The Beautiful Haired Monkeys

The Concise Oxford English Dictionary defines a monkey as "a small to medium-sized primate typically having a long tail and living in tropical countries." While perfectly good for everyday speech, that's not a viable scientific definition, for two reasons. For a start, you can't just say "typically" - you have to define exactly what you're talking about.

The other problem, though, is more subtle, and relates to how we define groups of animals. As I've mentioned many times before, a natural group of animals is one that includes a single common ancestor and all of its descendants. Apes are descended from monkeys, so, scientifically speaking, either apes are a special kind of monkey, or there is no such thing as a "monkey". You can take your pick, but scientists more commonly use the former option. A gorilla, therefore, is a kind of monkey, and it manifestly doesn't fit the definition given above.

A quick survey of eight online dictionaries, incidentally, shows that only one gives the more technically accurate definition, even as an option. This is, if anything, a little odd, since it's hardly unusual to hear people describe, say, chimpanzees, as "monkeys". You'll also hear a lot of people correcting them ("no, they're apes"), but strictly speaking, apes are monkeys, so a better quality of pedantry is clearly required.

Having said all that, I have to confess that, when I say "monkey", I usually do mean "a monkey that isn't an ape." And when I want to include both apes and (other) monkeys, the word "simian" does the job perfectly well. By this definition, there are six families of monkeys, plus two of apes. The largest family are the Old World monkeys, found across Africa and Asia, and including everything from leaf monkeys to baboons. The other five are all New World monkeys, and are found only in the Americas. (There is, incidentally, a debate as to whether these are all families, or whether some are just subfamilies. But it's too tedious to go into right now).

There are a number of differences between the Old World and New World monkeys, other than where they happen to live. The noses of the American species are flatter, with their nostrils facing sideways, and this is generally thought of as the key difference. Hence the scientific name for New World monkeys as a whole: "platyrrhine" literally means "flat-nosed". Another significant difference is that it's only New World monkeys that have prehensile tails, which they can wrap around branches while climbing.

Because they are less closely related to us, New World monkeys were long thought to be the more primitive kind. In reality, of course, both groups arose at exactly the same time, since they're two different branches of the same tree, and there's no reason to suppose that they haven't evolved equally far in the 40 million years since.

The eight simian families, with New World monkeys shown in blue

At any rate, among the five families of New World monkey, the one that was thought to be most primitive was the Callitrichidae, or marmoset family. That's because they're unusually small, and, in evolution, animals within a group tend to get larger over time. There's no particularly obvious reason why this should be so, and there are a number of exceptions, but it does seem true more often than not. Mammals themselves, for instance, evolved from small shrew-like animals, and the first primates also seem to have been small lemur-like animals.

But, regardless of the general utility of such a rule, it seems reasonably clear now that it doesn't apply to marmosets and their relatives. Instead, marmosets have shrunk over time, becoming dwarfs. Why? Well, that's not entirely clear. There are advantages to being smaller, such as the need for less food, the ability to fit more of you up a tree, the ability to hide from predators, and the fact that you can breed faster because it doesn't take so long to grow up. On the other hand, there are disadvantages, too, such as the fact that it's easier for predators to eat you if they can find you, and the fact that you lose heat more rapidly, and so need a faster metabolism. Clearly, for marmosets, at least some of the former won out over the latter, but we don't know for sure which ones were more important.

The marmoset family also includes tamarins, and the two share a number of features that set them apart from all other monkeys. Most obviously, there's that small size. And they certainly are small - even the largest callitrichid monkey is about 10% smaller than the very smallest monkey in any of the other New World families. But there is, as we might expect, rather more than their diminutive bodies that makes them unique.

For a start, I said up above that New World monkeys are the only monkeys to have prehensile tails. That's true, but I didn't say that they all had such tails, and, in fact, while callitrichids do have long and fairly mobile tails, they aren't prehensile. They're also unique among simians in having claws, rather than nails, on their fingers and toes. Presumably, these help in scampering up trees, especially when your hand isn't necessarily large enough to hold onto a branch that can support your weight. Oddly, though, while they have claws on all of their fingers and on four toes of each foot, they do have typical primate-like nails on their big toes. Go figure.

