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.

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.

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).

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.

Mini-Monkeys: Marmosets of the Southern Atlantic Forest

Geoffroy's marmoset

Last month, I looked at the three species of marmoset inhabiting the northern half of the Atlantic Forest along the east coast of Brazil. Further south, the forests naturally become cooler - which is to say, that they're merely subtropical, rather than fully tropical. All of the marmosets inhabiting the Atlantic Forest are closely related, belonging to the genus Callithrix, and many of them are capable of interbreeding with one another. They all descend, most likely, from a single species that lived around the early Pleistocene, just before the Ice Ages, and which presumably crossed over what is now the relatively sparsely forested cerrado from the more fertile Amazon to the north and west.

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.

Pleistocene (Pt 13): Giant Bears and Speedy Cats

The most famous predators of the North American Ice Ages were surely Smilodon and the dire wolves. But, of course, these were hardly the only carnivorous mammals on the continent at the time. For one thing, it's worth remembering that something like half of the species in North America in the mid Pleistocene are still around today. Even where modern species had yet to evolve, their immediate ancestors often had, and would have looked very similar to modern forms. So Ice Age North America would have had its share of coyotes, bobcats, and cougars, not to mention smaller creatures like badgers. And let's not forget jaguars, which entered South America from the north early on in the epoch, and are still found in parts of Mexico today.

But surely Smilodon was the most fearsome predator of its day? Well, probably... kind of. But it most certainly has a contender for that crown.

Hunky Mother Squirrels

In general, among mammals, males are larger than females. We can see this in our own species: on average, men are taller than women, and, again on average, they also tend to have more muscles and a greater overall body mass. Compared with some other species, though, the difference isn't all that great. Seals are an extreme example, with males often many times larger than females. But, in many cases, the difference is more like that with us: it's a noticeable difference, but not a dramatic one.

The reason for this is generally thought to be male-male competition. Males often have to compete for mates, and the bigger you are, the more likely you are to win that contest. It doesn't even have to be physical; the mere fact that you've been able to 'waste' calories on bulking yourself up proves that you're physically fit, and therefore an attractive mate for females. Even the fact that you've survived long enough to grow to large size is a point in your favour.

How this manifests will depend a lot on the mating system of the species concerned. In species that are monogamous, it's not such a big deal, and it's even less so if females are highly promiscuous. It's most noticeable in polygynous species, where one male monopolises as many females as he can, perhaps defending a harem from all comers. This is what happens in seals, and it's also true, for example in deer. Stags, for instance, not only are quite a bit larger than hinds, but they have huge antlers that not only let them fight off their rivals, but also further demonstrate how many calories they've been able to waste growing something huge just for the heck of it.