Mini-Monkeys: The Odd One Out

Goeldi's monkey

Under the scheme that I have been using over the last year, there are 42 currently recognised species in the marmoset family. Of these, 22 are marmosets, and 19 are either tamarins or lion tamarins. It doesn't require much arithmetic to deduce that there must therefore be one member of the family that's neither.

This is Goeldi's monkey (Callimico goeldii). It is sometimes called "Goeldi's marmoset", or, less commonly, "Goeldi's tamarin", but these names are misleading and inaccurate. It is, undoubtedly, a member of the marmoset family, but it's equally clear that it is quite different from anything else within that family.

First, the similarities. Goeldi's is about the same size, at about 25 cm (10 inches) in body length, as other members of the family, and therefore much smaller than any monkeys outside the family. It has similarly luxuriant fur, in its case almost entirely black, with an almost mane-like ruff around the head. Like members of the marmoset family, but unlike other monkeys, it has claws, rather than nails - something it uses to cling on to rough bark. It also lives in roughly the same area, in the high altitude western margin of the Amazon, from Colombia down to Bolivia and in to western Brazil.

Mini-Monkeys: Monkeys with Manes

Golden lion tamarin

The tamarins of the Amazon jungle and the forests bordering it to the north are sometimes known as "true" tamarins, to distinguish them from their relatives further south. Even when the marmoset family was first recognised as distinct from those of other South American monkeys, and all marmosets were placed into a single genus, the tamarins were already divided into two: the "true" tamarins in the north, and the lion tamarins in the south.

More recent molecular analysis, of the sort that also showed the clear difference between the Amazonian and Atlantic Forest marmosets, has shown that our initial instinct on the tamarins was correct. Indeed, it seems to be the case that lion tamarins are actually more closely related to marmosets than they are to other tamarins, something that makes the distinction unavoidable. Having said which, there's no dispute that, anatomically, they look much more like tamarins than they do their apparently closer relatives.

What this means is that it's the tamarins, not the marmosets, that most likely resemble the original members of their family. The marmosets are specialists, having changed further from their ancestor's body form because of their heavy reliance on gum as a food source, and the need to modify their teeth, jaw muscles, and bowel structure to accommodate this odd diet. Lion tamarins, which never did that, retain the original "tusked" teeth and fruit-digesting colons of their ancestors, just as true tamarins do.

Mini-Monkeys: Punk Monkeys of Colombia

Cotton-top tamarin

The Amazon rainforest is the region of dense tropical jungle that is drained by the Amazon River and its tributaries, and it is where the majority of marmosets and tamarins are found. In the northwest, it is bounded by the Andean mountain chain. Beyond this, on the northern side, there are more tropical forests, but these are much smaller than, and separate from, the Amazon proper. Once they would have stretched much of the way down to the Carribbean coast, interrupted only by patches of more open grassland and extensive swamps. Today, however, they form a belt of forested land, mostly in hilly terrain, that gives way to the more inhabited parts of Colombia, dominated at first by subsistence agriculture, and then by cities and areas of more intensive farming around the larger rivers.

The tamarins that live here, in the northern Colombian forests, must be descended from some group that crossed the Andes, presumably through some of the lower, shorter, passes near what is now the Venezuelan border, or else along the coast. There are three species here today, all apparently descended from that same original group, and including one of the first of any tamarin species to be formally described, back in 1758. This is the cotton-top tamarin (Saguinus oedipus), and it's at once one of the best known members of the marmoset family, and one of the most threatened.

Mini-Monkeys: The Midas Touch and More

Midas tamarin

A great many species of marmoset and tamarin have been named and described over the years, and the process continues today. The very first three to be described were the common marmoset, the cotton-top tamarin, and the Midas tamarin. Two of these are well-known, well-studied species. But the third, despite the fact that it was first described, alongside the other two, all the way back in 1758, remains rather more of a mystery.

Not a complete mystery, by any means. Midas tamarins (Saguinus midas) are among the most common monkeys in the Guyanas, the three relatively small countries that lie east of Venezuela on the north coast of South America. They're also found further south, in Brazil, as far as the north bank of the Amazon. They're not at all endangered, partly because there's relatively little logging in that part of the Amazon, and partly because they don't seem that bothered by disturbed forest if they do have to live in it. No, the real issue is that most researchers just happen to have focussed on other species. Which, come to that, may not be entirely unrelated to the fact that they live in an area that nobody visits...

