The Drill

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Still image taken from video footage of a large dominant male dril (J.Jay Bioko 2011)

The Bioko Island drill Mandrillus leucophaeus poensis

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 including crabs, sea turtle eggs and even 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 45-65 pounds with the smaller females weighing about 20 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.


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Male Bioko drill (J.Jay Bioko 2012)

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 planned to extend throughout the Gran Caldera and Southern Highlands Reserve (GCSH), which began in 2009 with the Belebu-Ureca road, will increase access to what have 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.

Little Studied Species At Risk

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The Illegal Bushmeat Trade, Joel Sartore 2007

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.

Time is running out

While studying the drills in 2010-11, The Drill Project was able to follow several family groups, including one led by this very large and dominant male (pictured at top of page).   We hope to see more of this incredible animal and his bustling family, but unfortunately their time is running out.  Roads are already in the construction phase to be built right into the heart of their range, opening it up to more commercial hunters.  To help save these endangered animals, The Drill Project needs your support, to fund this much needed Documentary.  Please visit our donations page for more information.  Any amount donated will move us that much closer to reuniting with these incredible animals, to tell their amazing story.  Thank you for your support.

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.


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