The Mysterious L’Hoest’s Monkey of the Bwindi Impenetrable Forest

Tharcisse Ukizintambara, doctoral student, Center for Tropical Ecology and Conservation, Antioch University, New Hampshire, USA
September 2006

Uganda’s Bwindi Impenetrable National Park (BINP) is a haven for L’Hoest’s monkey (Cercopithecus lhoesti). Despite living on the exposed edges of this forest, L’Hoest’s monkey is among the world’s least-known primates. As such, populations of this mysterious “mountain monkey” might mistakenly be believed to be in good health.

In fact, so little is known of this animal’s status in the wild that L’Hoest’s monkeys could face hardships like starvation, predation, competition with other species, or other catastrophes—or even become extinct—due to inadequate management policies aimed at human activities. Such policies may result in human behaviors that increase habitat loss, disease transmission, hunting pressure, and human-wildlife conflicts along forest edges. In addition, subtle factors such as forest edge effects (that is, the changes in habitat that occur at forest edges—such as more exposure to sunlight—as opposed to forest interiors) can play a role in slowly bringing a species to extinction. Losing the “near-threatened” L’Hoest’s monkey forever before we have even begun to understand it would indeed be a tragedy.

Tracking a secretive monkey

L’Hoest’s monkey is one of Africa’s most attractive primates, with intriguing behavior and geographic distribution. Climate changes over the past two million years or so are believed to have isolated three closely-related species. The sun-tailed monkey is confined to central Gabon (not far from Africa’s Atlantic coast), while the Preuss’s guenon is found further north in Cameroon, Nigeria, and Bioko Island; to the east, L’Hoest’s monkeys are found in the eastern Democratic Republic of Congo and the western parts of Rwanda, Burundi, and Uganda.

L’Hoest’s monkeys frequently come to ground but are nonetheless secretive (they rarely emit alarm calls). They feed mostly on terrestrial herbaceous vegetation, pith, fruits, flowers, eggs, and small young animals. L’Hoest’s monkeys are adept at catching flying insects. They will carefully clean dirty dormant seeds that they have excavated from the soil before eating them.

L’Hoest’s monkeys are reported to occur almost exclusively on the edges of BINP, a forest island within an ocean of farmland. The area around BINP has one of the highest human population densities in the world (over 400 people/km2). Due to the high rates of habitat loss, most forest primates in this region will likely soon find themselves living on forest fringes and may thus face extinction resulting from high predation rates and human encroachment. However, some species may be able to adapt to forest edge conditions, depending on local land use and management strategies.

From daily “follows” of one group of L’Hoest’s monkeys living on the forest edge and one group in the interior that my four field assistants and I have been habituating for four months, we are learning a lot of very interesting things. The edge group includes in its diet green beans, gum from black wattle trees, papaya fruits, and products from other exotic trees introduced in and near the forest edge zone. Only adults venture out of the forest in search of additional (but risky) food sources, while juveniles play “the sentinel” from the forest canopy. At any alarm, noise, or movement, the “raiders” retreat quickly back into the forest canopy to escape threats from farm-guards, baboons, and feral dogs.

In contrast, the interior group usually avoids the forest canopy, where they are exposed to threats from the crowned hawk eagle, an interior forest raptor that preys on monkeys. The interior group seems to be especially careful in choosing sleeping sites. One place near the hilltop where the interior group has been sleeping for the most recent two weeks of my study must be one of the safest sleeping spots. When disturbed, individuals drop down and remain motionless, concealed under terrestrial vegetation cover. Ground predators such as leopards became extinct in BINP more than ten years ago. Compared with the edge group, the interior group forages more in association with other species—such as blue monkeys, black-and-white colobus monkeys, and even red duikers (a kind of forest antelope)—so that there are more eyes and ears on the alert for raptors.

Two ways to survive

At this point, we expect that our edge and interior groups of L’Hoest’s monkeys could be behaving differently in BINP. We hope that more data from this on-going study will help us to estimate the extent of these differences. Two different strategies for surviving L’Hoest’s monkeys are feasible:
1) “Edge-thriving” populations could adapt to the effects of environmental degradation to the extent that they may lose their original identity and become “pest” (or “crop-raiding”) species, subject to translocation or eradication.
2) “Edge-resilient” populations could survive on forest edges and exploit the interface between forest and non-forest (or non-crop) habitats. Resilient populations could serve as a model for conservation management and provide easily accessible subjects for ecotourism, especially in Bwindi, a really rugged and impenetrable montane forest.

To be able to help protect the future of these wonderful monkeys, more accurate information is needed. This is why studies such as ours are timely and important and deserve more scientific and financial support.

This study has been supported by the following organizations: Rufford and Margot Marsh Foundations, Primate Conservation Inc., Wildlife Conservation Society, Royal Zoological Society of Scotland, Center for Tropical Ecology and Conservation small grants, and the Institute of Tropical Forest Conservation.