Sunday, 11 December 2016

Bovines: Prehistoric Bison and Buffalo

Steppe bison, from a cave in Altamira, Spain
In the course of this series I have described 22 species of bovine. Whether that number accurately reflects the number of species alive today depends, in part, on what you consider to be a species - not least whether or not long-term domestication is a sufficiently dramatic thing to constitute the creation a new species. But that's the number I came up with, and it's worth noting that no less than seven of these species - almost a third - are currently considered "endangered", and at high risk of extinction. Indeed, the kouprey may well already be extinct, while the species to which domestic cattle belong is extinct in the wild.

(On that last note, incidentally, I received a note after posting on the subject reminding me of the Chillingham Cattle of Northumberland. These are sometimes claimed to be aurochs, or genuinely wild cattle, but this has no foundation, and, while they have been living wild for centuries, they are actually feral animals, likely descended from escaped medieval stock).

At any rate, there have been many other species of bovine that have gone extinct over the millions of years since their first appearance, in most cases for reasons that have nothing to do with humans. So, albeit with a focus on the genuinely cow-like ones, rather than the spiral-horned antelopes and their kin, today I'm going to look at some of them.

One species that came rather close to being wiped out by humans, but wasn't, is, of course, the American bison. Before Europeans arrived, vast herds of the animals did, indeed, sweep across the American west. But bison are the only native bovines in the Americas, whereas there are many species in Asia and Africa. So how did they get there?

It turns out that they arrived fairly late on, as such things go, about a quarter of a million years ago. This is roughly the time that the last-but-one Ice Age was beginning to get under way, and the bison presumably crossed over the Bering Straits at a time of low sea level, heading from Asia into what is now Alaska, and from there to the rest of the continent. This was a hugely significant event in terms of the wildlife of North America, and is considered to mark the dividing line between the two different phases of the Pleistocene used in that continent.

The animal that headed across was not, however, the bison that we know today, but instead the steppe bison (Bison priscus). This had already existed in Asia since the Ice Ages got started, almost two million years ago, and was very widespread. This had long horns, and was probably taller than the modern bison, but not quite as heavily built. In fact, we have remarkably good information on what it looked like, because we don't just need to rely on fossils.

Firstly, they lived long enough in Europe that there are contemporary paintings of the animals on cave walls, clearly distinguishable from the aurochs that would also have inhabited the area, as well as some evidence that Neanderthals may have hunted and eaten them. And, secondly, we have bodies, preserved as mummies in deep frozen ice. The most famous of these is "Blue Babe", an intact body found in Alaska in 1979, and now on display in a museum in Fairbanks. It is, however, by no means the only such carcass, with several others known from both North America and Siberia. Much older specimens have also been analysed in sufficient detail to evaluate the average temperature of the climate in which they lived.

Steppe bison survived for a long time, eventually being replaced in Europe by the aurochs and European bison alongside which they had once lived. In America, however, they evolved into something else entirely. Here, it's worth noting that, back in my post on bison, I mentioned a couple of genetic studies that appeared to show that domestic cattle were more closely related to European bison than the American sort, while yak showed the opposite relationship. In other words, that European and American bison were not each other's closest relatives. I expressed some scepticism at the time, which was also reflected in the comments.

As it happens, just a few months after I posted that, a further study, this time using steppe bison DNA, has shown the same thing. The authors also propose (but by no means prove) that this is probably not due to the alternative explanation, that aurochs and European bison interbred significantly after their separation. The estimate here is that yak split from steppe bison, and their descendants, the American bison, around 300,000 years ago, while the last common ancestor of American and European bison lived over 900,000 years ago. But, as I say, it's still not definitive, and there's no doubt that the two kinds of bison look a heck of a lot more like each other than either of them looks like a yak.

