Geographic Distribution and Habitat
The L’Hoest’s monkey, also known as the mountain monkey, can be found in eastern Democratic Republic of Congo (DRC), Burundi, Rwanda, and western Uganda. It is unknown just how far south their range extends, but they are likely also present along the eastern bank of the Congo River. There are isolated populations located in multiple national parks in DRC and Uganda, which is east of what is considered its main range.
L’Hoest’s monkey can be spotted in both the primary and secondary levels of lowland, submontane, and montane tropical forests within the 600-mile-long (1,000 km) Albertine Rift, also known as Africa’s Western Rift Valley. The rift is one of the most biodiverse regions in Africa—more than half of the bird species and about forty percent of the mammals live in that region of the continent. This species occurs at altitudes ranging from 2,952 to 8,202 ft (900–2,500 m). They are widespread in the lowland forest block of eastern DRC. L’Hoest’s monkeys are sympatric with Hamlyn’s monkeys (Cercopithecus hamlyni), especially in the isolated populations west of the Albertine Rift.
It is important to note that the L’Hoest’s monkey was reclassified by the International Union for Conservation of Nature (IUCN) as belonging to the genus Allochrocebus in 2013. Prior to 2013, the species was classified within the Cercopithecus genus, which is the genus many sources still refer to as its taxonomic name. Some controversy regarding classification remains among scientists. Before that, the L’Hoest’s monkey, along with the Preuss’s Monkey (Allochrocebus preussi) and sun-tailed Monkey (Allochrocebus solatus), were classified as a single species. To clarify: L’Hoest’s monkeys are a distinct species and their taxonomic name as of 2013 is Allochrocebus Ihoesti.
Size, Weight, and Lifespan
L’Hoest’s monkeys have long limbs and a long tail, usually reaching a total of 12.5–27 in (31.7–68.6 cm) in length with an additional 19–39 in (48.3–99.1 cm) for the tail. Females weigh approximately 7.7 lbs (3.5 kg), while males weigh approximately 13.2 lbs (6 kg).
In the wild, their lifespan is unknown, but in captivity one individual lived to be 24 years old.
L’Hoest’s monkeys are large, strikingly patterned primates with short, thick fur in a variety of colors; a reddish-orange patch of fur covers their back, a fluffy white section stretches from their throat to their ears like a giant beard, and their limbs and stomach are black.
Their head, back, and tail are speckled with white, giving them the appearance that someone stood over them while eating a particularly powdery funnel cake. Eyes are orange. Cheek pouches store food and narrow feet aid in terrestrial locomotion.
L’Hoest’s monkeys are sexually dimorphic in size. Both genders are mostly the same in coloration, although males have a bright blue scrotum. Females are also smaller and lighter than males.
The L’Hoest’s monkey is omnivorous; their dinner menu is full of terrestrial herbs (35%), fruits (24%), mushrooms, flowers, and small vertebrates (approximately 30%). Invertebrates such as insects, earthworms, spiders, ants, and grasshoppers are also consumed, but at a much lower consistency (approximately 10%). In areas where L’Hoest’s monkeys overlap with cultivated land, they are often guilty of sneaking snacks from unsuspecting farmers—in others words, they pillage the fields and gardens like the worst kind of neighbors.
Groups of L’Hoest’s monkeys will forage together, mainly in lower forest strata (the part that has mushrooms, herbs, and arthropods, which are often found in shallow streams, under fallen leaves and trunks, and on branches). In the upper strata, they consume small fruits, buds, flowers, young leaves, and herbaceous stems.
Behavior and Lifestyle
L’Hoest’s monkeys are diurnal and most active in the early morning and the late afternoon. Mostly terrestrial, they are typically found on the ground, traveling and foraging for food. As mentioned, they forage at different levels throughout the forest. However, their sleeping habits are somewhat unique. They sleep sitting upright in trees, holding onto each other or onto branches. Each member picks out a preferred sleeping spot that becomes one they return to each night.
They will flee to the trees when they feel threatened by a ground predator, remaining very still once they’ve reached a safe hiding spot. When hunted by crowned hawk-eagles, L’Hoest’s monkeys will flee from the trees and escape on the ground.
Perhaps not a “fun” fact, but an interesting one: L’Hoest’s monkeys can become infected with Simian Immunodeficiency Virus, or SIV, which becomes HIV when transmitted to humans.
