About 5,600 years ago, on what is now Alaska's St. Paul Island, an isolated population of woolly mammoths gradually disappeared, and now scientists think they know why.
Decreased water levels in the island's lakes, along with decreased quality of the water, likely doomed the mammoths, according to a new study by University of Alaska Fairbanks researchers.
"Freshwater resources look like the smoking gun for what pushed them into this untenable situation," said study co-author Matthew Wooller in a statement.
Woolly mammoths became isolated on the island after the Bering Sea land bridge was covered by water during a period of rising sea levels. The island gradually became smaller, hampering the mammoths' chances to find new places with ample water.
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Wooller and his colleagues extracted core samples from the bed of a freshwater lake on St. Paul Island, testing the remains of aquatic insects preserved in the sediment. Key chemical signatures retained by the insects remains allowed the scientists to read the lake's water level and quality before, during and after the time of the mammoths.
The analysis of the core told the researchers that water abundance and quality had indeed both diminished.
Meanwhile, chemical analyses of mammoth bones and teeth indicated that the island had grown progressively drier in the run-up to the animals' die-off.
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The drier conditions and decreased lake levels "paints a dire picture of the situation for these mammoths," said Wooller.
Woolly mammoths disappeared from mainland sites some 10,000 years ago. About the size of today's African elephants, the last among them died out about 4,000 years ago on Russia's Wrangel Island, north of Siberia in the Arctic Ocean, an island also cut off by the submerged Bering Sea land bridge.
The St. Paul Island mammoths would have enjoyed the vegetation of the time, which was much as it is today, Wooller said in 2014. There's no evidence humans occupied the land at the same time as the mammoths.
Wooller and his team have published their findings in the journal Proceedings of the National Academy of Sciences.
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