Sunday 18 November 2012

Pleistocene (Pt 5): Giant Deer and Woolly Rhinos

There were a number of Ice Ages during the Pleistocene, but the most severe of them all seems to have been the most recent one - the Last Ice Age. Or, perhaps more accurately, the Last Ice Age So Far, since there's no particular reason to assume there won't be another one along in a few thousand years time. This was the time when the earlier steppe mammoths were replaced by their more famous descendants, the woolly mammoths. But woolly mammoths did not live in isolation. With what other creatures did they share their world?

Even today, deer are relatively common animals in the wilder forests of Europe. In the Pleistocene, before the spread of farms and towns, they would have been even more so. At the height of the last Ice Age, however, there were relatively few forests in Europe, and many of the deer we are familiar with - red deer, roe deer, and so on - would have been sheltering in warmer climes. Reindeer and moose, on the other hand, were doing well, with the former, in particular, being widespread across the continent.

Throughout the course of the Pleistocene, however, there had been another kind of deer in Europe, one that is no longer with us. These were the "giant deer" of the genus Megaloceros. Some species, isolated on islands created by the rising melt-waters of the glaciers between the Ice Ages, were unusually small, but, in general, they had been getting larger as the Pleistocene went on. Before the last Ice
Age, the largest species, slightly bigger than the primitive moose of the day, had been M. verticornis. Now that was replaced by an even larger species, the Irish elk (Megaloceros giganteus).

With adult males standing around six or seven feet tall at the shoulders, Irish elk were no bigger than an exceptionally large moose. What would really have drawn the attention of observers, though, were the enormous antlers. With a combined spread of 3.5 metres (11.5 feet), these weighed as much as 45 kg (almost 100 pounds) and required an unusually thick and muscular neck to support them. Being so large, it's likely that the antlers were used more for display than for actual fighting, and, contrary to popular belief, they don't seem to have handicapped the males very much.

They would probably have been a bit of a bugger if the animal tried to walk through dense forest, but there wasn't any dense woodland  in Europe for much of this time, so that wasn't really an issue. Indeed, by examining the large number of fossils that we have of the animal, we can say that the majority of them seem to have died young, before the antlers reached their full extent, and presumably as a result of the almost absurdly cold winters. The same studies also show that, as with many living deer, males and females formed separate herds during the winter. This suggests that they rutted in the spring, with the males showing off the magnificent size of their antlers to impress and attract females to mate with.

The name "Irish elk" is something of a misnomer. The first fossils were discovered in Ireland, but we now know that they were found right across Europe, and even into western Asia. Indeed, while the species originated around 400,000 years ago, they didn't reach Ireland until almost the end of the Pleistocene, around 12,000 years ago - for the vast majority of their existence, they lived elsewhere. There was also a closely related species, called either M. yabei or Sinomegaceros yabei, living further east, in China.

But are they even elk?

The word "elk" is often used in Europe to refer to moose, and that was what the scientists who originally named the animal were thinking of. The antlers of Irish elk are broad and flat, not the relatively narrow branching spines that are found on, for example, red deer. Apart from their great size, they do look quite like the antlers of moose, and, taken together with the size of the animals and their general bodily proportions, it's not too hard to see why the connection was made.

However, Irish elk died out so recently, on a geologic scale, that some of their remains haven't fully fossilised. This makes it possible to extract DNA from their bones, and carry out the same sort of analysis that we do on living species of deer to determine their relationships. These studies all agree that Irish elk are not closely related to moose. Some have suggested that they are much closer to red deer, and hence, ironically, to the animal that Americans call an "elk".

The majority of recent studies, however, agree that, while they belong broadly to the "cervine" group of deer, which includes the red deer, those are not their closest relatives. Rather, they represent a relatively early branch within the cervines, and their closest relatives today are probably fallow deer. The many physical similarities between Irish elk and moose, therefore, are probably just due to them being the same size - there are only so many ways a really large deer can be shaped.

