Drill (Mandrillus leucophaeus)

MORPHOLOGY: The average body mass for an adult male drill is around 20 kilograms and for adult females it is 12.5 kilograms (Hill, 1970; Fleagle, 1999). The drill has a dental formula of 2:1:2:3 on both the upper and lower jaws (Ankel-Simons, 2000). The drill, like all cercopithecines, has cheek pouches for storing food while foraging. The dorsal pelage color is olive-green, with the face and ears being black. The ventral side is colored white or gray-white (Groves, 2001). The anterior region of the scrotum of the male is colored pink and the posterior region is a lilac color. The region around the anus is colored red. The bright color of the skin will become more pronounced when an individual is excited (Nowak, 1999). The glans penis of the male is colored pink. Infants are born with a distinctive light colored face that darkens to black at 8 months (Boer, 1987). This is a sexually dimorphic species. There are swells on either side of the nasal area, pronounced maxillary ridges. This species has two subspecies each having slightly differing pelage colorations: Mandrillus leucophaeus leucophaeus: The hairs on the sides of the crown on this subspecies are ringed yellow and black (Groves, 2001). The legs of this subspecies are less buffy than the arms (Groves, 2001). Mandrillus leucophaeus poensis: The hairs on the body are more grayish toned and the hairs on the side of the crown are yellow-brown with a black tip (Groves, 2001). The legs of this subspecies are more buffy than the arms (Groves, 2001).

RANGE:
The drill is found in the countries of Cameroon and Nigeria and on the island of Bioko, Equatorial Guinea. This species occurs from the Cross River in Nigeria south-eastwards to the Sanaga River in Cameroon (Lee et al., 1988). This species lives in mature primary forests that are lowland, riverine, or coastal (Lee et al., 1988). The drill is infrequently found in secondary forests (Lee et al., 1988). On Bioko Island, the drill occurs most often in lowland areas and is found in altitudes from sea level to 1200 meters (Butynski and Koster, 1994).

This species has two subspecies each having different ranges:

• Mandrillus leucophaeus leucophaeus: This subspecies is found from Southeastern Nigeria to north western Cameroon (Groves, 2001). This subspecies occurs between the Cross River and Sanaga River, and slightly south of the lower Sanaga River at Ongue creek (Groves, 2001).
• Mandrillus leucophaeus poensis: This subspecies occurs on Bioko Island, Equatorial Guinea (Groves, 2001).

ECOLOGY:
The drill is a frugivorous species, but it will also eat insects, leaves, roots ground plants, and cultivated crops. On the island of Bioko, the drill was found to eat snails, especially the African giant land snail, Achatina spp. (Butynski and Koster, 1994). Marine turtle eggs are also foraged upon by this species (Butynski and Koster, 1994). There are records of this species husking and breaking open coconuts, and hunters have reported that crustaceans found in forest streams are consumed (Butynski and Koster, 1994). This species mainly forages on the forest floor where they search the leaf litter for fallen food items not consumed by more arboreal sympatric primate species (Estes, 1991; Fleagle and McGraw, 1999). The group of the drill ranges from 14 to 200 individuals (Lee et al., 1988; Gartlan, 1970). This is a diurnal species. This is a semi-terrestrial species (Lee et al., 1988). Humans, Homo sapiens, are a prime predator of this species (Rowe, 1996).

LOCOMOTION:
The drill moves through the forest floor quadrupedally (Fleagle, 1988).

SOCIAL BEHAVIOR:
The drill has a unimale social system, with the leader male receiving most of the copulations. These small groups come together with other groups to form troops of up to 200 individuals (Lee et al., 1988). Solitary males are known to occur (Gartlan, 1970). Males are thought to disperse from their natal group based upon evidence in captivity (Boer, 1987). Gartlan (1970) observed a high rate of group aggression in the wild for the drill. In captivity, during an introduction of a foreign infant into a cage of an adult male, that was not the father of the infant, the adult male displayed aggressively towards the infant, injuring the infant seriously (Boer and Sommer, 1992). The adult male did not however behave aggressively towards any of the infants that he fathered (Boer and Sommer, 1992). Thus adult males may only tolerate infants that have occurred during their residency, and infanticide may occur in the wild (Boer and Sommer, 1992). Young drills engage in play of which bouncing is the most common behavior during play (Hearn et al., 1988).

