Mammals
Mandrill
Mandrillus sphinx · also called mandrill
The mandrill is the largest monkey on Earth and, by a wide margin, the most colourful mammal — a dominant male’s face is a mask of scarlet and electric blue, his rump the same, and the brighter he is, the higher he ranks. Its forests hold the biggest social groups of any primate.
Overview
Mandrills live in dense lowland and montane rainforest in a small region of west-central Africa — southern Cameroon, Equatorial Guinea, Gabon, and the Congo. They forage mostly on the ground, turning over leaf litter for fruit, seeds, fungi, roots, insects, and small animals, and climb trees to feed and to sleep.
The largest monkey
A male mandrill weighs 30 kilograms or more; a female only 10–15. That two-to-threefold size gap is among the most extreme in any primate. A male’s canines can reach 4.5 centimetres — the wide “yawn” that displays them is a threat, not aggression toward troopmates.
Colour as rank
A dominant male grows the brightest red nose stripe, blue cheek ridges, yellow beard, and red-and-blue rump, all tied to his testosterone and rank. If he loses status, the colours fade within weeks. Females and rivals read his fighting ability and fertility from his face at a distance, cutting down on fights, and females prefer the brightest males — Darwin’s textbook example of sexual selection.
Huge hordes
The core unit is a “horde” of females and young that can exceed 800 — with reports over 1,300 — the largest stable groups of any primate. Males are mostly peripheral, joining to breed.
Status
The mandrill is Vulnerable and declining, chiefly from bushmeat hunting, on top of a naturally small range and habitat loss. Its sister species, the black-faced drill, is even more endangered.
Mandrill: common questions
Is the mandrill the biggest monkey?
Yes. A large male mandrill weighs 30 kilograms or more, with exceptional individuals recorded over 50, which makes it the heaviest of all monkeys — heavier than a baboon, and heavier than any other monkey species. (The apes — gorillas, chimpanzees, orangutans — are larger still, but they are not monkeys.) Female mandrills are far smaller, only 10 to 15 kilograms, so the species also has one of the most extreme differences in size between the sexes of any primate, roughly two to three times.
Why is the mandrill so brightly coloured?
The colour is a signal of male dominance and reproductive status. A mandrill male at the top of the hierarchy develops the most vivid red nose stripe, the bluest cheek ridges, and the brightest red-and-blue rump, and these colours are directly linked to his testosterone level and his rank. When a male loses status — is defeated by a rival or drops in the hierarchy — his colours visibly dull within weeks. Females and low-ranking males can read a male's fighting ability and fertility from his face and hindquarters without any contact, which reduces the number of actual fights, and females prefer to mate with the brightest males. Charles Darwin cited the mandrill as an example of sexual selection producing extreme, costly ornament.
Do mandrills really form troops of over a thousand?
They form the largest groups of any primate. The core social unit is a large "horde" of adult females and their offspring, and long-term studies at Lopé in Gabon have counted stable hordes of over 800 animals, with reports of aggregations exceeding 1,300. Adult males are mostly solitary or peripheral for much of the year and join the horde during the breeding season, competing for access to females. The reasons mandrills can maintain such huge groups are debated but probably involve the patchy abundance of fruit in their forests and safety in numbers against leopards.
How is a mandrill different from a baboon?
Mandrills were long classified as baboons and are close relatives, but they are now placed in their own genus, Mandrillus, along with the drill. Compared with a baboon, a mandrill is more heavily built, has a shorter tail (a stub only a few centimetres long), lives in dense rainforest rather than savanna, spends more time actually in the forest interior, and of course has the vivid facial and rump colouration that no baboon has. Behaviourally, mandrills form those enormous hordes, whereas baboons live in troops of a few dozen. The drill, the mandrill's sister species, is similar in shape and size but has a black face and is even more endangered.