Pliocene (Pt 15): Life on the Australian Grasslands

Kolopsis, a diprotodontid

At the dawn of the Pliocene, Australia was a relatively green continent, with plenty of rich, tropical and subtropical, vegetation. That changed as millions of years passed, with the continent becoming steadily drier and the grasslands and semi-desert of the Outback came into being. This was, of course, bad news for many of the animals that had lived there in the wetter past, many of which went extinct, but it also saw a noticeable increase in the number of grazing animals, for which wider grasslands were clearly a boon.

Elsewhere in the world, this sort of thing was benefiting animals such as horses, goats, and antelopes. But Australia was different. It wasn't, of course, the only island continent of the day, but it was the oldest by some margin, having separated from its neighbours long before South America split from Antarctica, or before animals stopped crossing between Eurasia and North America (even ignoring the Ice Age crossings of the Bering land bridge, which were, at this point, still in the future).

Scaring Off Snakes

Animals would, on the whole, prefer not to be eaten. As a result, they have evolved a number of ways of avoiding this fate. Being particularly large and fearsome is one tactic - very little eats lions, after all - but that obviously won't work for more most creatures. Many other defensive measures are passive, such as camouflage, or involve hiding or only coming out at a time of day when the local predators aren't around much.

An approach that's essentially the exact opposite of camouflage is the "aposematic display", in which the animal has stark, highly visible, colour markings that warn predators it is dangerous. Of course, you really need something to back this up, or the predators will eat you anyway, and, moreover, find you quite easily. Among mammals, among the clearest example of this are the skunks, with their dramatic black-and-white colouring that warns potential predators that they might get a face full of stink if they try anything.

Pandas in Cages

Zoos have changed a lot since their inception in the 19th century. (Obviously, collections of wild exotic animals have been around for thousands of years, but the first "zoological garden" in the modern sense was arguably London Zoo, which opened its gates to the public in 1848). For a long time, animals were literally kept in cages, the easier for the public to view them. Conditions were, from the animal's perspective, for the most part pretty grim.

There are doubtless many zoos across the world, especially in poorer countries, where things haven't improved all that much. But, at least in the West, things have changed significantly. Animals frequently get the chance to roam in outdoor enclosures with grassy environments, rocks and trees to climb, ropes or other toys to play with, and so on. Furthermore, modern zoos do have an important role to play in issues such as conservation - there are species in the world today that would have gone entirely extinct had not some of them been kept in zoos. (For what it's worth, two mammal species - a deer and an antelope - are currently listed as "extinct in the wild" by the IUCN. Re-introduction efforts are underway for both, but it's a slow and difficult process).

Pinnipeds: The Difference Between Seals and Sea Lions

Otariid (above) and Phocid (below)

Mammals, being air-breathing animals, mostly live on land. Even when they hunt or graze for food in the water, they generally return to dry ground to do things like sleeping. Nonetheless, an aquatic lifestyle has evolved several times amongst mammals, and four of those lineages survive today. Perhaps the best adapted, because their young are born able to swim, are the cetaceans (whales, dolphins, etc.) and the sirenians (such as manatees). The other two are sea otters and pinnipeds.

There are three recognised families of pinniped alive today: seals, sea lions, and another that includes only the walrus. All are distinguished by having flippers instead of feet, and a lifestyle that requires them to climb out of the water in order to breed and raise their young.

Leaving the walrus aside, the names I've just given to the other two families (and thus the title of the post) are really a bit misleading. This is because a group of animals called "fur seals" actually belong to what I'm calling the "sea lion family" (technically the Otariidae), and in casual usage, the term "seal" is often extended to pinnipeds in general. As a result, if we really want to be accurate, we need some term to distinguish members of the other family from seals more generally. The technical term for these animals is "phocids", but other commonly used terms include "true seals" and "earless seals".

Fossilised Hedgehog Ankles?

Many moons ago, back when I was at University (and studying zoology formally, instead of just as a hobby), the great majority of invertebrate-eating, non-flying, placental mammals were placed in a single group, the Insectivores. Even then there was some suspicion that this was a bit of a "wastebasket taxon", a group of animals placed together because it wasn't obvious what else to do with them. Molecular phylogenetics and cladistics were still in their infancy in those days, and, so far as I recall, the latter was never even mentioned.

Since then, things have changed, giving us a better picture of evolutionary relationships overall. Among other things, the number of animals that really belonged in the Insectivora was whittled away, as we discovered where the things we'd thrown into the wastebasket really belonged. The rump of the group, a genuine evolutionary unit that now goes by the less accessible name of "Eulipotyphla", does, however, still contain its most familiar members, animals such as shrews, moles, and hedgehogs.

Part of the problem is that these are relatively unspecialised mammals, in the sense that they remain broadly similar to what we think the first ever placentals looked like. Moles, of course, have adapted to a burrowing underground lifestyle that makes them fairly distinctive, but this is rather less true of their relatives. This also makes things difficult when we're looking at fossils, and we're trying to figure out just how far down the placental family tree any given suspected small insectivorous animal actually is.

Socially Awkward Mice

Many mammals have complex social lives, in which they have to interact with others of their kind in a multitude of different ways. Even those that predominantly live alone still require some degree of social interaction, especially during the breeding season, or when raising young. But for those that regularly encounter other members of their species, social interaction is particularly important, and its sophistication in primates was likely one of the key factors in the rise of humanity.

In the case of humans, of course, much of our social knowledge is built up as children, learning social rules through observation and experience as well as through more explicit instruction. And, while out ability to use language to impart detailed information is something that's essentially unique to us, the need for a suitable environment to fine tune social behaviour isn't something that just arose out of nowhere. Raising an animal in isolation from others of its species might not be as cruel as trying to do the same to a human child, but that's not to say that it wouldn't have some effect on them.

Ice Age Survivors in Hungary

Asiatic wild ass

The end of the Ice Ages saw a number of extinctions sweep across the world, particularly the Northern Hemisphere. Those animals that did not die out as the climate suddenly became warmer often moved further north, changing the composition of the local fauna wherever they had previously lived. This was no "mass extinction" of the sort associated with the end of the dinosaurs, but, on a smaller scale, it was nonetheless significant.

However, as we know all too well, it was by no means the last time that animals went extinct, let alone the last time that they have been eliminated from some local area. Many of these later extinctions were, of course, due to human activity, especially as we colonised new continents or discovered new islands.

But not necessarily all of them, since the climate has not, in fact, remained constant since the Ice Ages ended. After the sudden thaw that marks the boundary between the Pleistocene and the current, Holocene, epochs, warming continued, but slowed. It reached a peak somewhere around 4,000 to 5,000 BC, and then began a slow cooling that has continued almost until the present. Even that cooling trend has had its fluctuations and reversals, most notably the Medieval Warm Period of the 10th to 13th centuries AD.