Sunday 20 April 2014

Mini-Monkeys: Madeiran Marmosets and More

Santarem marmoset
Back in the 1970s,  a survey of the monkeys of South America identified just two species of "typical" marmoset living in the Amazon jungle: a naked-eared form, and a tassel-eared form. Both species were fairly variable in appearance, and each was considered to include three distinct subspecies. Today, better knowledge of things like their genetic make-up means that we have elevated all of those subspecies to full species status.

The "tassel-eared marmoset" had first been described in 1811 by Geoffroy Saint-Hilaire (although the date is officially given as 1812, because it took a year to get the description published). Because it was the first to be so described, the other two were considered subspecies of it, rather than the other way round. Once they were split off, we were left only with the species to which the originally collected specimen belonged. While still sometimes called the "tassel-eared marmoset" or some variation thereof, it is now more commonly called the Santarem marmoset (Mico humeralifer) to distinguish it from its kin.

It turns out, however, that most of the studies on tassel-eared marmosets had actually been done on one of the other subspecies. As a result, despite first being described over two hundred years ago, we still know very little about it. Named for the city closest to where the first specimen was collected, it lives on the south bank of the Amazon River between the Tapajós River in the east and the much smaller Maués River in the west (not visible on the map linked to - it's about half way to the Madeira). It's less clear exactly how far south it gets, although it's probably no more than about 60 miles. Which isn't exactly a huge area.

Physically, it's pretty easy to tell apart from the silvery marmoset, its taxonomic counterpart among the naked-eared species. For a start, it doesn't have hairless ears, instead having large pale tassels sprouting from each. It's also a completely different colour, having a sort of mottled black-and-white coat, with faint white stripes on the thighs and rings on the tail.

Presumably it behaves in much the same manner as its close relatives, and we know that it eats more or less the same things, and lives in groups of a dozen or so individuals, but that's about the extent of it. We don't even know if it's endangered or not, partly because of the uncertainty about exactly how big an area it lives in. (Although it is, by Amazonian standards, quite an urbanised area, so the signs aren't good).

In 1842, Johann Wagner described a second tassel-eared species, the golden-white marmoset (Mico chrysoleucus). Demoted to subspecies status later on, it's now back where it belongs. Colour aside, it looks pretty much exactly like the Santarem species, and it lives only a little bit further west, on the east bank of the lower Madeira River. It's basically white over most of the body, with pale golden-orange limbs and faint gold stripes on the tail. And... yeah, that's about it. That's what we know about it.

Golden-white marmoset
The one we do know something about lives a little further south, between the Aripuana and Roosevelt Rivers. When Philip Hershkovitz merged the Santarem and golden-white species in 1977, he also identified, for the first time, what he believed was a new subspecies of the same animal. Since raised to full species status, although it's often called the "Rio Aripuana marmoset", an alternative name is Hershkovitz's marmoset (Mico intermedius), in his honour. (I'll note, in passing, that Wikipedia currently appears to have the scientific names of all the three species I've listed so far slightly wrong; this seems to be due to someone misreading the sources in the articles).

Hershkovitz's marmoset, or whatever we want to call it, has pale silvery forequarters that fade, though mottled patches around its middle, to plain dark brown hindquarters and tail. The tassels on the ears are relatively small, compared with those of the other two species. It likes to live among dense vegetation, which means, perhaps counter-intuitively, patches of relatively open woodland, caused by past human activity from the which the forest has not fully recovered, or just clearings made by fallen trees or large patches of water. These sort of environments have heavier undergrowth than the primary forest, since sunlight can actually reach the ground. Since the marmosets don't live in the highest branches of the forest canopy, this heavy undergrowth gives them places to forage and bundles of tree creepers and the like in which to spend the night.

The area in which they live also has clear wet and dry seasons, and the marmosets' diet changes through the year to reflect this. There are some trees, such as Cecropia palms, that provide fruit more or less year round, but most don't, and even those that do have the least fruit towards the end of the wet season. At this time of year, therefore, the monkeys switch from a fruit-based diet to one where up to two-thirds of their food comes from gum. Although the fruit is presumably easier to get at when it's around, this again shows Amazonian marmosets relying on the constant availability of gum as a back-up plan that keeps them fit through hard times. Like other marmosets, they also eat a wide range of insects to get their protein.

Hershkovitz's marmosets live in groups of up to fifteen, each claiming an area of around 28 hectares (70 acres), although there's likely quite a bit of variation in this. For the most part, they stick to the central 50% or so of this area, only occasionally patrolling the boundaries. However, should they encounters others of their kind at the edges of their territory, they are apt to scream at one another for anything up to three hours at a time, all the while glaring at their rivals, scratching their tails (this apparently looks menacing to them), and waving their genitals while 'mooning' each other. If all of that somehow fails, a good chase is clearly in order, although outright violence seems rare.

