The Bioko Island Drill

Mandrillus leucophaeus Poensis

Found primarily in the West/Central African Rain Forests, the drill monkey is an IUCN endangered species throughout its range, both on the mainland (Mandrillus leucophaeus leucophaeus) and on Bioko Island (Mandrillus leucophaeus poensis). The continually declining Bioko Island drill population is currently being hunted for the commercial bushmeat trade at an alarming rate; in the past 13 years over 4,500 drill carcasses have been sold in the Island's illegal bushmeat market located in the capital city of Malabo. Based on recent surveys the estimated area of drill range has decreased from approximately 525 km2 in 1986, to 220 km2 today (Butynski and Koster, 1994; Cronin et al. 2010). This is most likely a result of the intense hunting pressure and high rate of capture of drills for the commercial bushmeat trade. The development of new roads now extending throughout the Gran Caldera and Southern Highlands Reserve (GCSH), have increased access to what has historically been the least accessible areas of Bioko Island. Although local wildlife patrols run by the Bioko Biodiversity Protection Program (BBPP) have been successful as a passive deterrence to hunting activities in the GCSH in the past, the high price paid for bushmeat on Bioko Island will likely drive illegal hunters into these areas and further jeopardize Bioko's remaining drills.

Diet: Drills are omnivores and forage primarily on the ground. Their diet primarily consists of fruit (frugivorous) but may also rely heavily on plants and invertebrates, depending on the habitat and season.  As opportunistic foragers,  drills have been observed eating a variety of food sources even including crabs, sea turtle eggs and domesticated crops.

Habitat: Drills can be found in lowland, submontane rainforest up to 1,000 meters. On Bioko, they can be found in mature old growth and secondary forests.

Range: In addition to the southern parts of Bioko Island, the drill's are known to live in western and south-western Cameroon and south-eastern Nigeria, between the Cross River and Sanaga River.

Size: Average males can weigh between 45-75 pounds with the smaller females weighing about 20 pounds. Drills in captivity have been recorded up to a hundred pounds.

Groups: Wild drill monkeys live in multi-male, multi-female groups which average approximately 20 individuals and are typically lead by a single dominant male. However, the number of individuals within a group can vary dramatically, from less than five to recorded aggregations of well over a hundred drills. Drill social organization and group size dynamics remain poorly understood and highly debated, particularly in respect to the impact of ecological and environmental factors on these aspects of their natural history.

In biology, the range or distribution of a species is the geographical area within which that species can be found. Throughout the drill's range, they remain one of the least studied monkeys in Africa. Drills have been the subject of far fewer field studies than their only congeneric, the mandrill. Additionally, all focal studies completed on the drill have been performed on the mainland subspecies, not the Bioko drill - the focus of The Drill Project. Information on the Bioko Island drill is found embedded in more general discussions of Bioko Island primates, particularly within reports on the local bushmeat trade.  With the exception of the Drill Rehabilitation and Breeding Center in Nigeria, drills have bred poorly in captivity but have provided useful behavioral information.

Butynski, T. M. and Koster, S. K., (1994) Distribution and conservation status of primates in Bioko Island, Equatorial Guinea. Biodiversity and Conservation, 3, 893-909.

Cronin, D. T., D. Bocuma Meñe, T. B. Butynski, J. M. E. Echube, G. W. Hearn, S. Honarvar, J. R. Owens, and C. P. Bohome. 2010. Opportunities Lost: The Rapidly Deteriorating Conservation Status of the Monkeys on Bioko Island, Equatorial Guinea (2010). A Report to the Government of Equatorial Guinea by the Bioko Biodiversity Protection Program, Drexel University, Philadelphia, Pennsylvania.

Beginning in 1963, every plant and animal species on our planet has been continually evaluated (every 5-10 years) and listed by risk of extinction, by the world's authority on the conservation status of species: The IUCN, or International Union for Conservation of Nature. The list is called The IUCN Red List of Threatened Species.