VOCAL COMMUNICATION:
crowing call: This begins with discrete units of 0.04-0.06 seconds, which are separated by intervals of 0.25 seconds (Gartlan, 1970). These intervals shorten and merge into a continuous call (Gartlan, 1970). The duration of the entire call may be for 1.8 seconds (Gartlan, 1970). This call sounds like "crowing". This call is heard when smaller unimale groups are coming together to form large troops.

two-phase grunt: This call consists of two deep grunts of 0.4 to 0.5 second duration separated by 0.5 to 0.6 second interval (Gartlan, 1970). The main energy of this call is from 0-400 hertz (Gartlan, 1970). This call is emitted by adult males (Gartlan, 1970). This call is associated with group movement and is heard during or before group progression (Gartlan, 1970).

two-phase huffing grunt: This call consists of a series of rapid grunts or pants lasting 0.08-0.20 seconds separated by an interval of 0.07-0.18 seconds (Gartlan, 1970). The main energy of this call is between 50-950 hertz (Gartlan, 1970). This call is emitted by adult, subadult, and juvenile males (Gartlan, 1970). This call is rarely associated with subadult chases (Gartlan, 1970).

squeal: This call is sometimes associated with two-phase grunt and two-phase huffing grunt (Gartlan, 1970). One situation in which this call is heard is during subadult chases (Gartlan, 1970). This is an aggressive call (Gartlan, 1970).

alarm bark: This call is emitted by all individuals of the group except for infants (Gartlan, 1970). This call consists of short staccato barks that occur singly or in pairs, and are given once or in brief intervals for periods of long time (Gartlan, 1970).

OLFACTORY COMMUNICATION:
sternal gland rub: This is when an individual will rub the sternal area against a substrate (Mellen et al., 1981; Hearn et al., 1988). The chin is usually raised when performing this behavior (Mellen et al., 1981).

VISUAL COMMUNICATION:
submissive crouch: This is a subordinate behavior (Hearn et al., 1988).

submissive present: This behavior serves to communicate that an individual is subordinate to another (Hearn et al., 1988).

look away: This is when one individual will look in the opposite direction of another individual. This display communicates submissiveness on the part of the performer (Hearn et al., 1988).

smile: This behavior is used when an individual makes a tense but friendly approach to another conspecific (Hearn et al., 1988).

tension yawning: This is done by an adult male drill (Estes, 1991). This is when the mouth is opened fully to reveal the canines (Estes, 1991). This is done when a rival group or a predator is approaching (Estes, 1991).

piloerection: This behavior serves to communicate tension (Hearn et al., 1988). This is when the hairs of the body are raised to a vertical position.

lip smack: This behavior is seen during social grooming and during copulation (Hearn et al., 1988).

present for sex: This is done by the female and serves to communicate that she is ready to be copulated with (Hearn et al., 1988). The females shows her anogenital region to the prospective male.

threat jerk: This display is a threat display. The head is jerked forward, while the eyelids are retracted, the medial crest is raised, and the lips are compressed forward. This is done sitting or standing. This display may be performed by adult males and females, subadult males, and juveniles (Gartlan, 1970).

branch-shaking: This is when an individual will jump on to a branch and start to rattle it up and down sharply (Gartlan, 1970). This is done several times with the entire body weight and the branch is held with both hands and sometimes the feet (Gartlan, 1970). This is a threat or intimidation display (Gartlan, 1970). This display is performed by adult, subadult, and juvenile males (Gartlan, 1970).

threat rush: This serves to communicate threat on the part of the performer, and is when one individual will rush towards another (Hearn et al., 1988).

herding: This is when an adult or subadult male will alter the placement of juvenile individuals and direct them back towards the group (Gartlan, 1970). Juveniles most often give squeals when this is done to them (Gartlan, 1970).

