monkey will have sexual swelling on her rear end. This lasts for half of her menstrual cycle, signaling to males she is ready to mate. As for the dominant male, he has unusually large testes, which may similarly be to attract the attention of a potential mate.
Olive colobus monkeys are seasonal breeders, mating between April and August. With pregnancies lasting 150-180 days, this means most births happen between November and February.
Female olive colobus reproduce every two years, giving birth to one baby at a time. The newborn olive colobus looks a lot like an adult–just a little bit lighter and a little bit more yellow and, of course, much smaller.
Not one to follow the crowd, an olive colobus mother is the only Afro-Eurasian monkey species that carries her newborn dangling from her mouth. This lasts for the first few weeks after birth, until the baby is strong enough to start clinging to the mother’s abdomen instead. The mother is the primary caregiver, providing milk, grooming, and protection. It’s unknown if the male plays a role in raising the young.
As a flower and seed eater, the olive colobus contributes to the overall health and diversity of the forest. When an olive colobus defecates, the seeds they ate come back out, helping the plant grow somewhere new. These sources are only a small portion of their diet, however. As leaf eaters, they also help to regulate plant growth, cycle nutrients through the ecosystem, and influence forest structure. Additionally, they act as a food source for potential predators such as chimpanzees, leopards, crowned eagles, and, unfortunately, human hunters.
The International Union for Conservation of Nature lists the olive colobus monkey as Vulnerable (IUCN, 2018), appearing on the IUCN Red List of Threatened Species. This is an escalation from a previous status of Near Threatened, because the olive colobus monkey has likely experienced a population decline of more than 30%, from 1993 to 2020, spanning three generations.
Their primary threats come from habitat loss and hunting. Between 1990 and 2015, olive colobus monkeys saw a 22.5% reduction in the West African forests they call home. The greatest loss has been in the eastern end of the range in Nigeria. Olive colobus are also heavily hunted for their meat.
Fortunately, the olive colobus can survive even in small forest fragments and adapt to some human disturbance—these factors have likely played a role in slowing their decline. This does not address the core threats, though, so the population of olive colobus monkeys is only expected to keep declining. West Africa also has one of the highest rates of forest habitat loss in the world. Combined with steady human population growth and continued hunting, things are likely to only get worse for the olive colobus.
The olive colobus monkey is listed as Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments to ensure that international trade in specimens of wild animals and plants does not threaten their survival. Appendix II lists species that are not under immediate threat but may become so without proper trade regulations. They are additionally listed as a Class A species under the African Convention, meaning they receive the highest level of protection, with hunting or capture only allowed under special authorization. Despite these agreements, olive colobus monkeys are still threatened by the flourishing trade and consumption of animal meat in West and Central Africa.
Members of this species are found in a few protected areas, such as in Côte d’Ivoire (Taï National Park and Tanoé-Ehy Forest), Ghana (Bia, Kakum, and Nini-Suhien National Parks, Bomfobiri Wildlife Sanctuary, Ankasa Game Reserve, and Sukusuku Reserve), Liberia (Mount Nimba Strict Nature Reserve and Sapo National Park), and Sierra Leone (Gola Forest Reserves and Tiwai Island Wildlife Sanctuary).
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Written by Jiayu Liang, December 2025