The Edinburgh Zoo welcomed a baby L’Hoest’s monkey during the height of quarantine due to the 2020 coronavirus pandemic. The playful youngster was named Butembo after a city in his native home of DRC.
L’Hoest’s monkeys typically live in small groups comprised of 10 to 17 females, their offspring, and one male. Scientists have occasionally observed larger troops of up to almost 40 individuals. These are one-male-multi-female groups that travel long distances, forage, and sleep in nesting sites together. Bonds grow strong within the group through allogrooming and the allomothering of young, which is unsurprising since most of the females are related to one another.
Other males live in all-male “bachelor” groups. Because only one male has the opportunity to mate with the females in his group, there is a great deal of competition among the males. Those from the bachelor group seek to topple the regime of the male holding court within a one-male-multi-female group. That singular male must constantly defend his position from other the males. As a result, males rarely stay in that top position longer than one or two years, and sometimes even just weeks or months.
L’Hoest’s monkeys rarely associate with other guenon species.
L’Hoest’s monkeys communicate and coordinate while they flee from predators on the ground, though the exact method they use to do so is unknown. They use five different alarm calls, emitting different sounds for different types of danger.
Females engage in presenting when they are ready to mate. For visual threat displays, L’Hoest’s monkeys will stare with their eyes fixed on the subject, raise their eyebrows, stretch their facial skin, move their ears back, and occasionally bob their head. They will also often open their mouth without displaying their teeth.
L’Hoest’s monkeys are polygynous, meaning a single male lives and mates with multiple females. After five months of gestation, female L’Hoest’s monkeys will give birth to a single offspring every other year at the end of the dry season. Infants are born fully furred with open eyes; their fur darkens as they age and reaches adult coloration around two to three months. Infants are held by multiple females within the group after birth and they nurse until their mothers produce another offspring, although the frequency of nursing decreases significantly after the first couple months. Weaning occurs at an average of one year of age and males, upon sexual maturity, will depart their natal group.
Adorably, young L’Hoest’s monkeys will entwine their tail with their mothers’.
Due to their semi-frugivorous diet, L’Hoest’s monkeys play a role in seed dispersal, which regenerates their forests.
L’Hoest’s monkeys have faced a noticeable population decrease in the past and their future looks most unfortunately bleak. The majority of the L’Hoest’s monkey’s range overlaps a region that has faced years of violent human conflict, which has done little to assuage the threats leveled against them. Two of their biggest threats consist of bushmeat hunting via snares and shotguns (which L’Hoest’s monkeys are particularly susceptible to) and habitat loss mostly due to artisanal mining (which also, in turn, promotes bushmeat hunting and poaching because the process of mining opens up easier access to their habitat).
It is likely the continuing political turmoil will result in further population reduction of more than thirty percent from the years 1995 to 2025, an unfortunate statistic that has granted L’Hoest’s monkey with the International Union for Conservation of Nature classification of Vulnerable (IUCN, 2016).
Deforestation is also occurring on the eastern edge of their range in DRC due to agricultural expansion. The population size of L’Hoest’s monkeys as of 2016 was unknown, but it is undoubtedly shrinking due to the aforementioned threats.
A non-human-caused threat is chimpanzees, which have been observed preying on L’Hoest’s monkeys in DRC.
L’Hoest’s monkeys are the most important primate prey of leopards in the Ituri Forest. They are also preyed upon by crowned hawk-eagles.
L’Hoest’s monkeys occur in multiple protected national parks over their range, such as Virunga and Kahuzi-Biega national parks (DRC), Kibira National Park (Burundi), Nyungwe National Park (Rwanda), Kibale Forest National Park, Kalnizu Forest Reserve, Ruwenzori Mountains National Park, Maramagambo Forest Reserve, and Bwindi Impenetrable National Park (Uganda).
L’Hoest’s monkeys are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) and on Class B of the African Convention of Nature and Natural Resources (where they are classified as Cercopithecus and not Allochrocebus as mentioned earlier). They are also protected by national legislation in Burundi, Rwanda, and Uganda and only partially protected in DRC. According to IUCN, the most immediate action needed to protect L’Hoest’s monkeys is the control of bushmeat hunting through stricter and more effective law enforcement.
L’Hoest’s monkeys are also a part of the European Endangered Species Programme, a breeding program managed by the Edinburgh Zoo.
Written by Rachel Heim, September 2020