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.

Sunday, 13 April 2014

Pleistocene (Pt 14): Ice Age Africa

Giant warthog
In the earlier parts of this series, when I described the animals of Ice Age Europe, I said that the continent, at that time, had a feel reminiscent of present-day Africa. But, if that's so, what was Africa like during the Ice Ages?

The answer is, perhaps disappointingly, "not that different to how it is now". Indeed, of all the settled continents, it's probably Africa that has changed the least since the Ice Ages. You might think that this has something to do with Africa being close to the equator; in particular, that it's too close for whopping great sheets of ice to have rolled across the countryside.

Which they didn't, so that much is true. Indeed, the southern hemisphere in general had far less ice cover than Europe, Asia, and North America. That's due mainly to the way that the continents happen to be arranged, with glacial ice sheets only being able to get as far as southern South America (there are fjords in places like Tierra del Fuego). Presumably, sea ice extended much further across the Southern Ocean than it does now, but that would have little effect on land-based animals.

But that's not to say that Africa, or Australia, were unaffected by the Ice Ages. Africa was colder than it is today, and, to begin with at least, rather drier, too. The Sahara and Kalahari deserts were more extensive than they are today, with the semi-desert belt of the Sahel being quite a way south of its present position, running through what are now fairly lush countries such as Guinea, Nigeria, and northern Kenya.

Sunday, 6 April 2014

Deep Dives to Dark Depths

There are something like ninety species of cetacean, which are usually grouped into fourteen families. Perhaps the most obscure and little known of these families - and among the most obscure of all mammal families - is that of the beaked whales. It's actually quite a large family, with over twenty species, representing almost a quarter of all known cetacean species, and around a half of all those that are noticeably larger than dolphin-sized. And, since they typically weigh a ton or more, they're hardly small themselves.

The main reason we know so little is where they live. These are deep-water animals, that rarely come in close to shore, and so just aren't seen very often. Combine this with the fact that they're mostly of little interest to whalers, and it becomes apparent that studying them is neither easy, nor likely to be commercially motivated.

Saturday, 29 March 2014

The Antlers of Early Deer

Dromomeryx, a palaeomerycid

Probably the most distinctive thing about deer is that they have antlers (and not horns). However, especially when we're looking at fossil species, it's important to remember that not all deer have antlers. Granted, the enormous majority do, and those that don't have lost them during their evolutionary history, rather than being holdovers from some ancient form that never had antlers in the first place. (This isn't true of musk deer, but that's partly why they aren't considered to be true members of the deer family).

And even that's assuming your fossil belongs to a male. (Or a female reindeer, oddly enough).

Antlers first appear on deer fossils in the early Miocene, on the general order of 20 million years ago. Still, in the grand scheme of the Age of Mammals, which has so far lasted 66 million years, that isn't all that long. It's a lot closer to us than it is to the dinosaurs, at any rate. So we ought to have a reasonable idea of what some of these first antlered forms look like.

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.

Sunday, 16 March 2014

Clearing the Wastebasket

Vulpavus ovatus, a "miacid"
Apart from whales, dolphins, and their kin, all large carnivorous placental mammals alive today belong to a group called, appropriately enough, the carnivorans. The carnivorans include several families, and they can be divided into two smaller groups, which are broadly described as either "dog-like" or "cat-like". The dog-like carnivores include, besides the dogs themselves, such families as the bears, weasels, and seals, among others. The cat-like carnivores are a slightly smaller group, including cats, hyenas, and a host of animals that look more or less like mongooses.

But, if we go back in time, we find a number of large, carnivorous mammals that, for various reasons, don't really fit. Some of them, such as Hyaenodon, are different enough that we can say that they're definitely not carnivorans in the modern sense. Others, such as sabre-tooth cats and dire wolves, clearly are, because they're so similar to animals we have around right now. And then there's others that are kind of in-between. So where do we draw the line?

One way is to look at what's called the "crown group". The idea here is that you take every living representative of the group you're interested in (in this case, carnivorans as a whole) and trace it back to its last putative ancestor. Anything descended from that animal, including all the extinct ones, belongs to the crown group, and anything that isn't, doesn't.

Sunday, 9 March 2014

Clicking Clans

There are two broad types of whale alive today: those with teeth and those without. Generally speaking, it's the members of the latter group that are physically larger: these are the great filter-feeding, krill-slurping, baleen whales. The toothed whales, in contrast, tend to be much smaller. The biological group of toothed "whales", after all, includes dolphins, porpoises, and all their strange fresh-water relatives. Even the big ones, such as killer whales, are smaller than the great majority of baleen whales.

But there is an exception, and its by far and away the largest of all the toothed whales. This is, of course, the sperm whale (Physeter macrocephalus), and, while it's merely average when compared with the great baleen whales, at up to 50 tons in weight, it's nothing to be sniffed at. (The second-largest, incidentally, is probably the relatively obscure Baird's beaked whale, at 12 tons, which is itself about half again the weight of the animal in the #3 spot).