No scientists have been successful in a long-term study of a drill group in the wild - this means that very little information exists on wild behavior.  Only two scientists; Steve Gartlan (a study in the 1970s) and more recently, Christos Astara in Korup National Park have managed to follow wild drills for short periods of time through the forest and have published what information they learned (see the 'further reading' section below).  A number of surveys in drill range have documented the status of drills in parts of the forest (see 'further reading' below).  At the Pandrillus Drill Rehabilitation and Breeding Center in Nigeria, where drills live in enclosed natural habitat forests, a large volume of information about life-history, behavior and reproduction in the large social groups of drills found there was the subject of Dr. K.L. Wood's (of Tengwood.org's) dissertation, and is available for purchase here.

As they move toward adulthood, young males no longer spend as much time with their mothers and sisters.  Typcially, they become a target of the dominant male's aggression as subadults (depending on how much they are following their hormonal urges!) and begin to become more peripheral in the group.  Solitary males have been observed in the wild, so it is believed that a male may leave a group to find opportunities elsewhere.  A male may also stay in the group and take his chances at besting the reigning dominant male.

In a drill group, adult males fight for dominance and control of breeding opportunities.  The dominant male performs a behavior called 'mate gaurding'; this means that he monopolizes matings with receptive females and is aggressive toward other males who approach these females.  Being dominant is an exhaustive position, to say the least - a group of females comes into estrus during the same season and the dominant male must not only satisfy all these females, but also keep a close eye on the efforts of subordinate males to sneak off into the forest with willing females!  Because of this, a dominant male's tenure is believed to be about 3 years.  After this, another male usually bests him in a contest of wills.  Sometimes it is a peaceful takeover - obtained by means of 'threat displays'.  A drill 'threat yawn' involves the male showing his teeth to another male - it may look like a yawn (and maybe it is - no one can yet fully explain the causes of yawning) or it may be a threat display - the sight of the 2 inch long canines and the musculature associated with the jaw gape of an adult male drill show another male just what he will be facing in a fight!  Often, this assessment of one another's weaponry may be enough to actually prevent a fight. 

Actual fights are rare, but do happen.  When posturing, males will sit facing one another and yawn and gnash their teeth.  In actual aggressive contact, a male will hold out a hand and try to grab the other - if they make contact, they will grasp one another's arms and grapple, with the winner likely to be the one who gets in the best bite.  However, a male can't forget about the group - a successful dominant male is backed by the larger social group, especially the females, who watch the contest closely.  The dominant females may even influence the outcome by standing between the two during a fight, or supporting one over the other.  Juveniles in the group don't get involved, but seem to find male contests highly entertaining stuff.  They will sit nearby and follow the specktacle as if they are in a theater, watching a movie!

Primates use aggression to reinforce the existing hierarchy, but they also use more subtle tactics.  Here is how it works - a tasty food source is discovered by a lower ranking animal. A more dominant animal will approach the lower-ranking animal - and the lower-ranking animal will walk away.  This behavior is called 'displacement' and it is the dominant animal's non-violent way of letting another know that they are in charge.  Sometimes a high ranking animal will displace another for no more reason than to sit where he/she is sitting.  These little everyday reminders reinforce what appears to be a stable hierarchy in the group, where each drill knows its place.

Drills have a rigid social hierarchy.  Babies grow up under the protection of their mothers, sisters, and brothers.  In an altercation, a female will protect her offspring.  Matrilines (a family of related females) support eachother against unrelated matrilines.  A matriline has a distinct ranking within the larger social group.  (i.e. one matriline will rank over another matriline, which is stronger than everyone but the first, and so on...).  There is also a rank among the females within a matriline (i.e. first, second, third, etc.) - a mother will usually support a younger daughter over an older one (which makes sense, as the younger is usually smaller and needs to be protected more).

The female ranking system is thought to remain stable over time, but some things can impact this hierarchical order - such as which female the dominant male likes best!  When a female is in estrus, she exhibits a genital swelling that the male is attracted to - a high ranking female often directs aggression at a low-ranking female that has become 'attractive' to the male.  Dominant males will often break up a fight between females by supporting one female over another, depending on his preferences.  In altercations between females, it is important which female is stronger than the other, but always, the outcome of an altercation depends on who else in the group supports you!  