Irish elk survived the end of the Pleistocene and the Ice Ages, but not by very much. For once, humans seem to be in the clear, at least in Ireland: the last one died 1,600 years before the first people reached the island. Instead, it was simply the warming climate that was the most likely culprit.

Before the last Ice Age, there had been two species of rhino in Europe. They were closely related, but one, Stepanorhinus kirchbergensis, lived in lush forests, while the other, the smaller S. hemitoechus, was better suited to eating tough grasses and hardy plants. Unsurprisingly, the forest-dwelling species was the first to die out when the climate changed for the worse, but even the latter could not survive the very worst Ice Age winters, and eventually followed its relative into oblivion.

They were replaced by Coelodonta antiquitatis: the woolly rhino.

The Coelodonta lineage dates right back to the beginning of the Pleistocene, and perhaps slightly earlier. However, the earlier species lived exclusively in Asia, mainly in eastern Siberia. They were relatively unspecialised rhinos, feeding on a mix of different plants and being, by the standards of rhinos, slender and fast-moving. The later, woolly, rhino was the product of a long evolution leading towards an efficient tundra-dwelling animal, and it was this that enabled it to spread across most of Asia, and into Europe.

The bulky body with its short limbs and ponderous stance helped the animal retain heat, but its most obvious adaptation to the cold was its long hair. Today, African rhinos have very little hair, but the woolly species had a thick coat of dark fur that protected it against the cold. We know this both because early Europeans painted pictures of them on cave walls, and because, like woolly mammoths, we have some well-preserved mummies of the animal to examine.

Those mummies also enable us to perform a lot of studies that would be impossible if all we had were fossilised skeletons. As with the Irish elk, we can extract the animal's DNA and see just how it related to other rhinos. It turns out that its closest living relative is the Sumatran rhino (Dicerorhinus sumatrensis), and this makes a lot of sense. It's an Asian species, and, unlike the other Asian rhinos, it has two horns on its nose, something we know to have been true of woolly rhinos. Interestingly, Sumatran rhinos, unlike the African species, are really quite hairy, albeit not as much as the woolly rhino was.

The woolly rhino was, however, much larger than the living Sumatran species. While the Sumatran rhino is the smallest of the living species, the woolly rhino, standing over six feet high at the shoulder, was, if anything, slightly larger than the largest of living species (the Indian rhino), and would have weighed something like two and a half tons. It had much longer and more prominent horns, thicker fur, and high-crowned grinding teeth ideal for eating tough grasses rather than soft tropical leaves.

Some of the mummies have even included stomach contents, so that we know that, as their teeth suggest, woolly rhinos ate tough grasses and Arctic sagebrush. One study, examining the growth rings inside the animals' horns, suggests that that they ate woodier plants during the winter, when the weather was particularly harsh.

Humans do seem to have hunted woolly rhinos on occasion, and the British Museum has a piece of woolly rhino rib with a carving of what appears to be a hunter on it. However, it seems that climate change was likely at least as responsible for their eventual extinction. Woolly rhinos died out in Britain around 35,000 years ago, during one of the relatively mild periods between the worst parts of the last Ice Age. Although they were adapted to cold climates, they could not survive on vast icy glaciers devoid of plant matter, and retreated to the continent when the ice reached its last maximum.

Further south, they hung on rather longer, surviving in Spain, for example, until around 20,000 years ago. As the climate warmed again, this time as the Ice Age ended for good, they disappeared from Europe altogether. They needed not just a cold climate to prosper, but apparently a dry one, too - grassy tundra rather than pine forest, for example - and it seems that even northern Europe at this time was no longer suitable. However, they had always lived further east as well, and the climate of Siberia remained much more to their liking.

 They probably didn't make it out of the Pleistocene, though; the most recent studies indicate that they died out around 14,000 years ago, sheltering in the coldest parts of north-eastern Siberia just as the last Ice Age finally ground to a halt.

[Paintings by Mauricio Antón (© 2008 Public Library of Science, released under the Creative Commons Attribution licence) and Charles R Knight (copyright expired)]

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