As is typical for marmosets, only one female in the group gives birth, and everyone, males included, shares in looking after the resulting twins. However, there does appear to be some evidence that the groups are polyandrous, with the breeding female being sexually active with at least two of the adult males. This likely gives her, and her children, more protectors, since her partners aren't quite sure which of the kids are theirs.

Back in 1977, that was where we stood. Since then, however, not only have we increased the original two species to seven, we have also found entirely new forms of marmoset that we just didn't know about until recently. The first to be announced were in 1992, consisting of one "bare-eared" species, and one "tassel-eared".

The bare-eared one is the black-headed marmoset (Mico nigriceps) which, as the name suggests, has black skin on its face, rather than the pink or orangeish colour that most others have. The fur is mostly grey, with yellowish patches on the hindlegs, and a black tail. It lives a little further up the Madeira River than the golden-white marmoset, and, like it, inhabits jungles east of that river. The other is the Maués marmoset (Mico mauesi), which lives close to the Santarem species, but apparently in denser forest, with more high branches but less undergrowth. It is dark in colour, with some pale patches, and a black ring around its face, although the most distinctive feature is that the tassels on its ears are blackish and point straight up, rather than flaring out to the sides.

The following year, in 1993, we discovered another one. Sort of. Marca's marmoset (Mico marcai) was described on the basis of three skins collected near the mouth of the Roosevelt River. These showed an animal with a brownish body and chestnut hands, head, and tail, as well as a dark, hairless face and ears with a pale patch on the muzzle. None were seen alive for twenty years, when a few were finally spotted in 2013 near where the skins had been collected.

Moving forward five years, the next to be discovered was the Satéré marmoset (Mico saterei), in 1998. It has silvery-gold fur on the forequarters and head, but is dark chocolate brown elsewhere, with the two colours contrasting sharply. It lives between two eastern tributaries of the Madeira River, apparently in both disturbed and undisturbed forest. Like all the other forms discovered since the Maués species, it has bare ears, but its genitals are said to be somewhat unusual. For one thing, they're bright orange, which they normally aren't in marmosets, but they also have strange looking flaps of skin hanging down from them (from the scrotum in males, and just in front of the genitalia in females). Nobody knows why, but perhaps they like showing off.

Rondon's marmoset
Then, in 2000, Marc van Roosmalen and co-workers announced the discovery of two new species, the Manicore marmoset (Mico manicorensis) and the Rio Acari marmoset (Mico acariensis). They live near the rivers for which they are named, which are broadly in the same area that we've already been talking about, in between the Tapajós and Madeira Rivers. They also look somewhat similar, with grey bodies and black tails, although there are clear differences, with the former, for example, having rusty-orange thighs. I have to mention here, though, that van Roosmalen does not have a perfect record when it comes to identifying new species - some have been overturned on closer examination - and it has been argued that the Manicore marmoset, at least, may just be a slightly odd-looking Marca's marmoset. For the time being though, these two species are generally accepted as valid.

Moving almost up to the present day, we have the 2010 announcement of Rondon's marmoset (Mico rondoni), named for the Brazilian explorer Cândido Rondon. Oddly enough, we know more about this than we do about most of the earlier discoveries. That's partly because these marmosets were actually first spotted in 1985, when they were assumed to be Snethlage's marmosets, despite living much further west, on the mid-reaches of the Madeira River. (We now know that Snethlage's species only lives east of the Tapajós).

Rondon's marmosets are a solid grey in colour over most of the body, noticeably darker than the silvery marmosets to which they appear most closely related. They have a black tail, brownish markings on the hind legs, and black fur on the top of the head, with a distinctive white spot on the forehead. For the most part, what we know of them matches what we know of other species of Mico, such as the silvery and Hershkovitz's marmosets. One interesting feature, though, is that nearly half of them seem to live together with groups of Weddel's saddle-back tamarins, a species I will come back to in a later post.

Despite travelling together, and eating more or less the same food, the two species avoid competition by foraging in different ways. The tamarins look for food in holes and crevices about 6 metres (20 feet) above the ground, while the marmosets search along branches about 11 metres (36 feet) up. Furthermore, the marmosets, being marmosets, gouge the bark off trees to get at the gum, while the tamarins look for easier sources - not that they'll avoid eating at the gashes the marmosets have made for themselves, if they can get away with it.

Shortly, I will turn to look at the other half of the marmoset family, including those tamarins. But, before that, there are just two more marmosets to go...

[Photos by Fábio Manfredini and "DuSantos", from Wikimedia Commons. Painting by Joseph Wolf, copyright expired.]

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