Both marmosets and tamarins also tend to have luxuriant hair, and the scientific name 'callitrichid' that refers to them both means "beautiful hair" in Greek. The hair of callitrichids is also often colourful, and, in some species, forms distinctive tufts that help to make these amongst the most beautiful of all primates.

A key way to distinguish different kinds of mammal, especially when you've only got a skeleton, is to examine their teeth. Old World monkeys have the same general pattern of teeth as do humans: two incisors at the front of the mouth, then the canines, then two premolars, and finally three molar teeth. This is less than the typical mammalian pattern, and likely associated with our lack of a long snout. New World monkeys, while still having less teeth than the mammalian default, do, however, have an extra pair of premolars, giving them 36 teeth in their jaws, rather than the 32 of their Old World cousins (and us).

Except, that is, for marmosets and tamarins. In almost every species, they do have 32 teeth... but it's the last molar that's missing, not the third premolar. The last molar is, of course, what we humans call a "wisdom tooth", and they probably lack it for the same reason it develops so late in us - there isn't enough room in their small mouths to fit it in. Mind you, they could have got round this problem by the simple expediency of shrinking their teeth in proportion to their small heads, but evidently that would have created problems with feeding. Their teeth are, as a result, proportionally larger than you might expect. In fact, the lower canines of tamarins are particularly long, a feature that's used to tell them apart from the marmosets proper.

Speaking of food, marmosets and tamarins are omnivorous, and they will eat things like insects and snails, as well as the odd tiny frog or lizard. The larger portion of their diet, though, is plant matter, with fruit being particularly important. Like other primates, they have the ability to see at least partially in colour, allowing them to distinguish ripe red fruits from the unsuitable green ones. They also eat a lot of gum, the thick sugary substance that some trees exude from their wounds in a manner analogous to an animal's blood clot. True marmosets, in particular, are highly adapted to this diet, and some of them eat little else.

Another oddity is their reproduction. Simians, including us, are adapted to have one child at a time. In most mammals, for example, the uterus has two chambers, one for each fallopian tube, making it easy to have litters of two or more offspring. We, however, only have a single chamber, ideal when you don't want more than one child. Callitrichids are no exception to this rule, yet about 80% of the time, they give birth to non-identical twins.

Since everything else suggests that they should just have singletons, as humans normally do, this is apparently something that appeared relatively late in callitrichid evolution. Of course, twins are hardly unknown among other primates, but it's certainly unusual for it to be the standard result of any pregnancy. Perhaps it's part of a drive to reproduce rapidly, ensuring against the threat of such small monkeys being eaten by predators.

Indeed, the twins actually share some of their blood supply in the womb. In cattle, this can lead to problems, if blood cells from a male foetus cross over into a female twin. The end result is to make the female calf into a sterile freemartin. This evidently doesn't happen with callitrichids, which is just as well, when it would affect about 40% of births. This may be because much of the crucial sexual development occurs after birth, by which time the female monkey is able to flush the problematic cells out of her blood.

Marmosets and tamarins live in social groups of up to twenty or so individuals, but the precise details vary enormously between different species. In general, both sons and daughters leave home when they reach adulthood, in contrast to many other mammals where females stay behind to form the hereditary core of the group. This makes the groups fluid from one generation to the next, as youngsters either bring new blood to existing groups, or gather together to form new ones of their own.

Including one oddity that doesn't quite fit the description I've just given, there are at least 42 species of callitrichid monkey. There are also a number of subspecies that we might be able to get away with raising to species status, which could raise the total higher. Over the coming months, I'm going to take a closer look at every one of them, starting in January 2014 with some of the true marmosets.

[Picture by Dario Sanches, released under Creative Commons attribution share-alike 2.0 generic license. Cladogram adapted from Opazo et al, 2006.]