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.
While gum is a good deal easier to digest than, say, grass, it is very thick and goopy, so that it passes through the gut quite slowly. It's a good source of carbohydrate and minerals, but much less so of protein, and it does need to be digested quite thoroughly to extract all the nutrients. All members of the marmoset family have large lower intestines, in which they can ferment food - the same approach is seen, for example, in horses. But in true marmosets, the lower intestine is especially long, including a large fermentation chamber (or caecum) and a deep internal folds. In comparison, the small intestine is relatively short. Perhaps unsurprisingly, they supplement their low protein intake by eating the odd insect here and there, and they will also eat fruit, if that's what's around.
The Callithrix monkeys all inhabit the Atlantic Forest, an extensive region of woodland that runs along the entire southern half of Brazil's east coast, and for some way inland. It's tropical to subtropical, but not quite as hot and moist as the Amazon Forest further north, from which it is separated by the more open terrain known as the cerrado - South America's equivalent of the African savannah.
Starting our journey at the northern end of the Atlantic Forest, at the most easterly part of Brazil, we encounter the very first marmoset to be scientifically described, way back in 1758. This is the common marmoset (Callithrix jacchus), easily distinguished by the fans of long white hair that sprout from each ear. They inhabit what is perhaps the driest part of the Atlantic Forest, and the one most subject to change with the seasons. (Not, in this case, summer and winter - it's too close to the equator for that to mean much - but there are very distinct wet and dry seasons).
It's probably because of this that, even by the standards of marmosets, the common species relies heavily on gum. Here, fruits are only available at certain times of the year, but gum essentially never runs out. This allows them to live in habitats that are, for marmosets, relatively barren and marginal, and they're even found away from the forest proper, in the scattered woodlands of the northeastern cerrado. As a result, forest disturbance worries them less than it might other monkeys, and they have even been successfully introduced into urban parkland in places such as Rio de Janeiro.
Common marmosets live in groups of up to thirteen individuals, typically with two or three adults of each sex, and the remainder made up of youngsters. The groups seem to be relatively peaceful, with a loose hierarchy based on age, rather than on who can beat up whom. Neither sex is obviously dominant over the other, probably because they're about the same size.
A typical day is said to consist of waking about half an hour after dawn, breakfasting on trees that you scraped the bark off the night before, playing, resting, and generally having a good time for a few hours, and then an evening meal before retiring for the night about ten to eleven hours after you got up. Because they only have to look for trees that produce gum, rather than for a good supply of fruit, the group doesn't have to travel far. Indeed, they spend most of their lives in an area of no more than about nine hectares (22 acres), and often only half that.
It obviously helps that they're small. Even the largest are no more than 20 cm (8 inches) from crown to rump, albeit with a long tail that adds as much as a further 30 cm (12 inches), and they weigh an average of just 320 g (11 ounces). This may also help to explain why they breed so rapidly.
Within the group, only one male typically breeds, but, unusually, it's quite common for there to be two breeding females - often mother and daughter. Everyone else is a 'helper', supporting the group with various communal activities. This includes helping to defend the territory from interlopers. Here, it's generally members of the same sex that drive each other, with opposite sex encounters often ending in a crafty bit of sex before anyone else notices. That this sort of thing happens may help familiarise groups with their neighbours, and perhaps make it easier when it comes time for female monkeys to leave home (a necessary move, when the only breeding male around is your dad). Groups have also been reported to mob predators, such as tayras, gaining strength in numbers by doing so.
Heading further south into the central region of the Atlantic Forest, we come to the black-tufted marmoset (Callithrix penicillata). In many respects, it closely resembles the 'common' species, with a similar diet and habitat. Indeed, like common marmosets, it is often found in the scattered woodlands of the cerrado, although, in its case, there is at least some suspicion that this may be increasing due to logging in the Forest proper. Physically, they're similar too, although slightly slimmer and taller, and with a paler body. The simplest way to tell them apart is, of course, the ears, whose moderately sized black tufts are easily distinguished from the large white fans of common marmosets.
