Pinnipeds: The Evolution of Seals

Acrophoca

There are seventeen species of "true" seal alive today. Most of them are reasonably widespread and numerous animals, and, despite the effects of seal hunters, are not under any particular threat. Populations of hooded seals are in a steady decline, but, although they are still hunted in places, a 1985 ban on the use of seal fur across most of Europe has hugely reduced the demand for the animals; the main reason for the current population decline seems to be the loss of their icy breeding habitats. The two species of living monk seal, and the Caspian seal, have not been so lucky, and are all considered endangered species. And an eighteenth species, the Caribbean monk seal, went extinct in the 1950s.

Early attempts to deduce how these various species relate to one another identified a number of different sub-groups among the seals, based largely on anatomical resemblances. On this basis, it was thought for a while that the Arctic and Antarctic seals were each others' closest relatives, with the more temperate species representing older, and perhaps more ancestral, forms. But this picture began to change in the 1990s, and its replacement has subsequently been confirmed by a number of different genetic analyses. While the exact details have still not been fully settled, we do now know the broad pattern of what happened, and I've provided one consensus family tree below. Using estimated rates of evolutionary change, calibrated with the age of known fossils, we can also get a rough idea of when some of the key events in this history unfolded.

First of the Guinea Pigs

Wild Brazilian guinea pig (Cavia aperea)

Domestic guinea pigs (Cavia porcellus) are widely found as pets in the western world, and are still used as meat animals in parts of South America. But, of course, they didn't come from nowhere, and six species of wild guinea pig are also generally accepted to exist, two of which have only been discovered since the 1980s. They are fairly widespread across tropical and subtropical South America, and it's still not entirely clear which one is the ancestor of the domestic sort, although suspicion tends to fall on the montane guinea pig (C. tschudii) of the Andes.

The guinea pig family (Caviidae) as a whole doesn't include many more species, and most of them, such as the rock and mountain cavies, are physically rather similar to their better known kin; exceptions include the large capybaras and the long-legged and oddly rabbit-like maras. In the distant past, many other species existed, some of them even larger than capybaras, and positively gigantic for a rodent. When it comes to the guinea pigs proper, however, we don't really have much of a fossil history, and none of the living species can be definitively traced back any further than the tail end of the Ice Ages.

Endangered Mammals of the Caribbean

Mammals are found on every major land mass on Earth, except for Antarctica. The same is, however, much less true for 'minor' land masses. Here, isolation from the mainland means that, in most cases, there are less mammal species on islands than there are on neighbouring bits of continent. There are bound to be at least some, in other words, that didn't make it across, or that, if they did, didn't survive and where never replenished. This is even true, albeit to a lesser extent, on some (but by no means all) peninsulas, with fewer species towards the tip than at the connection with the mainland - there are, for example, fewer terrestrial mammal species in South Korea than, say, Manchuria.

There are a couple of important exceptions here. For one thing, the rule doesn't really apply to bats, unless the islands are really remote (such as Hawaii, for example), and we're obviously not talking about marine mammals that use the shoreline, such as seals. Secondly, particularly large islands, especially those in the tropics, don't have this effect at all, being able to provide a perfectly adequate base to evolve a large profusion of unique animals. Madagascar, New Guinea, and Borneo are just, perhaps, the most obvious examples of this. It's probably also worth noting that many islands, even if they don't have very many species in total, often have ones that aren't found anywhere else on Earth, the separation from the mainland working in both directions.

Do Kangaroos Chew the Cud?

Mammals that eat a lot of grass, or other relatively low-quality vegetation, have to process their food thoroughly to extract the maximum amount of nutrition. I've previously discussed how this works in some detail, but, in general, there are two approaches. In "fore-gut fermenters", the stomach is divided into multiple chambers, allowing the animal to ferment its food, then bring the solid parts back up to the mouth to re-chew ("chewing the cud"), before sending it back to finish the job. This is the system found in cows, deer, camels, and many other animals, collectively known as "ruminants".

The second approach is to place the fermentation chamber behind the stomach, down in the colon. This is less efficient, but quicker, and is the arrangement found in horses, rhinos, and a number of other animals. Rabbits are also "hind-gut fermenters" of this sort, but take the additional step of re-eating the fermented food once it passes out of their back end for the first time.

I mentioned in passing in my previous post that kangaroos are fore-gut fermenters, but that they are not ruminants, and have a slightly different system. Today, I'm going to explain what it is that they do do. (Much of what follows applies equally to wallabies, and, to a lesser extent, rat-kangaroos).

Pinnipeds: Killer Seals of the Antarctic

Leopard seal

As one might expect, seals, for the most part eat fish and squid, although the precise details do vary quite a bit between different species, based on such things as their diving ability, as well as where they happen to live. Two species of seal in the Antarctic, however, do have a somewhat different diet, at least in part.

Leopard seals (Hydrurga leptonyx) are among the larger seals, with full-grown adults ranging from about 240 to 320 cm (8 to 10½ feet) in length. While this is still quite a bit smaller than elephant seals, the largest females - which are, unusually, larger than the males - can still weigh over half a tonne. They typically inhabit frozen waters far to the south, moving over the course of the year as the ice front expands and contracts with the changing seasons. Younger individuals seem to travel further north than full grown adults, and, while the winter range normally extends only as far as the various subantarctic islands, the Falklands, and Tierra del Fuego, they are occasional visitors to continental Chile, Tasmania, and some points even further north. A couple of reports from the middle of the last century, supported by actual skeletal remains, even refer to lone individuals getting as far north as the Cook Islands in Polynesia.

Miocene (Pt 3): The Elephants Reach Europe

Gomphotherium angustidens

At the dawn of the Miocene, Europe was still relatively isolated from much of the rest of the world. The only other continent to which is was attached by land was Asia, and even with that, perhaps due to the higher sea levels and narrower land bridges of the day, there was less exchange of animals than one might expect. As a result, for a few million years at the dawn of the epoch, Europe remained a haven for animals that had died out elsewhere, or at least retreated. Tapirs, for example, were still common; today they are restricted to South East Asia and the Americas. The European rhinos of the day were also relatively small and primitive.

While the millions of years that they survived might actually have been quite notable, had we been talking about the much shorter, later epochs, in the grand sweep of the Miocene, we can say that they were quite rapidly replaced by immigrants from the east. The early tapirs were gone (although more modern forms did return for a while, much later), but the rhinos were instead replaced by larger and more sturdy, distinctly Miocene species.

What is a Subspecies?

Microtus californicus californicus

I've often mentioned in past posts the difficulty of defining what a "species" is. This, despite the fact that, of all the taxonomic ranks, it's supposed to be the one that comes closest to actually having an existence in the real world. Although there are strict rules about how we name, for example, "families", quite where we draw the lines to separate those families from one another is arbitrary. We could place raccoons in the weasel family, or we could place American badgers outside that family, but, purely for the sake of convenience, we choose to do neither of those things. (For what it's worth, though, the rules would prevent us from doing both).

But a species is meant to be an actual thing, right? Certainly, when the word was coined by Linnaeus (in the biological sense; it has older meanings, too) in the 18th century, that was his intent. A hundred years later, Darwin elaborated on the term, realising, as earlier evolutionary theorists had, but Linnaeus hadn't, that species do slowly change into new forms over time. Today, the most common understanding, outside of scientific circles, is that two animals belong to different species if they can't interbreed to produce fertile offspring.