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.
The internal parts of the digestive system are also different, and, again, those of tamarins are the ones that look "normal", which is to say, much like those of other monkeys. That is, they lack the enlarged caecum and shortened small intestine found in the marmosets. These differences are there for the same reason that those in the teeth are: tamarins have a different diet from marmosets.
Like marmosets, tamarins eat gum that oozes out of the wounds in certain sorts of tree. Unlike them, though, it's not a major part of the diet, and they don't collect it themselves. The chisel-like front teeth of marmosets gouge the bark of trees, so that the gum oozes out over night, and the marmosets can have it for breakfast. Tamarins, where they live in the same area as marmosets, just take whatever's left over, and otherwise look for wounds caused by storm damage, insects, and the like. Their teeth are not adapted for tree-gouging, and, instead, the bulk of their diet consists of fruit and insects. Indeed, because they gulp down berries whole, without spitting the pips out, they are major dispersers of seeds through the jungle.
The jungle in question is mostly the Amazon, and all true tamarins live either there, or in extensions of the same tropical woodland outside the Amazon Basin proper. In fact, while there are less known species of tamarin than of marmoset, they are more widespread, possibly because their diet isn't quite so specialised.
Undoubtedly, there's something found in Ecuador, but the debate is as to whether it's a species or a subspecies. The argument for the former is that monkeys looking like the Ecuadorian sort are found in the same region as those looking like typical black-mantled tamarins - and if they were really the same species, they should just breed together and blur into one. The counter-argument is essentially that no, they don't live in the same area, so please stop saying that they do, and, anyway, all the genetic tests show that they're impossible to tell apart.
The current evidence seems to support the latter position, and if there is a difference beyond the Ecuadorian "Graells' tamarins" being greyer with a more grizzled coat, it's likely pretty slim. So I'm sticking with it.
Anyway, black-mantled tamarins seem to get up about 4 in the morning, and stay active until about 7 p.m., although they sleep rather more during the wet season. They spend about a third of their day looking for grasshoppers, more time than they spend doing anything else. Grasshoppers, of course, tend to be found low down, so these are monkeys that spend more time in bushy undergrowth than high in the trees, and they will even forage along the ground.
Their technique for finding grasshoppers seems to consist largely of poking their hands into foliage, and rummaging about until they catch something. This technique, which doesn't rely heavily on acute vision, is best for catching relatively large, slow-moving insects, and, besides grasshoppers, they also eat beetles, termites, and earwigs. Even so, it's difficult enough to do successfully that mothers have been observed catching grasshoppers for their children even after the latter are otherwise independent. Once they've found an insect, they just hold it down and bite its head off before eating the rest of it. Despite their preference for grasshoppers, though, they apparently don't like the prickly hind-legs, and leave those behind.
Like other tamarins, they aren't pure insectivores, and they always make sure to spend an hour or two each day eating fruit, with a preference for things like Amazon grape, figs, and Cecropia, although just about anything with a fleshy coating will do. They live in groups of up to fifteen, typically consisting of a single mated pair and their children, and they don't seem especially territorial, being willing to share their patch of forest with neighbouring groups if need be. Mated females give birth to twins around January each year, just before the rainy season, but almost half do so again in June. It takes about a year for them to reach full size, although they have to leave home in order to find a mate.
The classification of saddle-back tamarins is, however, even more disputed, and probably with better reason, than it is for the black-mantled kind. Officially, there are eleven different subspecies, all inhabiting different parts of their large geographical range. There is some pretty good evidence that two of these should be raised to full species status, and some weaker evidence for another three. Apart from the "original" subspecies that all of these would be being split off from, nobody has yet got round to even checking the other five, so there could be more argument ahead yet.
Sticking with what we've got for the time being, all the various different kinds of saddle-back tamarin do look pretty much the same. All have a marbled back, reddish hind legs, dark forelimbs and head, and a pale face, although, naturally, there is some variation within this pattern. They may spend even more of their day searching for insects than black-mantled tamarins, although, like them, they also eat fruit - especially figs. Also like the black-mantled species, but unlike most other marmosets and tamarins, they frequently climb on and leap between, vertical tree trunks, rather than using horizontal branches. It's likely that this relates to their searching for ground-dwelling insects, and the fact that tamarins have claws, rather than nails, does make it easier for them to grip onto the bark.
