Miocene (Pt 15): The Coming of the Rain Shadow

Ramoceros

As the world passed into the Late Miocene, worldwide temperatures dipped from the high summer of the Middle Miocene towards... well, if not autumn, perhaps a late summer, since the world was still overall, probably warmer than it is today. For North America, however, perhaps a more significant effect was caused by the slow shuffling of the continents. A large chunk of rock that corresponds, broadly speaking, to present-day California began to tilt, causing the eastern edge to rise up.

We're not talking a huge tilt, of the sort that you'd notice if you were standing on it - it's been estimated that the eastern end was rising by somewhere around 0.25 mm (1/1000 of an inch) per year relative to the western one. Give it a few million years though, and that starts to be noticeable. While this process really only accelerated during the subsequent, Pliocene, epoch, even at this time, the rise of the eastern end of that block of crust was slowly, but surely, creating what we now call the Sierra Nevada Mountains.

Miocene (Pt 10): The Beasts with Three Horns

Syndyoceras

At the dawn of the Miocene, 23 million years ago, North America was, in many respects, quite different from the way it is today. It was, at the time, separated from South America by a wide expanse of sea, and, perhaps more significantly, the Sierra Nevada and Cascade Mountain ranges, which today run from southern British Columbia down into Mexico, did not yet exist as any more than a series of particularly high hills. While the Rockies were already mountainous, so that there was at least some rain shadow, in general, the continent was wetter and more forested than today, lacking the great grassy prairies that are so dominant today.

As the climate warmed, sea levels rose, flooding low-lying coastal regions and turning major valleys into bays. For much of the epoch, for instance, Florida was almost entirely underwater. Further north, however, it was still connected to Asia (and Greenland, although that's less significant in the grand scheme of things). Perhaps because the land bridge was both far to the north and relatively mountainous, not many animals seem to have crossed it, but that changed in the Early Miocene as the climate began to improve, and only increased as the epoch wore on. And, for whatever reason, most of them were heading east - into America.

Pleistocene (Pt 9): Before the Bison

Megatherium

The Pleistocene in North America can be divided into three 'land mammal stages', defined by the kinds of animal that inhabited the continent. The first, which was only recently added to the Pleistocene, is the one that comes before the Ice Ages proper, so that the Ice Ages themselves are, as I mentioned in part 8, divided into just two. As I said back then, the first of these starts with the arrival, not only of the ice, but also of the first American mammoths. The second, called the Rancholabrean after the famous tar pits, is much shorter, and essentially refers only to exceptionally bitter cold of the Last Ice Age. In terms of animal life, though, it is defined by the arrival of bison.

Today, when we think of America before the white man arrived, vast herds of bison are often part of our mental picture. And that, on the whole, is pretty much accurate, and is why their first appearance is deemed so significant in the ongoing evolution of North American wildlife. But what was the American wilderness like before there were any bison? What dominated the continent during the earlier, "Irvingtonian", stage?

As always, of course, the most common animals were the smallest. But, important though the teeth of (for example) voles are for the precise dating of geological deposits from this time, the eyes of any time travelling visitor would inevitably have been drawn to much larger animals. Obviously, mammoths and mastodons are a large part of the answer, and the presence of large elephantine animals crossing the plains would have been enough to tell our time traveller that, while he might still be in Kansas, it isn't really the same one he left. But there are many other beasts that would provide just as quick a clue.

Deer, peccaries, and horses are all examples of animals that aren't so unfamiliar today, although in many cases, the exact Pleistocene species were different from those alive now. Horses, for example, had, by this point, evolved to the modern single-toed form, but the last North American native horses died out around 12,000 BC, when the Pleistocene ended, leaving the ancestors of the domesticated forms behind in Asia. Exactly how many species of wild horse there were in America at this time, though, is unclear, with as many as fifty having been named - in my view, it's somewhat unlikely that they're really all distinct. There were, however, at least two broad types, one similar to the modern domestic horse, and the other, the so-called stilt-legged onager (Equus francisci, among others), which looked much more like a wild ass.

Pronghorn populations

The antelopes are not a true group of animals; generally speaking, the term just refers to any member of the cattle family that isn't a cow, sheep, or goat. There are many different kinds of antelope, found not only across Africa, but also in Asia.

Under this definition, however, the pronghorn (Antilocapra americana) is not really an antelope. At first glance, this North American animal certainly looks like one, with cloven hooves, horns, and a four-chambered stomach. But a closer look reveals some differences. A look at the cloven hooves of any member of the cattle family reveals that they have four toes: two that form the hoof itself, and two smaller, vestigial ones that don't reach the ground. In the pronghorn, these evolutionary remnants have vanished altogether (although there are still cannon bones within the leg), so that they only have two toes on each foot.

