How Mammoths Became 'Woolly'
Last month we had a look at perhaps the most spectacular of Ireland's Ice Age mammals, the woolly mammoth. Of course, there were many other mammalian stars of Ice Age Ireland, but before we move on to these we might just have one last look at the woolly mammoth and investigate the evolution of its one unmistakable feature – that great woolly coat. This is of interest not only in that it tells us a little more about Ireland's woolly mammoths, but also because it reveals much about the evolution of the venerable order they belong to, the Proboscidea.
Today, there are only two living species of this great order left on Earth, namely, the African elephant (Loxodonta africana), which we met last month, and the Asian elephant (Elephas maximus). Although there are slight differences between these two species in terms of size and body proportions, essentially they have the same characteristics, being very large five-to-six-tonne mammals that are virtually hairless and possess a long trunk. Indeed, it is this last feature, the elongated nose or proboscis, from which their order gets its name.
However, although the two remaining species of the Proboscidea share a very similar form, the order was once far more diverse, and although many of the extinct forms were quite similar to the living species, there were also some, especially when we consider the earlier forms, that were not.
Modern elephants have evolved to become almost hairless as their great size means that they can maintain a regular body temperature without a furry coat as the ratio of their surface area to volume is much lower than in smaller mammals (that is, they have greater bulk to retain heat and proportionally less surface area for it to be lost). However, the earliest proboscideans on Earth were, like most mammals, probably clothed in a thick coat of fur as they were far smaller creatures than their living relatives.
The jazzily-named Eritherium azzouzorum, for instance, which lived in Morocco around 60 million years ago – and is the earliest known proboscidean in the fossil record – was not much larger than an average Irish housecat, weighing only around five to six kilograms and having a shoulder height of about 20 centimetres. Even around five million years after Eritherium, in earliest Eocene Morocco, some proboscideans were still very small, with Phosphatherium only about the size of a cocker spaniel. However, other proboscideans from this time already demonstrate the tendency of some members of the order towards far greater size, with some weighing as much as around 200 kilograms.
Between this time, in the early Eocene, and the time the first proboscideans left Africa in the early Miocene, around 20 million years ago, the members of the order Proboscidea developed their typical 'elephant-like' features to become very different to the earliest forms, which looked somewhat like furry tapirs. Their heads became larger, their necks shorter, and the elongated snout became longer and longer to turn into the trunk. Incisors lengthened into great tusks and legs transformed into pillars to support the ever expanding bulk of their bodies. As they became bigger and bigger, a rich coat of fur would have become less and less necessary to maintain their body temperature and most of it would have been lost.
However, as the proboscideans left Africa and expanded their range into northern lands they were at times exposed to far colder conditions than their ancestors would have experienced in their original African homeland. This was especially true when the freezing conditions of the Ice Age set in, spurring in some forms the re-evolution of a mighty fur coat, most conspicuously in the mammoths, whose earliest members would probably have been as hairless as today's elephants and, like them, likely had much larger ears to dissipate heat, unlike the smaller, more human-like ears of later forms like the woolly mammoth.
Like all of the other great proboscidean lines, the mammoths – the genus Mammuthus – originated in Africa, the earliest known form being Mammuthus subplanifrons, which dates to around four million years ago. Soon after this, though, the mammoths broke free of their ancestral continent and the earliest known European mammoth, Mammuthus rumanus, may even have inhabited Ireland as it is known to have lived in Britain in the late Pliocene around three million years ago, at a time when Ireland and Britain are thought to have still been connected.
During the Ice Age, Mammuthus rumanus was succeeded by a number of later species, with Mammuthus meridionalis emerging first, then the steppe mammoth, Mammuthus trogontherii, and, finally, the woolly mammoth, Mammuthus primigenius. Although this transition from M. meridionalis to M. primigenius was once thought to have been a simple process of one species evolving into the next, mammoth evolution is now thought to have been far more complex, and a quick look at how this occurred may tell us something about the re-evolution of the furry coat.
M. meridionalis spread far and wide in the early Pleistocene and its presence in Britain sometime in the window from 2.6 to 1.8 million years ago means that it was possibly present in Ireland also as these two lands may still have been connected at this time. At the other end of its range, though, one population of M. meridionalis in eastern Asia, perhaps in China, evolved into M. trogontherii between two and one-and-a-half million years ago. By 1.2 million years ago, M. trogontherii had spread to northeast Siberia and it may have been the expansion of the mammoths' range into such freezing northern lands which spurred the evolution of the woolly coat, as some experts have suggested that M. trogontherii was the first mammoth to sport such an overcoat.
If M. trogontherii truly was the first mammoth to re-evolve a fur coat, this seems all the more remarkable when we consider that this species, like other early mammoths, was far larger than the woolly mammoth, with a shoulder height of up to 4.5 metres and a weight of around 15 tonnes! The necessity for such a large animal to have a furry coat clothing a vast, heat-retaining, bulky body attests to just how cold it could be in places like Siberia in the depths of the Ice Age.
By 600,000 years ago, M. trogontherii had completely replaced M. meridionalis across its entire range, but by this time one population had also begun to give rise to a new species. Around 700,000 years ago, in the lands of Beringia, which stretched from northeast Siberia across the emergent Bering Straits to the Yukon in Canada, a population of M. trogontherii began to evolve into early forms of Mammuthus primigenius. This new species may possibly have inherited its hairy coat from its parent species M. trogontherii, but as it is the first mammoth to unquestionably possess such a feature it is only right that it should be known as the woolly mammoth.
There is one last thing to be revealed about the woolly mammoth's fur coat, though, and it relates to its colour. Popular depictions of woolly mammoths, such as in the relatively recent Ice Age series of animated films, portray them as being bedecked in a rather rusty red coat. However, the red colour in the mammoth hair which has been recovered is due to a chemical process which took place in the hair after death and, in life, the hair in the great shaggy coats of the woolly mammoths was actually more or less black.
By around 200,000 years ago, from their homelands thousands of kilometres to the east, the woolly mammoths had made their way to western Europe, trekking along the vast grasslands of the Mammoth Steppe which once stetched from the Yukon of Canada to Britain. Here they replaced the remaining members of their parent species M. trogontherii (although perhaps not without some level of interbreeding) to become the only mammoth species left in the world.
Eventually, they would make their way to new lands in the very west of Eurasia, half a world away from where their species had first arisen on the Earth. And with their imposing stature and kingly robes, it is only fitting that they would become the rulers of late Ice Age Ireland.