An unusual feature of saddle-back tamarin life, however, is their tolerance for other species. They share much of their range with other tamarin species, most notably the moustached tamarin (to which I'll get in a later post), although there are others. Normally, we'd expect these animals to be rivals, but, in fact, they cooperate closely.
They sleep among their own kind, but come the morning, they call out to another, and assemble into a large group with one family from each species. They then proceed to spend the day together, jointly looking out for food, warning each other of approaching predators, and so on. (Even a single-species group are willing to gang up on tree boas to scare them away). These mixed-species are surprisingly stable, with the families preferring to sleep relatively close to one other, and acting essentially as a single group, defending a common territory from outsiders. Of course, not all saddle-back tamarins live like this, but in at least some areas, as many as 80% of them do, which is hardly insignificant.
The question, of course, is why do they do it? There are certainly benefits to living in a larger group, such as it being easier to watch out for predators. Sharing with another species, rather than another family of your kind, does at least mean you don't have any sexual rivals for your partner. Even so, being such similar animals, you'd think they'd compete for food, but apparently not. Nonetheless, it is true that they eat exactly the same sorts of fruit, and they almost never fight over them, unless they're particularly hungry for some reason.
However, the moustached tamarins look for insects by... well, looking for them, instead of poking about with their hands in the hope of finding something, as the saddle-backs do. This means that they tend to eat insects that are visible against bark - mostly green ones, it seems - rather than the larger, but better camouflaged, species that the saddle-backs eat. Indeed, their searching about up on the horizontal branches may even drive some insects down to where the saddle-back tamarins forage in the undergrowth, inadvertently helping them out. (Incidentally, saddle-back tamarins do have full colour vision, but they use it to find ripe fruit, not coloured insects).
A similarly friendly and cooperative attitude can also be seen in the mating habits of saddle-back tamarins. Groups tend to consist of up to ten individuals, with a single breeding female. She's often monogamous, but more commonly has two sexual partners. The two males don't compete with another, rarely fighting, showing every sign of friendliness, and calmly letting the female mate with whichever of them she feels like at the time.
The advantage they gain from this seems to be that they can help each other raise the kids. Which, when you remember that they always have twins, does have some sense to it - and, presumably, the males can't tell which children are theirs, and so just help out together. Indeed, the males take a greater share of the work looking after the young than the females do, although they're helped out by the babies' siblings and half-siblings.
This is, however, merely the standard set-up, and saddle-back tamarins seem to have a very flexible social structure. As noted, some are monogamous, but there are also a few rare groups that have a single adult male and two breeding females. For hormonal reasons, young females seem unable to become fertile so long as they are living with their mothers, moving into a neighbouring group as soon as a vacancy arises. Half of them never get the chance, and so never breed, while those that do so hold the position for about three years, giving birth once, and sometimes twice, each year.
It does, however, have the advantage of being (to human eyes) rather more visibly distinct. The mantle on the shoulders and forelimbs is, as the name suggests, golden-orange, not dark brown, and the hindquarters are much paler, with less of the distinct marbling found in the saddle-back species. They live around the Rio Napo on the Ecuadorian/Peruvian border, and, were it not for the colour, seem almost indistinguishable from their saddle-back relatives.
The other species is the white-mantled tamarin (Saguinus melanoleucus), the status of which is, if anything, even less clear. It certainly looks different enough - it's almost pure creamy-white, with black ears, facial skin, and genitals. Living in a small patch of western Brazil, and probably just over the border into Peru, despite its appearance, genetic tests have found it almost impossible to tell apart from one of the subspecies of saddle-back tamarin, so its possible that it's no more a species than a black panther is.
Officially, the white-mantled tamarin has two subspecies of its own. One of them, however, is known only from a single pelt, and nobody's completely sure where it came from. Besides which, it appears to have belonged to a white-mantled/saddle-back hybrid, and if so, doesn't count.
[Pictures by John Norton, "Mistvan", "Ksnordstrand", and Stefan Deinst, from Wikimedia Commons]