Saturday 29 January 2022

Monkeys with Many Stomachs

Rhesus monkeys have entirely
normal stomachs
The word "monkey" entered the English language relatively late, in the 16th century. It's not at all clear where it came from, and it doesn't have a direct counterpart in most other European languages. Instead, they, like medieval English, used the same word for both monkeys and apes (and that word was, in our case, "ape"). From a modern, scientific, point of view, these other languages are correct, because there is no scientifically definable group of animals that fits with what most people mean by "monkey".

This is due to the rule, often mentioned on this blog, that the members of any scientifically defined group of animals have to be more closely related to all the other members of that group than they are to anything else. The diagram further down this post showing how all the major groups of simian are related illustrates why this is. At the top, we have the macaques and the leaf monkeys, both of which are indisputably monkeys. But their closest relatives are the apes (which, of course, includes gibbons as well as "great apes"). Only then do we get to the other monkeys, which means that, for example, a macaque is more closely related to a gorilla than it is to a marmoset.

Either marmosets aren't monkeys, which is daft, or monkeys aren't a real group of animals. Or apes are a special kind of monkey, which does actually work, and fits with some colloquial usage, even if it's not what the dictionary will tell you. 

But hang on a second. All those monkeys at the bottom of the chart live in the Americas, while the two groups at the top do not. Moreover, those two at the top are more closely related to each other than they are to the apes, which means that they really are a natural group of animals, descended from some original, non-ape ancestor. That group is referred to as the Old World monkeys, or, more technically, the Cercopithecidae (literally, "tailed apes") and they are traditionally placed together in one family.

The Cercopithecidae were named by English zoologist John Edward Gray in 1821 when he distinguished them from what he considered to be two families of New World monkey. (Neither of the latter two families are still used today in his original sense, although he did name what is now the spider monkey family three years later). He had not, of course, failed to notice that this new group lived on different continents to the others, but that wasn't the basis on which he distinguished them. Because the two groups are anatomically distinct.

Some of the distinctions he noted, such as the presence of cheek pouches, are no longer considered reliable indicators. But others are, and the one that's most commonly cited today is that New World monkeys have nostrils that face to the side, while the Old World sort, like apes, have nostrils that face either forwards or downwards, depending on the shape and size of the nose.

In general, we can say that Old World monkeys are medium to large monkeys with elongated trunks (in comparison to apes), narrow nostrils, and ischial callosities. The last of those features refers to thick pads of tissue on the hips and buttocks, often covered with skin that is much less hairy than that on most of the rest of the body. They allow the monkey to sit comfortably on a comparatively narrow branch, and while all Old World monkeys have them, they are most pronounced on the species that like to sit in this fashion while feeding from other, nearby, branches. The oldest evidence for them dates back 15 million years to the Miocene, well after the split with the New World monkeys, which lack them.

Old World monkeys have the same number and arrangement of teeth as humans do, whereas all the New World species have an extra set of premolars giving them 36, rather than 32, teeth in total. The canine teeth are large and pointed, with the upper canines of males often being particularly impressive. The first premolar tooth in the lower jaw is also specialised, having a narrow cutting edge for slicing through tough material, while the molar teeth also have a double row of sharp ridges that are not found in apes. 

While the New World monkeys are spread among a number of different families, all non-ape monkeys native to Africa and Asia belong to the Cercopithecidae. That's a lot of species; the exact number may be open to debate, and new ones are still being discovered, but the current version of the Mammal Diversity Database lists 160 that are widely accepted as genuine. We can, however, as the evolutionary tree I've produced shows, divide them into two subfamilies.

One of these, following the standard rules for naming subfamilies, are the cercopithecine monkeys. I've listed them as "macaques, etc." but that "etc." is quite a large one including, among others, the guenons and the baboons. These are the monkeys that really do possess cheek pouches, as Grey mistakenly thought all Old World monkeys do, and they are omnivores that include a large amount of fruit in their diet. These enable them to stuff as much food into their mouths as they can as quickly as possible, before retreating to some safer spot and taking the time to chew and swallow it. Supporting this, they are most commonly used when in the company of other monkeys trying to eat the same food resource

The second subfamily are the colobine monkeys. These are also known as "leaf monkeys" because, rather than fruit, they mainly eat leaves. Compared with nutritious, sugary, fruit, leaves are not a great food resource. Hoofed mammals require a number of sophisticated digestive adaptations in order to extract enough nutrition from them to prosper. There's a reason that most monkeys prefer to eat fruit.

The howler monkeys of South America, which also have a heavily leaf-based diet, deal with this in what's probably the simpler way. Like horses, rhinos, and a number of other large herbivorous animals, they have a greatly enlarged caecum, part of the hindgut, which can break down and ferment food already partially digested by the stomach. This is clearly an effective mechanism, but there's a more efficient alternative.

That's the solution taken by ruminants, such as cows, sheep, and so on: have a multi-chambered stomach. Colobine monkeys haven't quite gone the whole distance in this direction, since they don't ruminate, or "chew the cud". (Although at least one species does come pretty close, as we'll see in a later post). But they do have a complex stomach.

In most species, this has three chambers, but some have a full set of four. The first chamber that's universally present is the saccus, which, like the rumen of a cow, is a large, expandable, chamber that holds food while it is fermented by microbes. Those monkeys that have an extra chamber have it in front of the saccus; called the praesaccus, it seems similar and it's not really clear what advantage having it gives them. Behind this is the tubus, which produces the gastric acid that further breaks down the food. Finally comes the pars pylorica, which produces mucus that helps to neutralise the acid and stop it burning holes in the intestines. It may only be this last chamber that's the "real" stomach, although the previous two (but not the praesaccus) do also have some glands we'd only expect to see in a stomach.

With so many species, it should come as no surprise that Old World monkeys have a wide range of different behaviours, social structures, breeding habits, and so on. They typically live in tropical and subtropical wooded habitats across Africa and southern Asia, but even then, the types of woodland they inhabit can vary significantly between species. There are far too many species for me to describe all of them in my usual annual series of monthly posts, but by grouping some of them together, I should be able to cover one of the two subfamilies.

So, through this year, I will take a tour of the colobine or leaf-monkeys a group that, as we will see, contains at least three significant sub-groups of its own. The most obvious of those are the ones from which the wider subfamily takes its name: the colobus monkeys. So that's where I'll start next month.

[Photo by Timothy Gonsalves, from Wikimedia Commons. Cladogram adapted from Springer et al. 2012.]


  1. Apes are monkeys, birds are reptiles, and cows are fish! Adopt proper phylogenetic nomenclature now!

    Well, "fish" may be better redefined as meaning only actinopterygians*, and "reptile" is perhaps best abandoned as a category**, but admitting apes as monkeys should be fairly simple, it seems to me, even if it makes English majors skin crawl.

    * This may be sort of happening already, I've seen expressions like "fish and sharks" in popular books.

    ** I was once told one can't abolish "reptile" because regular people won't ever be talking about "archosaurs" or "squamates". But somehow, not very long ago, regular people got by without any of those three classicisms.

    1. To any Terry Pratchett reader, the risks of describing apes as monkeys need no elaboration....