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Long after most of their kind had died out, one group of woolly mammoths was still surviving on an Alaskan island. Now it’s clear why they finally bit the dust: a warming climate caused their lakes to dry up.
Mammoths were in crisis at the end of the last ice age, when human hunters were able to spread into their habitat in the northern hemisphere. Most mammoths on mainland Asia and North America went extinct over 13,000 years ago, either due to climate change, hunting or a mix of both.
But a few hardy mammoth herds clung to islands in the Arctic devoid of humans for several millennia.
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One of these populations retreated to the Bering land bridge between Siberia and North America. As the ocean rose, the mammoths became stranded on the small, low Alaskan island now called St Paul.
What happened to them remained a mystery until Russell Graham at Pennsylvania State University and colleagues pinpointed both the time and cause of their extinction: 5600 years ago, they ran out of water.
“It’s probably one of the best-dated prehistoric extinctions around,” says Graham.
The team reached the precise date using three lines of evidence. Radiocarbon dating of mammoth bones gave them one clue. Collecting mammoth DNA from cores of sediment at the bottom of a lake and dating the sediment layers gave another.
The final piece of the puzzle had to do with dung. Certain types of fungi grow only in the scat of large mammals, so when the mammoths – the only large animals on the island – vanished, so should the spores. All three sets of data agreed on the same extinction date, give or take 100 years.
“Having those three lines of evidence – and they all coincide on 5600 years ago – that’s wonderful and unique as far as I know,” says Adrian Lister at the Natural History Museum in London.
“The mammoths made the situation worse by eating all the plants around the lakes and causing the shores to erode”
To sort out the cause of death, the team returned to clues buried in the lake sediments. Right around the extinction time, the plankton living in the lakes switched from deep-water species to shallow-water ones. This pointed to steadily evaporating fresh water.
There was also an increase in plankton that could handle murky water, suggesting the mammoths were making the situation worse for themselves by eating all the plants around the lakes and causing the shores to erode. “They were actually causing the lake to infill – it’s sort of an ironic situation,” says Graham.
Mammoths, like their living cousins the elephants, needed many litres of water every day to survive. Low supplies would have done them in quickly, says Lister.
The very last mammoth population lived on the larger Arctic island of Wrangel until about 4000 years ago, but it’s still unclear whether the climate or humans finished them off, says Love Dalén at the Swedish Museum of Natural History in Stockholm. He hopes to solve this question in future studies.
In the meantime, the demise of St Paul’s mammoths reveals one thing we might expect from modern climate change, says Graham. While sea level rise poses a threat to islands and low peninsulas like Florida, the most immediate impact of warming might be on drinkable water supplies. “People are watching the sea come up, but the real problem might be behind them with the fresh water,” he says.
Read more: Humans in the clear over mammoth extinction?
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