TACTILE COMMUNICATION:
social grooming: This is when one individual picks through the fur of another conspecific using the fingers, tongue, and/or lips (Mellen et al., 1981).

play grapple: This is a behavior seen during play (Hearn et al., 1988).

jump on animal: this is a behavior seen during play (Hearn et al., 1988).

play push: This is a behavior seen during play (Hearn et al., 1988).

tumble: This is a behavior seen during play (Hearn et al., 1988).

REPRODUCTION:
The drill gives birth to a single offspring. During estrus the perineum of the female swells up. Also during pregnancy the perineum turns red (Hadidan and Bernstein, 1979). In captivity it was found that females will reach sexual maturity at about 3 years of age and that pregnancy lasts from 179-182 days (Boer, 1987). Males in captivity were found to reach sexual maturity at 6 years of age and could remain sexually active into their 20's (Boer, 1987). In captivity it was also found that infants are weaned at 15-16 months and the interbirth interval is between 17-19 months (Boer, 1987).

REFERENCES:
Ankel-Simons, F. 2000. Primate Anatomy: An Introduction. Academic Press: San Diego.

Boer, M. 1987. Recent advances in drill research and conservation. Primate Conservation. Vol. 8, 55-57.

Boer, M. and Sommer, V. 1992. Evidence for sexually selected infanticide in captive Cercopithecus mitis, Cercocebus torquatus, and Mandrillus leucophaeus. Primates. Vol. 33(4), 557-563.

Burton, F. 1995. The Multimedia Guide to the Non-human Primates. Prentice-Hall Canada Inc.

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

Estes, R.D. 1991. The Behavior Guide to African Mammals. University of California Press.

Fleagle, J. G. 1988. Primate Adaptation and Evolution. Academic Press: New York.

Fleagle, J. G. 1999. Primate Adaptation and Evolution. Academic Press: San Diego.

Fleagle, J.G. and McGraw, W.S. 1999. The foraging adaptations of the Cercocebus-Mandrillus clade. (abstract) American Journal of Physical Anthropology. Suppl. 28, 128.

Gartlan, J.S. 1970. Preliminary notes on the ecology and behavior of the drill, Mandrillus leucophaeus Ritgen, 1824. in Old World Monkeys. eds. J.R. Napier and P.H. Napier. Academic Press: New York.

Groves, C.P. 2001. Primate Taxonomy. Smithsonian Institution Press: Washington, D.C.

Hadidan, J. and Bernstein, I.S. 1979. Female Reproductive Cycles and Birth Data from an Old World Monkey Colony. Primates, Vol. 20, 429-442.

Hearn, G.W., Weikel, E.C., and Schaaf, C.D. 1988. A preliminary ethogram and study of social behavior in captive drills, Mandrillus leucophaeus. Primate Report. Vol. 19, 11-17.

Hill, W.C.O. 1970. Primates: Comparative Anatomy, and Taxonomy, vol. 8: Cynopithecinae. Edinburgh University Press.

Lee, P.C., Thornback, J., and Bennett, E.L. 1988. Threatened Primates of Africa. The IUCN Red Data Book. IUCN Gland, Switzerland and Cambridge, U.K.

Mellen, J.D., Littlewood, A.P., Barrow, B.C., and Stevens, V.J. 1981. Individual and social behavior in a captive troop of mandrills (Mandrillus sphinx). Primates. Vol. 22(2), 206-220.

Nowak, R.M. 1999. Walker's Primates of the World. The Johns Hopkins University Press: Baltimore.

Rowe, N. 1996. The Pictorial Guide to the Living Primates. Pogonias Press: East Hampton, New York.

Last Updated: October 9, 2003.
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