18th century illustration by Charles Reuben Ryley.
source.
Everyone knows monkeys but I think most people think about cute white-faced capuchins or macaques or the langurs of India. Monkey have a lot of diversity and live in many different habitats.
The largest monkeys are especially interesting and consist of the mandrill and the drill. People may assume mandrills are baboons and they do look pretty similar (both are monkey species), but mandrills are their own group. I think most people have probably never even heard of drills! Drills are also large monkeys closely related to mandrills.
Because I got carried away, this post will only address mandrills, and I'll do a second post about drills.
Mandrill biology:
Mandrills (Mandrillus sphinx) have a very restricted distribution and are native to west and central Africa, mostly around Gabon. Unfortunately Gabon has historically been under civil war and guerilla warfare, so its notoriously difficult to study this species. Mandrills are probably most familiar to people as Rafiki from the Lion King (although Rafiki has a long tail for some reason; mandrills have very short tails). They are unmistakeable because of that crazy red and blue face coloration. They also have colorful bums!
What exactly mandrills are has been a historical question. When described by a european for the first time, it was considered a type of hyena (wow close but no cigar). It was initially placed in the same group as baboons, which is understandable as they are pretty similar. It wasn't until modern genetics got at it that the mandrill was realized to be more closely related to mangabeys, an African monkey. In 2011 mandrills were considered in their own group, in the genus Mandrillus.
Mandrills are mostly in tropical rainforests, making it difficult to do observational studies of them due to the high cover. They aren't huge climbers and spend a lot of time on the ground, eating fruit and seeds, and a little bit of everything else. Like baboons they live in pretty big social groups called hordes, which can reach up to hundreds of individuals. They are active in the day and sleep in trees at night.
They are listed as Vulnerable on the IUCN's red list of endangered species, but are currently protected at Lope National Park in Gabon, which is a UNESCO World Heritage Site.
Mandrill males and females look different, with males being much larger, having much brighter colors on the face, and the snout and canine teeth are more developed. Both males and females have blue and red on the face, interestingly. Differences are even reflected in the skulls of the sexes, where males have distinctive bone ridges on the snout (where the blue would be). In males the rump and butt area are a swirl of multiple colors.
I described this in a previous post, but the red coloration results from blood flow under the skin, and the blue is something really unique resulting from structural coloration.
Like all monkeys, their social behavior is one of the most interesting things about their biology. Mandrills surprisingly to me live in really large social groups of hundreds of individuals, not unlike what baboons do. At a national park in Gabon, there was an average of about 600 individuals (!!!) per group and they can cover a home range of 70 square miles (or 182 square km), which feels insane.
Mandrills are matrilineal, meaning a mother's lineage determines your social status in the group, which is not uncommon for monkeys. Dominant females are the most important core unit of the group, and any offspring they have will also be dominant. The groups are structured around related females and their offspring.
Males aren't always part of the big groups, it's only when receptive females are available that they join to mate, and then the males leave. Males wander around a lot and lead partly-solitary lives.
Males can get pretty big and can weigh up to 100 lbs (54 kg), which is pretty huge for a monkey.
Why the crazy colors?
It's all about the mating game. Dominant males have brighter facial coloration than subordinate males, as male coloration has been linked directly to testosterone levels. This may be a signal to females that a male has high social status. Like many monkeys, mandrills social status is fluid throughout their lifetime, where younger males start as subordinate, then move up in the ranks as they get a little older. Dominant males can be replaced by competitors also - it's kind of stressful to be a male because you are always jockeying for your position, and if you don't die via leopard, you probably eventually get forced out by a stronger male once you get old.
What's interesting is that these visual signals - the colors - are true measures of testosterone and dominance, meaning that they can change over time. They aren't born having brighter or duller colors. When a male gains alpha rank, their circulating testosterone increases, subsequently increasing the red color in the face. When they lose their position and decrease in rank, they have a loss in the redness. It doesn't seem to affect the blue color. Females also prefer redder males.
Males weigh more than 3x the females and have one of the strongest sexual dimorphisms of any primate. Males also have specialized vocalizations and scent markings. All mandrills have a scent gland on the chest, and males rub it against trees.
Basically, males invest a lot of time and energy in these displays. Visual displays in the size and insane rainbow colors, audible displays in grunting, and odor displays in scent marking. In terms of visual displays especially, no other primate (not even many other mammals) have so stark a sex difference. but its for a good reason.
Males have to establish a dominancy hierarchy among themselves, since they want to monopolize access to females. In other animals, this is done by fighting - the male that wins gets access. But the thing is that fighting is dangerous and can lead to costly injuries or even death.