Mini-Monkeys: Monkeys with Moustaches

Emperor tamarin (bearded subspecies)

Monkeys of the marmoset family, which include tamarins, are known, not just for their small size, but also their luxuriant fur, which often forms extravagant tufts on the head. Arguably, none of these tufts are more distinctive than the drooping moustaches of emperor tamarins (Saguinus imperator).

Emperor tamarins live in the lowland jungles just east of the Andes, in the border regions between western Peru, northern Bolivia, and eastern Brazil. Perhaps because of this remote location, they were discovered remarkably late for an animal so distinctive; they were first described by Brazilian zoologist Émil Goeldi in 1907 (and we'll be coming back to him in a later post). On the plus side, this distance from civilisation has kept them relatively secure, with loggers and the like only recently having reached this far into the jungle.

Compared with some other members of their family, they are not especially colourful. Their bodies are grey, reaching near-black on the face and hands, and they have a reddish tail with a grey tip. What makes them so noticeable, of course, are those long, pure white, moustaches, which are present in both sexes.  Indeed, the more widespread of the two subspecies also has a thin and straggly beard hanging from its chin. The moustache apparently reminded Goeldi of Kaiser Wilhelm II, although to be honest, it's hard to see why (one would have thought that some of the Chinese emperors would have made a better fit).

Mini-Monkeys: Chilling Out With the Mantled Tamarins

Golden-mantled tamarin

Over the last five months I have looked at the 22 widely recognised species of marmoset. However, as I mentioned back when I started, not all members of the marmoset family are actually marmosets. Almost all of the others are tamarins, the majority of which belong to the genus Saguinus, also known as the 'true' tamarins.

So what is the difference between a marmoset and a tamarin? On the face of it, they look quite similar, and, indeed, being members of the same family, that's not entirely inaccurate. They're roughly the same size, have the same luxuriant fur, often with extravagant tufts, and have the same basic body plan, with a long non-prehensile tail, clawed toes, and so on. The most significant difference though, and the one on which they were first separated back in the early 19th century, is in the shape of their teeth.

Unlike marmosets, tamarins have prominent canine teeth in their lower jaw. These are often called "tusks", although by a strict definition, tusks have to project outside the lips (as they do in wild boars or elephants), which these don't. In fact, they're actually no larger than those of marmosets, they just look that way because the incisor teeth that separate them at the front of the mouth are of a more normal size. In marmosets, the incisors are elongated and cylindrical, forming a straight line with the top of the canines, but in tamarins, the closer in size to what you'd expect in monkeys, making the canines stand out.

Mini-Monkeys: The Smallest Monkeys of All

Pygmy marmosets

As we've seen over the last few months, the marmoset family contains a number of species of unusually small monkey. Typically about 22 cm (9 inches) in length, plus tail, marmosets and tamarins are far smaller than any other kind of monkey. They're dwarfs, perhaps evolved that way so they don't have to eat as much food, perhaps to hide from predators, and perhaps for some reason we haven't thought of. But, if they're smaller than other monkeys, what is the smallest member of their own family? In other words, what is the smallest species of monkey in the world?

The answer is the pygmy marmoset (Cebuella pygmaea), and it most certainly is small. With fully grown adults measuring just 14 cm (5.5 inches) long, they're only two thirds the size even of other marmosets. The difference in weight is, of course, even more dramatic: typical marmosets weigh between about 350 and 400 g (12 to 14 oz.), while the pygmy species weighs a mere 85 to 140 g (3 to 5 oz.) As is typical for marmosets, the females are slightly larger than the males, but, in this case, not by very much.

Mini-Monkeys: Madeiran Marmosets and More

Santarem marmoset

Back in the 1970s,  a survey of the monkeys of South America identified just two species of "typical" marmoset living in the Amazon jungle: a naked-eared form, and a tassel-eared form. Both species were fairly variable in appearance, and each was considered to include three distinct subspecies. Today, better knowledge of things like their genetic make-up means that we have elevated all of those subspecies to full species status.

The "tassel-eared marmoset" had first been described in 1811 by Geoffroy Saint-Hilaire (although the date is officially given as 1812, because it took a year to get the description published). Because it was the first to be so described, the other two were considered subspecies of it, rather than the other way round. Once they were split off, we were left only with the species to which the originally collected specimen belonged. While still sometimes called the "tassel-eared marmoset" or some variation thereof, it is now more commonly called the Santarem marmoset (Mico humeralifer) to distinguish it from its kin.

