Their top coat grew to be half a metre long.
Over around 300,000 years, woolly mammoths evolved smaller ears and thicker coats to keep them warm on the Siberian steppes, according to new research.
The giant mammals evolved mutations that reduce heat loss – which also explain their iconic coat and a thick layer of fat beneath the skin.
It adds to evidence they went extinct due to global warming.
Mammoths used their 15-foot-long tusks to dig under snow for shrubs and grasses, but when the Ice Age ended, it became too wet for the plants they fed on – and they starved to death.
They were covered in two layers of fur – the shaggy outer one half a metre long to keep them warm in temperatures as low as -50C.
Woolly mammoths also had a lump on their back for fat stores that provided energy when food was scarce – similar to a camel’s hump.
First author Dr David Diez-del-Molino, of the Centre for Palaeogenetics in Stockholm, Sweden, said: ‘We wanted to know what makes a mammoth a woolly mammoth.
‘Woolly mammoths have some very characteristic morphological features, like their thick fur and small ears, that you obviously expect based on what frozen specimens look like, but there are also many other adaptations like fat metabolism and cold perception that are not so evident because they are at the molecular level.’
The Swedish team compared their genomes with modern day elephants to find out what made them unique. They found trademark features became more defined as the species roamed Earth.
The analysis included 28 modern-day Asian and African elephants and 23 woolly mammoths. One set of remains belonged to 7000,000 year-old ‘Chukochya’ – the oldest known woolly mammoth.
Senior author Professor Love Dalen said: ‘Having the Chukochya genome allowed us to identify a number of genes that evolved during the lifespan of the woolly mammoth as a species.
‘This allows us to study evolution in real time, and we can say these specific mutations are unique to woolly mammoths, and they didn’t exist in its ancestors.’
Many genes were adaptive to living in cold environments. Some are even shared by unrelated mammals living the Arctic today.
Dr Diez-del-Molino said: ‘We found some highly-evolved genes related to fat metabolism and storage that are also found in other Arctic species like reindeer and polar bears, which means there’s probably convergent evolution for these genes in cold-adapted mammals.’
The study, published in the journal Current Biology, is the largest of its kind – identifying genes common to all woolly mammoths rather than individuals.
‘We found some of the genes that were previously thought to be special for woolly mammoths are actually variable between mammoths, which means they probably weren’t as important,’ said Dr Diez-del-Molino.
Overall, Chukochya shared roughly 92 per cent of its mutations with more modern woolly mammoths.
Thick fur, fat metabolism and cold-perception skills were already present during divergence from the steppe mammoth. But the traits developed further in Chukochya’s descendants.
‘The very earliest woolly mammoths weren’t fully evolved,’ said Professor Dalen. ‘They possibly had larger ears, and their wool was different – perhaps less insulating and fluffy compared to later woolly mammoths.’
More modern woolly mammoths also had several mutations that boosted immunity in response to emerging viral pathogens.
Working with ancient mammoth DNA comes with a slew of hurdles.
‘Apart from the field work, where we have to battle both polar bears and mosquitos, another aspect that makes this much more difficult is that you have to work in an ancient DNA laboratory,’ said Professor Dalen. ‘That means that you have to dress up in this full-body suit with a hood and face mask and visor and double gloves, so doing the lab work is rather uncomfortable to put it mildly.
‘I would like to highlight Marianne Dehasque, the second author of this paper, who did the herculean effort of performing lab work on most of these samples.’
All the mammoths whose genomes were included in the study were collected in Siberia. The researchers now hope to compare them against North American woolly mammoths.
Professor Dalen added: ‘We showed a couple of years ago that there was gene flow between woolly mammoths and the ancestors of Colombian mammoths, so that is something that we will need to account for, because North American woolly mammoths might have been carrying non-woolly mammoth genes as well.’
Woolly mammoths were around 13 feet tall – as big as African elephants. The reason they went extinct has been debated for decades.
Some experts say they were killed off by human hunters – while others blame the naturally warming climate.
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