Groups of black-tufted marmosets commonly consist of only five or six members, although groups of a dozen or so individuals have been reported. At least in the cerrado, each group travels over a wider area than common marmosets do, with home ranges of up to 18 hectares (45 acres); this is probably less in denser forest. Although gum is available year-round, they spend more time searching for insects during the dry season than during the wet one; presumably, that's when it's harder to find useful protein supplements. That at least some black-tufted marmosets (all female in the study, although there were so few examined, it's hard to tell if that means anything) have full three-colour vision like that of humans, may be related to their success in foraging in tough times.
When it comes to predators, black-tufted marmosets seem particularly frightened of oncillas, a local jungle cat similar to ocelots. While they are also wary of animals such as hawks and rattlesnakes, even marmosets raised in zoos, who had never seen one before, just couldn't conceal their fear of a stuffed oncilla, no matter how long it sat there without attacking them. Until the researchers gave them diazepam, anyway, which probably doesn't happen much in the wild.
Apart from being smaller, the groups of black-tufted marmosets seem to have a similar structure to those of the 'common' species. They are based around a single monogamous pair, and there doesn't seem to be any fighting to decide who belongs to it. In black-tufted marmosets, as in humans, affection between individuals is boosted by the "cuddle hormone" oxytocin, which probably pays a significant role in forming the pair bond. When it comes to encounters with individuals outside the group, these also seem to be shaped by the possibility of sex, with same-sex strangers being more likely to be chased away.
Interestingly, this even extends to encounters between black-tufted and common marmosets, in the narrow band of territory where both are found. The two are capable of cross-breeding, and sometimes do so in the wild. In captivity, the fact that both kinds of marmoset are used to helping to look after children that aren't their own has been used to persuade them to look after foster children that don't even belong to the same species.
Wied's marmoset was first identified, as a subspecies of black-tufted marmoset, by Prince Maximillian zu Wied-Neuwied in 1826. There then followed over 150 years of argument as to whether he should have done any such thing... and, by extension, whether or not he should have a monkey named after him. Many said that it wasn't sufficiently different from other black-tufted marmosets to count as a subspecies. A particular influential analysis by Philip Hershkovitz instead concluded that it was actually a hybrid between black-tufted marmosets and another species living just to the south. We're now pretty confident that Wied had actually underestimated it's importance, and it was formally recognised as a species in its own right in 1986.
Wied's marmosets live in groups of similar size to those of their black-tufted cousins, although, presumably because they're looking for fruit, they travel over a much larger areas of 35 to 40 hectares (85 to 100 acres). Their social structure is also similar, and research has shown that, not only is it possible to determine the sex of a Wied's marmoset from it's call, but that the calls can provide quite complex social information. The latter, at least, is rather unusual, even in primates.
Like their relatives, all members of a group join in in looking after the young of a single breeding female. As with the others, this is at least partly due to a desire not to overwork the mother. For instance, it has been noted that baby Wied's marmosets keep waking their mothers up at all times of the night (who'd have thought, eh?), and that their fathers, who appear to be attentive enough during the day, really can't be bothered when a good night's sleep is at stake.
But there's actually more to it than that. Like other marmosets, Wied's species gives birth to non-identical twins. It has been shown that so much genetic material crosses from one twin to the other in the womb that many, perhaps all, adults are chimeric. This means that some of the cells in their body contain, not their own genes, but those of their sibling. In a sense, they are actually a mix of two different individuals, at least at the genetic level.
What's really weird about this is that it also applies to the germ cells - sperm and eggs. Which means that, even if you're physically the parent of a given child, you're not necessarily that child's genetic parent. Yes, a male Wied's marmoset can get his partner pregnant by his brother. With that level of genetic mixing, it's no wonder they share in looking after the kids. After all, any child might be yours, even if you'd never had sex in your life.
We don't yet know whether or not this is true of other marmoset species. But it seems distinctly possible.
[Pictures by Tony Hisgett, "Miguelrangeljr", and "Grendelkhan", from Wikimedia Commons]