Mammoths

Abstract

What are mammoths? The woolly mammoth — the symbol of the ice age — is the popular image of a mammoth: a large hairy elephant that lived in the arctic. However, there were also mammoths much smaller than modern elephants, mammoths without hair, and mammoths that lived in a Mediterranean or even African climate. All mammoths are members of the genus Mammuthus, closely related to the last two remaining elephant genera. Where did they come from and where did they go to? The elephant family originated in Africa, some 6 million years ago. From there, rather like humans, they colonized large parts of the world. One branch, the mammoths, is first known from African fossils 4–5 million years ago; they almost certainly had a sparse coat like modern elephants. Entering Europe some 3 million years ago, they remained largely restricted to temperate, wooded habitats. However, populations that spread to Northern China and North Eastern Siberia adapted to the cold, treeless environments of the later Ice Age. The final product of this process, the woolly mammoth, M. primigenius, ultimately spread west into Europe and east across the Bering land bridge into North America. How did they live? For the woolly mammoth we have the unique resource of mummified animals from the permafrost of Siberia and North America. Many have preserved fur, although it is unclear if the variability in colour — it can range from orange to black — is due to natural variation or alterations of pigments during burial. They had shorter tails and much smaller ears than modern elephants, to preserve heat. They fed on tough, grassy vegetation, as testified by both gut contents of mummified mammoths and their high-crowned teeth packed with enamel ridges. The spirally twisted tusks weighed up to 80 kg each and — like in African elephants — were present in both sexes. Life history and migration can be inferred from the tusk's annual growth rings. Circumstantial evidence suggests that mammoths had social structures similar to those of living elephants — matriarchal female-young groups, plus separate, solitary adult males. Were all mammoths giants? The word ‘Mammoth’ has become synonymous with gigantic size. However, the woolly mammoth was no larger than a living Asian elephant (3 m male shoulder height), although earlier mammoth species towered up to 4 m and attained 10 tonnes in body mass. Conversely, some mammoths became dramatically reduced in size. On the California Channel Islands, a population of the American M. columbi became isolated and dwarfed to around 1.75 m. The cause of island dwarfing is debated: food shortage, or selection for shorter generation time. What do we know about mammoth genetics? Woolly mammoths were among the first Pleistocene species from which authentic ancient DNA was recovered. Although the sequences were very short, they started a controversy whether mammoths were more closely related to the Asian elephant, as most morphologists believed, or to the African elephant as the DNA sequences initially indicated. Recently, the complete 16,000 bp sequence of the mitochondrial genome showed the mammoth to be more closely related to the Asian than the African elephant; however, the difference is not big. If African elephants separated from the lineage leading to mammoths and Asian elephants about 6 million years ago, then mammoths and Asian elephants separated less than half a million years later. Woolly mammoths were also the first Pleistocene species for which nuclear DNA sequences were reported. Indeed, the mammoth may also become the first extinct species from which the complete nuclear genome is sequenced. What happened to them? Until 20,000 years ago there were still mammoths across much of Northern Eurasia and North America. But once the climate started warming around 15,000 years ago, their habitat — the dry northern grassland known as the mammoth steppe — began to disappear, and their range shrank dramatically. By about 10,000 years ago mammoths had disappeared from the mainland. But on Wrangel Island in the Arctic Sea, mammoths survived until 4,000 years ago, when the pyramids were already under construction. Although Neanderthals and modern humans have hunted mammoths to some extent, it is a matter of debate whether humans were involved in the final demise of the mammoth. It is possible that humans dealt the final blow to some relict populations, since prior to the spread of modern humans, mammoths had survived climatic changes. Can we bring them back? It has often been claimed that it might be possible to clone a mammoth (or fertilize a female elephant with frozen mammoth sperm). Now people speculate that once the complete mammoth genome is known, it will be possible to create transgenic elephants and to ‘mammothize’ them. However, all these ideas are shattered by the reality of ancient DNA preservation. Both cloning and fertilization would require intact cells, and even in the best preserved mammoths from permafrost DNA or proteins are highly fragmented. Moreover, the fact that not a single but thousands of genes would need to be exchanged, in combination with the long generation time of elephants, makes the idea of creating a transgenic elephant seem a dubious proposal. Most likely, mammoths will remain gone forever.