As the seas swelled, freshwater fell into short supply.
On a remote island in the middle of the Bering Sea, woolly mammoths continued to roam for thousands of years after their counterparts in North America and mainland Asia had gone extinct.
The population on St. Paul Island originated when several mammoths became stranded on a small piece of dry land after rising sea levels submerged the Bering Sea land bridge, between 14,700 and 13,500 years ago. For a time, these mammoths enjoyed a comfortable existence, with plenty of food and water and no predators.
New research published in the Proceedings of the National Academy of Sciences concludes this remnant population went extinct around 5,600 years ago, coinciding with a period of freshwater scarcity, which would have dramatically dwindled their once-lavish water supply.
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To figure out the timing of the island mammoth extinction, the researchers extracted their DNA left behind in radiocarbon-dated sediment cores. Study co-author Russell Graham of Pennsylvania State University tells The Guardian, “When the mammoths lived along the lake they probably waded into it, they probably defecated in it, urinated it in, bathed in it, so their DNA got into the lake water and settled to the bottom.”
The DNA analysis took care of the when, but why these mammoths went extinct is another question.
By examining prehistoric microbes and the chemical signatures retained in their exoskeletons, the researchers determined that freshwater sources on St. Paul Island were becoming scarce between 7,850 and 5,600 years ago due to climate change. Rising sea levels gradually shrunk the island, leaving few freshwater sources for the thirsty mammoths. And some of the sources that remained were being polluted with saltwater.
Mammoths would have congregated around the last usable watering holes, unwittingly destroying them in the process. Graham explains to the BBC, “They were milling around, which would destroy the vegetation - we see this with modern elephants. And this allows for the erosion of sediments to go into the lake, which is creating less and less fresh water.”
The mammoths were thus contributing to their own demise.
For the mammoths on St. Paul Island, water supplies were shrinking for 2,000 years prior to their extinction. This research underscores the vulnerability of small island populations to environmental change — a problem that would only be hastened by the rapid pace of climate change in modern times.
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