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.
Pygmy marmosets are widespread through the southwestern reaches of the Amazon jungle, a region that they naturally share with many other kinds of monkey. While they can't avoid their larger relatives altogether - and tamarins, in particular, sometimes follow them about to get at gum oozing out of the tree-wounds they create - they minimise the competition by preferring to travel through branches and undergrowth above floodplains, rather than on more solid ground. With their small size, silky fur, and large manes around their heads, they look rather like fluff-balls with long tails.
Despite their smaller size, pygmy marmosets are, in many respects, similar to the various other marmoset species. Indeed, they are so similar that it has been disputed whether they really deserve a genus, Cebuella, all to themselves. That debate started in the 1990s, and it hasn't really gone away yet, although it's less of an issue now that we have also split the Amazonian and Atlantic Forest marmosets apart from one another. It's worth mentioning, though, that, while they'd never meet in the wild (living on opposite sides of the continent), pygmy marmosets and common marmosets (Callithrix jacchus) kept in captivity can interbreed. So far as I'm aware, it's not yet clear whether these hybrids are fertile, although one would suspect not, since the two species have different numbers of chromosomes.
Like other marmosets, pygmy marmosets eat a lot of gum, which they obtain by scraping bark off suitable trees and waiting for it to ooze out. They spend something like four hours every day doing this, and gum makes up about two thirds of their diet, with the balance almost entirely made up of insects. (They also eat fruit when it's in season, and sometimes eat the odd small vertebrate, but these are very minor parts of their diet). Compared with some of their relatives, they are more likely to move by climbing up trees and vines than they are leaping from place to place, although, for all we know, that may just be because the gaps look bigger to them.
Pygmy marmosets live in groups that typically have six or seven members, led by a (mostly) monogamous mated pair. The mated female seems to be in overall charge, although her partner is at least dominant over the other males. In many respects, pygmy marmoset social life seems fairly sophisticated, with a range of different calls that they can choose from depending on both their social status, and the various other sounds around them in the jungle at the time. They also seem to have a very basic understanding of the concept of mirrors, although not to the true levels of self-recognition achieved by, for example, chimpanzees and elephants.
In addition to alarm and contact calls, they can communicate by leaving scent marks, either with urine, or using specialised glands on their skin. Older males can also exert their dominance over their subordinates by raising their tail, ruffling their fur, and waving their genitals - a display that's apparently enough to cow almost any potential rival. Females sometimes wave their genitals at males in a similar manner, although, in their case, they seem to intend it as an invitation, not as a threat. The male responds to such heavy-handed hints (and to the fact that the female isn't trying to beat him up) by, among other things, sticking out his tongue and vibrating it in what's presumably a suggestive manner. This, interestingly, is not something we see other marmosets doing.
As is common among marmosets, only the dominant female gives birth, as she seems to be able to prevent her daughters from ovulating. Pregnancy lasts 142 days, which is scarcely shorter than the 150 days or so for other marmosets, and usually results in the birth of twins. Most mothers give birth twice a year, favouring May to June and November to January as ideal times, but this becomes less common as they age. The young, unsurprisingly, are extremely small, measuring about 6 cm (2.4 inches), and weighing just 14 g (0.5 oz.) at birth. The mother and her daughters carry the infant for about three weeks, before handing the duty over to the males.
There is one final species of marmoset to look at, however. As it turns out, it's also a small one; the dwarf marmoset (Callibella humilis). At about 17 cm (6.5 inches), they're noticeably larger than the pygmy species, but not by overly much. Lacking the manes of their smaller brethren, they look more like "typical" marmosets, but, apart from their size, can be distinguished by having distinct white eyebrows on a dark face.
They were discovered only in 1998, and we still know very little about them. As with pygmy marmosets, there is some debate as to how distinct they really are. Their skeleton has been reported to be quite different from that of other marmosets, including the pygmy species, implying that their unusually small size is the result of a separate evolutionary event. On the other hand, their genetics seem to show that they're no different from other Amazonian marmosets (again, apart from the pygmy one), in which case, their correct scientific name would be Mico humilis.
Initially reported to live only in an extremely small area (less than 400 square miles) on the east bank of the Aripuanã River, a report published just a few months ago indicated that they may not be quite so restricted. Reports by their discoverers also hint at a very unusual social and reproductive structure, compared with other marmosets. According to them, dwarf marmosets are not territorial, with several families sharing the same general area, and all the adult females breed, rather than just a dominant matriarch. Stranger still, they appear to give birth to singletons, rather than twins, most of the time, and, presumably as a consequence of this, only the mother looks after her young, rather than sharing the duties out.
At any rate, with these two smallest of small monkeys, I have completed my survey of marmosets, and will turn next to their cousins, the tamarins...
[Photo by Frank Wouters, from Wikimedia Commons]