In the past, drills and mandrills were called 'forest baboons' and classified with savanna baboons in the genus Papio.  In the 1980s, they were recognized as being distinct from the baboons by the International Commission on Zoological Nomenclature (an organization responsible for the classification and naming of the animal kingdom) and were placed in their own unique genus - Mandrillus.  Studies that led to this reclassification looked at both morphology and molecular DNA. The drill and mandrill are thought to be a branch that diverged from the baboons  far back in evolutionary time.

The closest relative of the Mandrillus genus is another little known West African primate - the mangabey (genus Cercocebus).  Mangabeys are also forest dwelling, but are smaller - somewhere between the size of the long-tailed guenon monkeys and Mandrillus.  Mangabeys have long tails and are more arboreal than the two Mandrillus species, but have similar tooth and forearm morphology and are likely to have similar diets (i.e. they are both specialists in cracking seeds).

The drill is part of a unique genus, Mandrillus, which includes only one other primate, the most colorful mammal known to man - the mandrill. 

If mandrills are the most colorful mammal, then drills are the second-most colorful!  Both drills and mandrills have the same brilliantly hued backside - a feature which is unique in the primate world.  The genetalia of Mandrillus species is also brightly colored, especially in the dominant male.  As for the differences between these two species, the well-known mandrill's face is red and blue, whereas the drill's is jet black and shiny, without the longitudinal grooves mandrills have in the skin covering the bony ridges on either side of the nose (see the picture gallery below).  The dominant male drill has a bright red patch on his chin and pronounced cheek flanges, which are absent in the mandrill and the mandrill sports a yellow beard and ruff while the drill's is short and white.  Female and juvenile mandrills are less colorful than the male. The color of the pelage of these two species also differs - in the drill's latin name, the word 'leucophaeus' means 'dusky' and probably refers to the darker brownish hair of the drill, while a mandrill's body hair is usually described as 'olive'.

The drill monkey is one of the least understood and little-known primates in Africa.  Found only in Nigeria and Northwestern Cameroon, from the Cross to the Sanaga River, they inhabit an area roughly the size of Switzerland with possibly fewer than 3000 left in the wild. They prefer living in dense forest and will flee from humans. Drills are also not common in zoos, so most people have never seen a drill, or even a picture of one! This species has been called a forest baboon, but is part of a unique genus, Mandrillus, which includes only one other primate, the colorful mandrill.  Drills are also very colorful, with the same brilliantly hued backside as the mandrill and a bright red chin patch on  dominant males. They are not well-protected in any of their remaining habitat, and are hunted for bushmeat, while their forest home continues to be logged and farmed.  The drill is one of the largest monkeys and most endangered primate species in the world.  Like the Nigerian-Cameroon chimp, drills will go extinct if conservation action is not taken soon!

Drills are not common in zoos, so most people have never seen a drill, or even a picture of one!  The average person will not know what a drill monkey is if asked.  In contrast, most people will recognize a mandrill.  Why are there so few drills in zoos?  It is possible that because drills and mandrills are so similar, that zoos historically chose to exhibit mandrills because of their more colorful faces.  It is also possible that early collecting expeditions occurred more often in forests below the Sanaga, where mandrills rather than drills occur.  The drill's range and population size may also have always been smaller, meaning less ended up in captivity.  Drills don't reproduce well in zoos either.  In North America, there are only about 16 drills, found in 4 zoos.  In Europe, there are approximately 60 drills, in some 12 zoos.  German zoos have had the most success in breeding drills, but the zoo drill population is just as endangered as their counterparts in the wild!  If you live near a zoo that has drills, visit and tell them you think it is important for zoos to support field efforts (such as Tengwood.org) to save drills!

