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.
Like the two species living immediately to the north and west, Geoffroy's marmoset has prominent black tufts of fur on its ears, but, unlike them, it has an almost pure white face, making the tufts even more apparent. Other than that, it is physically similar to other Atlantic Forest marmosets, having, for example, the same ringed tail (although there are exceptions, most marmosets from the Amazon lack these rings).
Compared with other marmosets, Geoffroy's species seems to prefer relatively open woodland, which, in modern times, means that it is far more likely to be spotted in forests that have regrown after past logging than it is in untouched, primeval jungle. This is probably because of its predilection for foraging among the undergrowth and in the lower branches of trees - denser forest simply doesn't have much undergrowth, because sunlight can't get down that far.
About two thirds of their diet consists of tree gum, at least when it's at its most abundant, with the rest made up of fruit and insects - the latter particularly important, because it's their best source of protein. They spend almost half their day either feeding or moving about, and travel for about a kilometre every day, albeit within an area of only around five to seven hectares (12-17 acres) concentrated around some particularly tasty trees. They feed mainly in the morning, socialising for much of the rest of the day, and scraping bark of trees in the evening so that they've oozed out more gum in time for the next day's breakfast.
Like other marmosets, and New World monkeys in general, some Geoffroy's marmoset individuals have full three-colour vision, the same as their Old World kin (including us), while others can see only two colours. While one would think that being able to see a full range of colours would be an advantage - for example, in identifying ripe fruit - it can't be that large a one, or the monkeys without that ability would die out. Indeed, it turns out that each option has its own advantages.
Three-colour monkeys do, as you would expect, find it easier to identify orange food against a green background, but those with 'inferior' colour vision find it easier to spot camouflaged food, and to identify food and to forage in poor light. Presumably, when many things look the same colour anyway, you pay more attention to other visual cues.
Geoffroy's marmosets live in groups of up to fifteen individuals, each with just one breeding female, although other adults of both sexes are also present. The remainder of the group may assist in child-rearing, as is the case with other marmosets, but, being so small, there is a clear advantage in safety in numbers, and the monkeys have been shown to be quite good at assessing risks from predators such as snakes and birds of prey, remaining vigilant if they've seen or heard something suspicious in the area, but returning to their normal behaviour if it turns out that it wasn't a threat.
Compared with other local marmosets, the buffy-headed species has a more yellowish coat, and it really isn't just the head that's "buffy" in colour. The ear tufts are short, and creamy off-white, making them quite distinctive from the four northern species. They have a similar diet to their kin, but apparently eat more small lizards and frogs than other marmosets, as well as being keen to take nectar when fruit is in short supply.
However, while gum is, as usual, their main food supply they're quite picky about where they get it. 90% of the gum that they eat comes from just two plants: the yopo tree (a sort of giant legume with hallucinogenic beans) and the ara a gato vine. While the latter is a fair size, as vines go, it's apparently not suitable for the usual bark-scraping of marmosets, so the animals instead look for gum oozing from injuries previously made by insects and the like.
They live in groups of up to sixteen, which seem to be quite stable, and have four different alarm calls for responding to different kinds of threat. When facing something on the ground, they have one call for things that aren't very likely to reach them (such as snakes), which results in everyone bunching up together to look tough, and another for really scary predators (such as tayras) to which they respond by screaming as loud as they can in the hope they'll frighten it off. However, their main threats come from the birds of prey, and their alarm calls in those situations can translate to either "look out, there's something up there" or "holy crap - get under cover!", depending on what they've just seen.
Usually, one one female in the group breeds, although sometimes she allows a single daughter to do so as well. Everyone shares in looking after the young, with the mother only holding onto her children for a single day before letting others help her out - and seeming to almost lose interest in carrying them altogether after the first week.
The total population of buffy-headed marmosets is no more than about 2,500, and it's thought to be dropping. Since they only live in a small area, it doesn't take much to destroy their native habitat, which is under threat from agriculture and urban expansion as much as from more obvious threats such as mining. Because of their preference for very specific gums, even the introduction of eucalyptus plantations in the area has threatened their survival. As a result, this is the first endangered species in my survey of the marmoset family so far. It won't be the last.
In most respects, buffy-tufted marmosets are very similar to their buffy-headed cousins, and there's no doubt that the two are very closely related. It's almost impossible to tell the young apart, although the adults of the former have much darker bodies than the latter, off-setting their cream-coloured ear tufts and yellowish foreheads. They can interbreed to produce fertile offspring, but then, so can some of the others, and, so long as it doesn't happen very often, that's no longer considered proof of anything.
Like the buffy-headed species, buffy-tufted marmosets eat gum primarily from the ara a gato vine, although they may have a slightly broader selection of alternatives. They have also been observed following army ants around, apparently to eat any other insects or spiders that might be flushed out by the ants' passing. They live in similar-sized groups to buffy-headed marmosets, and live in similar terrain, although these days typically above 600 metres (2000 feet), probably because there's more human settlement in the lowlands in their part of the country. Groups include a single breeding male, and usually just the one breeding female, although polygynous arrangements have also been seen.
As we might expect, the ongoing survival of buffy-headed marmosets is presents a similar challenge as it does for their more northerly relatives. They do, however, appear to be more numerous, and in 2008, they were removed from the official endangered species list. Even so, their population is declining as their forest habitat is progressively destroyed, and there is no guarantee that they won't return to that status in future years.
[Photos by Frank Wouters, Giovanni Mari, and Kevin Ross, from Wikimedia Commons].