Mini-Monkeys: The Many Faces of the Silvery Marmoset

Silvery marmoset

The marmosets of the Atlantic Forest are, most likely, descended from a population that emigrated from the Amazon shortly before the Ice Ages. All of the other species of marmoset still live in those larger and more northerly jungles. But how many species is that?

In a wide-ranging and influential study of American monkeys in 1977, mammalogist Philip Hershkovitz identified two such species, one that had hairy tufted ears, and one that did not. He noted that the two species had more in common with one another than with their Atlantic Forest cousins. For example, while their teeth are adapted for scraping bark off trees to make nutritious gum ooze out, they aren't quite as adapted for this as they are in the more southerly species. He also noted that the two species had quite a lot of variety in their colouration, although that didn't necessarily mean much.

Since the 1970s, however, our knowledge of the miniature monkeys of the Amazon has increased dramatically. For one thing, we now place more emphasis on some of the minor differences Hershkovitz identified. For instance, today, we consider the Amazonian species to belong to a different genus from the common marmoset and its kin, using a name, Mico, first proposed by René Lesson back in the 19th century. In a similar vein, we have also raised many of Hershkovitz's subspecies to full species status. Finally, we have actually discovered species he simply didn't know about.

End result: there are now something like fourteen species of Amazonian marmoset known to science. And, given how recently many of them were discovered, there's no reason to suppose that we're done yet. There may well be more marmosets to find.

Mini-Monkeys: Marmosets of the Southern Atlantic Forest

Geoffroy's marmoset

Last month, I looked at the three species of marmoset inhabiting the northern half of the Atlantic Forest along the east coast of Brazil. Further south, the forests naturally become cooler - which is to say, that they're merely subtropical, rather than fully tropical. All of the marmosets inhabiting the Atlantic Forest are closely related, belonging to the genus Callithrix, and many of them are capable of interbreeding with one another. They all descend, most likely, from a single species that lived around the early Pleistocene, just before the Ice Ages, and which presumably crossed over what is now the relatively sparsely forested cerrado from the more fertile Amazon to the north and west.

Leaving behind Wied's marmoset, we now hop over the Jequitinhonha River to its southern bank, and a stretch of forest that heads about another two hundred miles or so down the coast. This is the home of Geoffroy's marmoset (Callithrix geoffroyi), also called the "white-headed marmoset". In fact, Geoffroy has quite a few species named after him, and is a key figure in the development of mammalian classification. Living in France around the turn of the nineteenth century, he was an early proponent of the theory of evolution - although, dying fifteen years before the publication of On the Origin of Species, he had most of the details of how it worked wrong.

Mini-Monkeys: Marmosets of the Northern Atlantic Forest

Common marmoset

The official name of every family of animals is derived from the name of the 'type genus' - a smaller group of closely related animals within it that are deemed to be 'typical', and which are used to define that family. Of course, what's 'typical' is a subjective opinion, and will only reflect what somebody thought at the time the family was named. So, really, it's more of a convenience than a truly meaningful statement. But, still, it's a convenience that we continue to use.

The type genus of the marmoset family is Callithrix, and the scientific name of the family is therefore 'Callitrichidae'. If we're honest, they're not really any more 'typical' than most other species in the family, but they're as good a place to start my survey of the world's marmosets and tamarins as anywhere else.

The Callithrix monkeys are true marmosets, a group that is distinguished from other members of the family by a heavier reliance on tree gum as a source of food. All callitrichids eat gum, but for true marmosets, it's a vital part of their diet that they eat all year round. Having evolved to do this, they have a number of physical adaptations that suit them for the lifestyle. Unlike their relatives the tamarins, they have no tusks in their lower jaws, meaning that the teeth at the front of their mouths form a relatively straight line. They feed by clamping the teeth of their upper jaw onto tree bark, and then scraping upwards with that flat row of teeth on the lower jaw.

Although they aren't born that way, over time, these teeth are honed to a sharp chisel-like edge. However, for this to work, they also need a remarkably wide gape, and their jaw and its muscles are adapted accordingly. Nor does it end there, because there's the business of digesting the gum.

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.