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).

Sunday, 2 March 2014

The Kindness of Wallabies

The most dramatic encounters between wild animals often involved physical conflict of some kind. Animals fight for all manner of reasons, by no means limited to the eternal struggle between predators and prey. Even within the same species, there is often conflict, typically over access to things such as food or mates.

Such interactions are called "agnostic". (In an oddity of the English language similar to the flammable/inflammable divide, "agonistic" and "antagonistic" sound like they should be opposites, but actually have exactly the same meaning). In the case of solitary animals, they are often the main form of interaction within the species other than mating. But, for animals that live in groups, it necessarily has to be different.

Even if we ignore any breeding season, when competition, especially among males, is likely to be intense, animals within a group are still likely to come into conflict from time to time. In order for the group to remain cohesive, there has to be some mechanism to defuse this tension, to ensure that the pressures of competition don't outweigh the benefits of staying together. Such "affinitive" behaviour helps strengthen social bonds, and reduce stress.