Mothers and daughters may remain together for a lifetime, or individuals may leave and join another group - no one really knows.  It seems that age mates (other drills born at the same time/season - known as a 'cohort') may be very important.  Drills give birth seasonally, which means that most babies are born in the same season - specifically, in the rainy season, when food is abundant.  Infant drills are weaned from their mothers at the tender age of 3-6 months; during this weaning period, young drills begin to leave their mother and join large play groups, made up of all the juveniles in the group.  Drill play groups are boisterous and fun to watch!  But there is a serious undercurrent to them - play is meant to teach young animals the skills necessary to survive in the adult world.  Hence, play fighting prepares a male for later in life, when the fighting is real. For drills, play also allows them to establish relationships with their peers that have a lasting effect.

Juvenile drills spend a lot of time playing.  As they grow bigger, they begin to join in the more serious business of adult interactions. For females, this happens at 2-3 years of age (when they experience their first estrus cycle).  Females have their first baby at around 3 years of age.  For a male, maturity takes much longer - they are usually 7 to 10 years of age before they become adult (an adult male drill is more than twice the size of a female!).  In the meantime, they continue to play and sometimes entertain themselves by causing mischief with the adults in the group.

Both drills and mandrills are wary of human observers and have remained elusive to study.  Very little is really known about their lives in the forest, so bits and pieces have been put together by the various scientists studying these primates.  Drills live in dense, primary forest and most of a drill group's day is likely spent traveling in search of food and they are known to have very large, seasonal home ranges (traveling up to 5 square kilometers a day).  In the wild, most primates, including drills, spend the majority of their time just finding enough to eat.  Overall, the forest is not an easy place to live - imagine searching the forest for food  with a large, hungry group of people who are searching in the same place.  It is a delicate balance between the safety of the social group and finding enough to eat.

The preferred food of the drill is forest fruits - when they can find them!  The majority of the diet will be fruit when it is in season, but the distribution of fruiting trees is patchy and drills must know where to go and when fruits are in season.  During the dry season, or times when fruit is scarce, drills must supplement their diet with other foods, such as young leaves and grasses, and the pith of tree branches.  Most of a drill's time is spent foraging in the leaf litter.  Here, drills find insects, which provide protein, and also seeds, which their hard molar teeth are thought to be specially designed to crack open.  In fact, drills, mandrills, and mangabeys are believed to be 'seed specialists'  (see Fleagle and McGraw's work on the specialized dental morphology of Mandrillus).  Seeds last for long periods of time on the forest floor and are also high in protein.  Primates that eat fruits and seeds, such as the drill and chimpanzee, play an important role in maintaining the forest via seed dispersal.

During travel, drills spend a large proportion of their time on the ground, resulting in reports that drills are 'terrestrial'.  In truth, drills are very good climbers.  Drills and mandrills have specialized forearm muscles and can cling vertically and leap long distances from tree to tree (see this video of mandrill locomotion in trees).  They also spend their nights very high in the trees - a drill sleeps most comfortably about 100 feet up and balancing on a precarious branch!  Drills sleep this way to avoid predators.  In the morning, the group climbs down and travels on their daily food foray.  The day also includes some rest stops, where the majority of socializing occurs.  Captive drill groups living in natural forest enclosures in Nigeria were observed to socialize mostly at the beginning (dawn) and end (dusk) of their day, usually around the sleeping tree.

Drill monkeys are one of the most endangered primate species in the world. The drill is found in a very small geographic area with a high human density (from the Cross River in Nigeria to the Sanaga River in Cameroon - see Tengwood.org's 'Rivers and Wildlife' article).  On this map, one can see that the drill has a much smaller range than the mandrill.  Drills are classified by the IUCN as endangered.  With their larger range and population size, in an area of Central Africa that is less populated by humans, mandrills are classified as threatened.

The drill monkey is one of the least well-known primates in Africa.  Found only from the Cross River in Nigeria to the Sanaga River in Cameroon, they inhabit an area roughly the size of Switzerland, with possibly fewer than 3,000 left in the wild.

No one has been able to observe wild drills well in their dense forest habitat, so our knowledge of them is limited.  Most of what is know comes from zoos and from a drill rehabilitation and breeding center in Nigeria known as Pandrillus.  This sanctuary rescues drills from the illegal trade in bushmeat and wildlife and places them in large social groups in natural, forest enclosures in the Afi Mountains of Nigeria.