Anyway, back to America. Here, the steppe bison evolved into the long-horned bison (Bison latifrons). This was enormous, standing as much as 250 cm (8 feet) high at the shoulder, and with whopping horns stretching up to 210 cm (7 feet) from tip to tip. They seem to have been widespread, and, perhaps pushed south by the advancing ice sheets, apparently reached as far as Georgia and southern California.

It's possible that they lived alongside other bison species, which left no modern descendants, but at some point, it's thought that they evolved into the so-called ancient bison (Bison antiquus). This looked much more like the modern species, although it was still slightly larger, and with longer horns, than the living animal. It was these animals that gave rise, as little as 10,000 years ago, to the American bison we know today. Ancient bison lived long enough to encounter, and be hunted by, the earliest Americans, with kill sites reported from Florida to Washington.

We know that, at at least some sites, they ate significantly less grass than living bison do, perhaps due to lack of any alternative, and that they may have migrated between different sites throughout the year. They also suffered from bovine TB long before, so far as we know, the human sort even existed.

The long-horned bison, large though it was, was not necessarily the largest bovine ever to have lived. Although it's hard to know for sure, because, honestly, there's not much difference, that honour may go instead to the similarly-named long-horned buffalo (Pelorovis antiquus). This lived throughout much of Africa during the Pleistocene Ice Ages, and had huge, sweeping horns similar in shape to those of modern water buffalo. It is likely a close relative of living African buffalo, and, presumably, the Asian species as well, and may have died out as recently as 2000 BC.

The long-horned buffalo is just one species of Pelorovis, as traditionally conceived. The others, however, lived somewhat earlier, during the late Pliocene and early Pleistocene around 2 million years ago, and were slimmer, with longer limbs, and horns that curved around and downwards, rather than out to the side and upwards. It has been suggested that the two types are, in fact, not close relatives at all, with the older forms possibly being the ancestors of Bos, the modern genus that includes domestic cattle and animals such as banteng and gaur. The discovery of a 1-million year old fossil with features said to be half way between early Pelorovis and modern Bos supports this argument.

An alternative candidate for the ancestor of our modern cows is Leptobos, which lived at around the same time, but further north, in open habitats from Europe to China. This was a more cow-like in general form, but quite a bit smaller, and with horns that grew in a more vertical direction than did those of Pelorovis. Features of its teeth and skeleton suggest, as we might expect, that it ate a lot of grass and was a moderately fast runner.

The earliest known of the cow-like bovines was Parabos, known to have lived in Europe between about 5 and 2.5 million years, during the late Miocene. It already had the bulky form of the modern species, and very large backward pointing horns, and so may have had a similar lifestyle, but we know relatively little about it. Spiral-horned antelopes, on the other hand, seem to go back a little earlier, to perhaps 6.5 million years ago, and originated in Africa, as did the ancestors of nilgai and four-horned antelopes, which are today only found in Asia.

Genetic information, however, suggests that the bovines represent the very earliest branch in the broader family tree of the horned mammals, so that their ancestry must go back some considerable way further than that. The earliest forms that we have any confidence are truly bovine, rather than some other kind of bovid, are represented by animals such as Miotragocerus, living in Europe and Asia up to 20 million years ago. This was a fairly small, antelope-like animal, in many respects resembling the modern four-horned antelope, apart from the admittedly significant fact that it had only had two horns. It seems to have fed on relatively soft plants, rather than grasses, and, judging from the shape of its hooves, may have lived in damp or even marshy environments.

Eotragus, living even earlier, has also been suggested as a possible bovine, although it's likely so close to the origin of all bovids that it's hard to know for sure. Our best bet is that the first bovines appeared not much more than 20 million years ago somewhere in southern Asia, with a location south of the Himalayas being the most favoured option. From there, they spread out to reach Europe and northern Asia very rapidly, with Africa following a few million years later.

[Photo by "Rameessos", from Wikimedia Commons; original artist unknown, but safely out of copyright. Cladogram adapted from Bibi 2013 and Stankowich 2009.]

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