When we think of woolly mammoths, what typically comes to mind is the classic hairy pachyderm that inhabited what is now Siberia’s tundra, megafauna that went extinct shortly after the end of the Pleistocene, nearly 12,000 years ago. But a number of mammoth species survived for thousands of years into the current era, including a colony of mammoths on Russia’s Wrangel Island that did not disappear until 1650 BC.

The mystery of why another latter-day colony of mammoths perished, only two millennia before on Alaska’s St. Paul Island, may have been solved by researchers. Lasting until 3750 BCE, the island’s mammoths, presumably having migrated there as rising sea levels contaminated other sources of fresh water, were able to thrive there as they had virtually no competition, sharing the island with smaller species such as Arctic foxes and shrews. Humans did not arrive on the island until the 18th century, so overhunting has also been ruled out — so what finally did the St. Paul mammoths in?

The international team, studying core sampled from the 100-square-kilometre island’s freshwater lakes, found that the mammoths had inadvertently decreased their water supply, with their footsteps eroding the lakes’ shorelines and shallowing the pools. In short, they eventually died of thirst.

“The amount of freshwater available decreased dramatically from about 10,000 years ago to 6,000 years ago,” explains Duane Froese, a Canada Research Chair on northern environmental change. The study also says that the dramatic reduction of the availability of fresh water in this case underscores the vulnerability of animal populations on small islands, especially in light of ongoing present-day environmental change.