The horns are different, too. In members of the cattle family they are never branched - unlike the antlers of deer - but the male pronghorn, as its name suggests, has a forward-pointing prong on each horn. Furthermore, they shed the outer layer of skin from the horns each year, like deer do, but quite unlike cattle. (Unlike deer, however, they don't shed the horn itself).

All of this places the pronghorn in a family all of its own. While the pronghorn family today consists of just one species, in prehistoric times, it was much larger, representing a branch away from the line that led to true antelopes that crossed over into the Americas, and developed on its own.

Pronghorn     Cattle Family     Deer Family
                                    etc.
    |              ^                 ^
    |              |                 |
    |              |                 |         Camel Family
    ----------------                 |              ^
           |                         |              |
           |                         |              |
           ---------------------------              |
                         |                          |
                         |                          |
                         ----------------------------
                                      |
                                      |


The pronghorn is not an endangered species; it is present in large numbers across a wide swathe of the western parts of the USA, and also into Canada and Mexico in places. This doesn't mean that it's had everything its own way; as with many animals in North America, and elsewhere, it used to be present in much larger numbers in the nineteenth century than it is now. But, if anything, it is more numerous now than it was at the end of that century, due in part to the introduction of such factors as regulated hunting seasons.

However, it can still be useful to look at the various factors that affect the pronghorn population, some of which can be extended to other, similar, animals. One study looked at the population of pronghorn in western Nebraska from 1953 to 1993, to see what conditions favoured the animals, and which didn't. It turns out to be quite a complex story, indicating that conservation isn't always a simple matter of fixing one or two problems and getting the same result everywhere.

One does, incidentally, have to be a bit careful with this sort of retrospective study. Because it's looking back over a long period, there might be all sorts of differences in the way that the individual studies that comprise it were conducted. Indeed, in this case, the 1993 cut-off was chosen because population studies after that date were just too different to be compared with the older ones. But, bearing that in mind, some trends do appear to be visible.

For many animals, the advance of agriculture is generally a bad thing. It disrupts the natural habitat, and leads to competition with livestock if you're a herbivore, or being shot by farmers if you're a carnivore. Pronghorns, however, don't really seem to have this problem. There is no doubt that they are more numerous in the wilder parts of their range, but, at the same time, the presence of nearby agriculture seems to help protect their population from such problems as unexpectedly harsh winters.

There are probably a couple of reasons for this. Importantly, pronghorns and domestic cattle tend to eat different kinds of plants, so that the cows aren't eating all of the pronghorns' food. Indeed, if anything, cattle grazing promotes the growth of herbs and grasses that the pronghorns can then feed on, so that in years when the cattle suffer, the pronghorns also struggle. Agriculture also increases the presence of winter wheat, and of cattle feed such as alfalfa during the harsh winter months, both of which are available for the pronghorns as well as their intended recipients.

However, an interesting thing happens when populations get very small, as happened at times for the pronghorns over the forty years examined by the study. Normally, when populations in a given area get larger, each animal produces less offspring that survive to adulthood. There is, after all, only a certain amount of food to go around, and animals are effectively competing with members of their own herd for survival. As the population shrinks, there is less stress on individual animals to find food, shelter, or other resources, and they produce more offspring.

But if the population shrinks enough, something called the Allee Effect often kicks in. Here, the usual trend of increased reproduction for reduced population density reverses, making the population dip still further. Continue this for long enough and the animal is wiped out in that area - and, if the area affected is big enough, that may even mean extinction. Perhaps the major reason behind the Allee Effect is inbreeding, due to a limited choice of mates.

There may, however, be other reasons as well. In the case of herd animals like the pronghorn, for instance, because the herds themselves will get smaller, less of the animals will be available to keep guard against coyotes while the others are feeding. We can also see this effect in play in one of the world's rarest large mammals, Przewalski's gazelle (Procapra przewalskii), which now lives only in a small area around a single lake north of Tibet (and, which, incidentally, very clearly is threatened by advancing agriculture). Now present in numbers too small to properly guard against the local wolves, they are increasingly seen sidling up to herds of the closely related, and more numerous, Tibetan gazelle, so that at least somebody is on the lookout, even if it isn't a member of their own species.

In the case of the pronghorns of western Nebraska, the population densities did, at times, dip low enough for the Allee Effect to kick in. But, in every case, the population recovered as conditions improved in subsequent years, or the hunting season was temporarily halted. So they're hardly in any danger of extinction yet. But even here, exactly how far a population has to fall before problems arise varies considerably from area to area. In this case, where agriculture is more common, the pronghorn herds could reach far lower population densities before the Allee Effect began to take its toll, but for other animals, it could be quite the reverse. And even where it isn't, it may mean that you have to take conservation action sooner in wild areas than in better farmed ones - quite the opposite of what you'd normally expect.

[Picture from Wikimedia Commons]