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...
Still, their being common does mean that we have a good grasp of the basics, even if much of our knowledge comes from watching them in captivity, rather than out in the wild. Midas tamarins are, for the most part, black. There's usually some grey mottling on the back, a bit like that seen in mantled tamarins, and young ones have some white patches on the head, but otherwise... just black. Except that is, for the hands. Which are bright golden yellow.
Hence, of course, the name "Midas tamarin". Indeed, there are many other names for this particular monkey, all making the same reference: golden-handed tamarin, yellow-handed tamarin, and red-handed tamarin are all different names for the same animal. To be honest, at least some of those probably more common names than "Midas tamarin", but you've got to admire that one for the poetry, so I'm sticking with it.
Beyond that, what we know indicates that they are (surprise, surprise) not that different from other tamarins. They eat about equal quantities of insects and fruit, and, when they get gum, it's largely by eating the seed pods of the local variety of Parkia legumes. They lick nectar off flowers, too. They spend much of their day foraging in the lower stories of the forest, heading up to the canopy occasionally to grab fruit, and taking mid-morning and mid-day naps before retiring to bed unusually early - about 4 or 5 o'clock, which is well before sunset in the tropics.
Since they spend so much time snoozing, it's important that they pick good sites, but they don't like to continually re-use the same ones, likely so that they're less predictable for predators. They try to find good solid trees, with strong, thick, vines among which they can hide, and try not to use the same one more than two or three days running, which leads to them having dozens over their 30 to 40 hectare (75 to 100 acres) territory. They live in groups of about five on average, with only one female breeding, much like other tamarins. And, apparently, they get stroppy when they're going through puberty.
We used to think that there were two subspecies of Midas tamarin, one of which lived south of the Amazon River in eastern Brazil. Examination of this southerly subspecies in the 1990s, however, showed that it not only looked a little different from its northern relative, but that it was genetically distinct, and that its teeth were as different as anything you would expect between two different species.
At any rate, the previous confusion between Midas and black tamarins is due to the fact that the two are very similar. Black tamarins are slightly smaller, but hardly so that you would notice, and the most obvious difference is the colour of their hands, hence their alternative name of "black-handed tamarins". Which is technically accurate, because their hands are indeed black, but slightly misleading, because so is the rest of them, aside from some brown markings on the back.
Perhaps because they live in a more accessible part of the jungle, black tamarins have been slightly better studied than their more numerous, but more remote, golden-handed cousins. Presumably for the same reason, their population has crashed by as much as 30% over the last two decades or so, and, unlike all the other tamarins I have described so far, they are considered under significant threat.
Ironically, they seem to be fairly tolerant of habitat disturbance - unfortunately, total habitat destruction is a rather different matter. Still, there are at least some areas where they don't seem quite so much at risk, and these include many that they share with the rather more successful silvery marmosets. The two seem unable to cooperate in the way that saddle-back tamarins do with some of their neighbouring species, probably because they forage for food in the same parts of the forest canopy, and hence keep getting getting in each other's way and competing for resources. One study shows that black tamarins eat a remarkable amount of fruit, as compared with insects, but it's unclear how true this is across the whole of their range. Certainly, like Midas tamarins, they seem to be important distributors of seeds, and females at least seem to have good colour vision (which they'd need to tell if fruit is ripe).
While black tamarins are considered threatened by loss of their habitat, they are not formally listed as an endangered species. We do not, however, have to travel too far upriver to find a tamarin that is. The pied tamarin (Saguinus bicolor) lives in a fairly small area of the central Amazon near to the city of Manaus. With a population of over two million this is the largest city in the northern Brazilian interior, and it will be no surprise to learn that its rapid growth in recent years is the main problem for the pied tamarins.
Pied tamarins are considered endangered because their population has dropped by at least 50% over the last three generations (assumed to be 18 years, in this case). The problem is that, historically, the entire species has been restricted to a patch of forest that is at no point further than 100 miles from the confluence of the Amazon and the Rio Negro, and only on the north side at that. And that river confluence is exactly where Manaus happens to have been built, meaning that the monkeys live only in a small area that is currently being developed for highways and agriculture.
Physically, pied tamarins are quite distinctive. They have black, largely hairless, faces, near-white forequarters, brown hindquarters, and reddish underparts. Like other tamarins, they eat a lot of fruit, as well as catching insects hiding among tree bark or leaves. This process of foraging flushes small lizards and larger insects out of the foliage, and there are reports of pied tamarins being followed about by double-toothed kites, specifically so that the latter can feed on whatever tries to escape the monkeys.
Breeding in pied tamarins seems to be similar to that in other species, and the fathers play at least as large a role in raising and feeding the young as the mothers do, once the young have stopped feeding solely on milk. One report from 2005 showed a margay (a kind of arboreal cat related to ocelots) hiding in dense vines and making sounds that clearly imitated those of a baby pied tamarin. Puzzled, an adult monkey went to investigate, and presumably would have been eaten had he not spotted the cat in time. In this case, the margay failed to get its dinner, but the fact that it used this tactic at all suggests that it must work at least some of the time.
Moving just a little further back downstream, we come to Martin's tamarin (Saguinus martinsi), a very close relative of the pied species. So close, in fact, that they were thought to be the same thing until as recently as 2000, and it's entirely possible that one of the supposed two subspecies of Martin's tamarin is really just a hybrid between it and its relative. The area that they live in is hardly any larger than that of the pied species, but just so happens to be relatively undisturbed, so that this monkey is not at all endangered. They have the same black, hairless, faces as do pied tamarins, but a more uniformly brown-ochre coat, albeit paler on the flanks and shoulders than elsewhere. Living in an isolated patch of forest, and not even been thought of as a species until 14 years ago, we know virtually nothing else about them.
To the west of Manaus, however, in the broad swathes of jungle between the Amazon River and the upper Rio Negro, and stretching from Brazil into Colombia, we find the last currently recognised species of Amazonian tamarin: the mottle-faced tamarin (Saguinus inustus). They're mostly black, with a few patches of brown here and there, and, like the pied species, have hairless faces. In their case, however, as their common name indicates, there are patches of pinkish-white mottling on the cheeks and across the face, that help to make them distinctive. The males also have pure white genitals, contrasting against their black fur.
They are another species that benefits from being in a relatively remote and untouched part of the jungle, and, like the others, this also limits our knowledge of them. However, while the forest is remote from the point of view of civilisation, it's an area with a significant population of Amazonian tribes, practising slash-and-burn agriculture, and the monkeys seem to be common around forest clearings created in this manner, as well as deeper in the jungle. This may be because many of the fruits they eat are particularly common there, and the locals don't hunt them, and sometimes even keep them as pets.
So it's another tamarin species that is surviving well in the modern world (at least, so far). That, however, is going to become a rarer sight as we move on to consider the remaining species of tamarin, outside the Amazon jungle...
[Photos by Frank Wouters, "Miguelrangeljr", and "Postdlf", from Wikimedia Commons]