All these displays make it very clear what each male has going for it. the largest, most brightly colored male is going to be the dominant one, and this can be assessed at a glance! No hand to hand combat required. Mandrills might be avoiding male-male fights by having these displays.
Another component of male's rainbowness is that they act like walking billboards. When you live in a really big group of hundreds of individuals, and especially if the males are transient and are only around some of the time, it's hard to know who's who in the hoard. Their coloration is a pretty clear indicator of male dominance at a glance - you don't have to interact with that male over the long term to see what his deal is. And since the colors can change slightly with testosterone, its a very much "up to date" indicator. This is true both for males who are looking to sort out their pecking order and for females who want to identify suitable mates.
Mandrill females also have some blue and red coloration, even though its less extreme. Female color can change across the menstrual cycle and may indicate when they are most fertile. It also seems to increase with age. So they are also advertising to other individuals what their status is very clearly. Slightly older females are likely to be more successful in reproduction because they have reproduced and cared for an offspring before, so they are not unexperienced, and they tend to have larger offspring. Very young females may have varying fertility. These are a few reasons why females are colorful, but overall it's a little unclear what the role of female coloration is.
The babies look very different without any face coloration and they don't really develop the longish 'snout' until later. They just look like generic monkeys to me.
There is a mandrill-themed rollercoaster in an amusement park in Chessington, UK, at the Chessington World of Adventures. A series of mandrills are in the center of the coaster carts with a series of carts being held on either arm. Apparently this is based on the most recent iteration of the Jumanji film - the one with The Rock.
So I went looking into how mandrills feature in the movie. The group gets chased by literally hundreds (a whole horde?) of them across a set of bridges. Interestingly the mandrills are almost all males and have long tails (?). They're also like gorilla-sized for some reason, to up the scary factor I suppose.
The nasal ridges are very prominent on males, as are the long canines, making for a really gnarly skull overall. You can rotate this model in your browser.
This wildlife camera video documents literally hundreds of mandrills passing by the camera. It's wild that they live in such big groups. You can see several females carrying young. For large mammals, we are used to thinking of them as solitary, and that seeing one is special because they are rare - like for tigers, bears, etc. It's so weird to see so many large mammals congregating together.
A mandrill at the Phoenix Zoo in Arizona is the oldest living individual at 37. She is a female named Nikki and was born in 1987! The median life expectancy is about 18 for mandrills.
A community of mandrills living in the Colchester England Zoo have developed a specialized gesture unique to their group. It's essentially a 'facepalm' where they cover the face with one hand, as if they are shading their eyes. It's a "do not disturb" signal they use when they want to be left alone. Other mandrills understand it and accordingly leave the individual alone! One individual, Milly, invented the gesture and used it alone for years before it caught on and spread.
Here is a video of one doing the facepalm.
Henri Rousseau painted a male mandrill in 'Mandrill in the Jungle', 1909. It looks like my terrier when he's begging for food.
Franz Marc painted one also, titled 'Mandrill', 1913, as part of the cubist movement - so it's barely recognizable as a mandrill, of course. It has some interesting abstractions and is bursting in color. You can clearly see the mandrill's ridged snout in there. I think cubism works pretty well for mandrills, actually.
A contemporary artist Dieter Braun has this retro-style piece of a front-facing male mandrill that is really striking. I like this one a lot.
Spark Mandrill is a robot-like boss in the video game Mega Man X, and has electricity powers. The vibrant face colors are preserved in the design.
Where are all the mandrills??
Mandrills are rare animals but I'm surprised they are not featured more in culture, art, and media, because these seem like pretty meagre pickings for a literal rainbow-colored giant monkey with fangs. You would think mandrills would be all over advertising or movies or videogames. They feature in zoos a lot, which is understandable, they're so showy. Why so few appearances in media?
Gorillas are rare also - maybe less constrained in their habitat range, but you see gorillas all over the place. Why not mandrills? Gorilllas in real life are relatively gentle and calm, but are portrayed in some media as dangerous. Mandrills would technically be a better fit for horror themes. They're even kind of clown-like! Where's my clown mandrill horror! Maybe books and films like Gorillas in the Mist really publicized gorillas as endangered, and that's why they are everywhere.
If you are a creator of any kind, please integrate a mandrill into your work. They need more representation and more people need to know about them. Maybe the increased awareness will also help with conservation funding.
If you would like to help mandrills, you can donate to the Wildlife Conservation Society which does some research in Gabon with mandrills.
The Mandrillus Project also takes donations and conducts research in Gabon.
lucky instar 
