Sunday, 23 March 2014

Mini-Monkeys: The Many Faces of the Silvery Marmoset

Silvery marmoset
The marmosets of the Atlantic Forest are, most likely, descended from a population that emigrated from the Amazon shortly before the Ice Ages. All of the other species of marmoset still live in those larger and more northerly jungles. But how many species is that?

In a wide-ranging and influential study of American monkeys in 1977, mammalogist Philip Hershkovitz identified two such species, one that had hairy tufted ears, and one that did not. He noted that the two species had more in common with one another than with their Atlantic Forest cousins. For example, while their teeth are adapted for scraping bark off trees to make nutritious gum ooze out, they aren't quite as adapted for this as they are in the more southerly species. He also noted that the two species had quite a lot of variety in their colouration, although that didn't necessarily mean much.

Since the 1970s, however, our knowledge of the miniature monkeys of the Amazon has increased dramatically. For one thing, we now place more emphasis on some of the minor differences Hershkovitz identified. For instance, today, we consider the Amazonian species to belong to a different genus from the common marmoset and its kin, using a name, Mico, first proposed by René Lesson back in the 19th century. In a similar vein, we have also raised many of Hershkovitz's subspecies to full species status. Finally, we have actually discovered species he simply didn't know about.

End result: there are now something like fourteen species of Amazonian marmoset known to science. And, given how recently many of them were discovered, there's no reason to suppose that we're done yet. There may well be more marmosets to find.

I'm going to start however, with the one we've known about for longest: the silvery marmoset (Mico argentatus). This was first described and given its scientific name way back in 1771, by Carolus Linnaeus, the man who essentially invented the idea of giving animals scientific names. A little over two hundred years later, this was the one that Hershkovitz identified as his 'bare-eared' species.

Indeed, the fact that it doesn't have the ear-tufts of the other species known at the time is immediately obvious. The monkey has silvery fur over most of its body, albeit with a black tail, but the ears and face are both virtually hairless and a distinct pinkish colour. For that matter, its genitals are an even brighter red, although attracting mates probably has more to do with scent marking.

Now that we've pared away the former subspecies, we can say that the true-and-original silvery marmosets live close to the mouth of the Amazon River, in moderately dense forests along its south bank. For reasons already noted, while they certainly do eat gum, they're not quite as skilled at getting at it as the Atlantic Forest species, which may limit their ability to survive in more 'marginal' habitats. (Insofar as such a thing exists in tropical rainforests).

Nonetheless, gum does constitute something like two thirds of their diet, being especially favoured in the dry season, when there's less fruit about. While they tend to stay close to a preferred set of gum-producing trees, they nonetheless spend much of their day wandering about, apparently in search of insects to give them some protein, although they'll eat some fruit during this time as well. This foraging takes place in the mid-level of the forest, below about 15 metres (50 feet), rather than right up in the canopy.

Each group of six to eleven monkeys travels over an area of up to fifteen hectares (35 acres), although there seems to be quite a bit of overlap of shared forest at the edge of each area. They travel less in the dry season, presumably because there's less to find, and staying close to the gum trees makes more sense. This is also where they sleep, typically high up and under coves of vines or dense foliage. As is typical of other marmosets, the groups consist of one adult male, and one, sometimes two, adult females - the others are either infants or non-breeding sub-adults.

Silvery marmosets are about as safe as anything can be in the Amazon, living in an area where logging is less intensive than it is elsewhere. There is some evidence that populations don't breed much with outsiders, even when they get the chance, and the overall population probably is declining, but slowly enough, and from a high enough starting point, that it's not yet cause for concern.

Black-tailed marmoset
Hershkovitz identified three subspecies of silvery marmoset, one of which is, of course, that described above. Of the other two, the one that's most obviously different is the black-tailed marmoset (Mico melanurus). From what he knew at the time, though, the differences were literally only skin-deep. While the silvery marmoset also has, as it happens, a black tail, the rest of the black-tailed marmoset is also much darker, and, while the ears and face are just as hairless, they're black, not pink. In all other respects, they do look quite a lot like the silvery species.

There are, however, other differences. The black-tailed marmoset is widespread, but lives for the most part along the southern margin of the Amazon Forest. Indeed, its the only "Amazonian" marmoset to be found outside of Brazil, because it also inhabits the wooded savannah that lies south of the Forest proper, in eastern Bolivia and far northern Paraguay. This may be in part because one of its main requirements seems to be heavy undergrowth, something you don't find in truly dense forest.

Perhaps because it wasn't identified as being a species until 1991, the black-tailed marmoset has not been extensively studied, and virtually everything we know about it comes from watching animals in captivity. (Although, so far as we can tell, being widespread and living well away from the bits of the jungle anyone bothers logging, it seems to be doing perfectly well in the wild). 

They appear to live in slightly larger groups, of up to fifteen members, than silvery marmosets do. We also know that they have a number of ways of showing they're upset with one another: glaring through half-lidded eyes, arching their backs and raising their hair like a cat, and raising their tails to contemptuously show off their genitals. Friendly gestures, on the other hand, include sniffing, licking, and lip-smacking, especially when they're soliciting mates.

The young are born as twins, as is usual for marmosets, and are carried by their parents for up to two weeks, which is less so. After that, they're looked after by non-breeding 'helper' adults, but even they start to lose interest after about four weeks, by which time the infants have stopped making baby-noises, and they seem to be pretty much independent by just six weeks of age.

The third subspecies identified by Hershkovitz is the imaginatively named golden-white bare-eared marmoset, sometimes instead called the "white marmoset", for fairly obvious reasons. It, too, is now a species (Mico leucippe), but it's easy to see how it could have been confused with the silvery marmoset. In fact, the only obvious difference is that the tail is a light golden colour, and not black. Other than that... slightly paler fur, and a more scarlet hue to the ears and face just about covers it.

Snethlage's marmoset
This species lives along a stretch of the east bank of the Tapajós River, one of the largest tributaries of the Amazon, and hasn't been seen elsewhere. This area has been impacted by the construction of the Trans-Amazon Highway in the 1970s, and, more recently, by tree-felling and the growth of soy plantations. From which we can conclude that, especially since it isn't a large region of forest to start with, the species is not as safe as its relatives. Although sufficiently little is known that we can't really say much more than that, while the species is considered "threatened", it's not officially endangered.

There was, however, one other type of bare-eared marmoset known to Hershkovitz. It had first been described back in 1920, but was so similar to the black-tailed marmoset that Hershkovitz didn't even consider it worthy of subspecific status. It was, he thought, no more a distinct subspecies than, say, red-headed humans are. Now that we have rather more data, it's generally accepted that it is a full species, commonly called Snethlage's marmoset (Mico emiliae). Both the scientific and common names honour Emilie Snethlage, a noted German-Brazilian naturalist of the day, and some sources prefer "Emilia's marmoset" as an alternative name.

Unlike the black-tailed marmoset, Snethlage's marmoset has a pink face, rather than a black one, and it lacks a stripe of whitish fur found on the thighs of it's relative. It's found in the eastern Amazon, roughly between the Iriri and Teles Pires rivers, which might imply that it's habits are closer to those of the silvery marmoset than the black-tailed, but, honestly, we don't have the data to know for sure.

Next month, I'll turn to the tassel-eared species, and to the many species that, back in 1977, nobody even knew existed.

[Pictures by Tony Hisgett, Hans Hillewaert, and Rich Hoyer, from Wikimedia Commons]

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