Until recently, Alaska's St. Paul Island was home to a mystery of mammoth proportions. Today the largest animals living on this 42-square mile speck of earth are a few reindeer, but once, St. Paul was woolly mammoth territory. For more than 4,000 years after the mainland mammoths of Asia and North American were wiped out by environmental change and human hunting, this barren turf served as one of the species’ last holdouts.
Only one group of mammoths lived longer than those of St. Paul: the mammoths of Wrangel Island, a 2,900-square mile island located in the Arctic Ocean, which managed to survive until about 4,000 years ago. In this case, scientists suspect we played a hand in the tenacious beasts’ demise. Archaeological evidence suggests that human hunters helped pushed already vulnerable populations over the edge.
But the mammoths of St. Paul never encountered humans, meaning they were shielded from one of the main destructive forces that likely killed their kin. So how did they meet their final end some 5,600 years ago?
Scientists finally think they have the answer. This week, an interdisciplinary team of researchers reported in Proceedings of the National Academy of Sciences that the mammoths essentially died of thirst. Using mammoth remains and radiocarbon dating, researchers found that dwindling freshwater due to climate change caused populations to dry up. Their results—which also show that the St. Paul mammoths persisted for longer than originally thought, until about 5,600 years ago—pinpoints a specific mechanism that may threaten other coastal and island populations facing climate change today.
Scientists had known previously that climate change must have played a role in the St. Paul mammoth extinction, but they had few clues as to the specifics. “This is an excellent piece of research, well-evidenced and well-argued,” says David Meltzer, an archaeologist at Southern Methodist University who was not involved in the study. “It’s just the sort of species- and region-specific work that needs to be done to fully understand the causes of extinction for this and other animals in the past.”
The researchers began by meticulously combing St. Paul’s caves for mammoth remains, which turned up a few pieces of bone and two teeth. Then, they took sediment cores from a lake on the island and analyzed them for four proxies that previous research has correlated with the presence of large animals, including ancient DNA and certain types of fungal spores. Radiocarbon dating allowed them to determine the age of the mammoth bones and teeth, and plant remains and volcanic ash from the sediment cores helped establish the precise time that the mammoths went extinct.
Finally, they pieced together a timeline of how the environment changed by using other proxies taken from the cores. These included oxygen isotopes, plant material and the presence of aquatic life such as phytoplankton and water fleas, all of which correlate with freshwater levels. The results revealed that mammoths lived on St. Paul until 5,600 years ago, plus or minus 100 years—one of the most precise extinction dates ever produced for a prehistoric animal.
The data painted a clear picture of what ultimately caused mammoths to disappear from the island, which was once part of the Bering Land Bridge but now lies stranded in the middle of the Bering Sea. Climate change led to a more arid island environment, increasing evaporation and limiting the amount of rainwater that collected in two lakes—the only sources of freshwater on St. Paul. At the same time, sea level rise caused what researchers refer to as a “saltwater wedge”: the tainting of groundwater by saltwater that bleeds in from below.
This revelation came as a surprise to the team. “We were sure that the mammoth’s extinction would probably have been caused by something environmental,” says Russ Graham, a vertebrate paleontologist at Pennsylvania State University and lead author of the paper. “But many of us, including myself, thought it might be some vegetational shift in response to climate change. We hadn’t anticipated that freshwater availability had caused the actual demise.”
The mammoth’s physiology—including thick hair impermeable to water, a body adapted to retain heat and the need to drink 70 to 100 gallons of water per day—made the animal less able to weather the drying landscape. If they act anything like elephants, their modern-day relatives , behavior also might have played a role in their demise. As water became scarcer, mammoths likely congregated near the island’s lakes, muddying them and exacerbating the freshwater’s disappearance. Whether St. Paul’s mammoths gradually died out or if a single catastrophic event did them in is unknown, but either way, the end result was the same: extinction.
“This is superbly done work that is deeply interesting and truly important,” says Donald Grayson, a zooarchaeologist at the University of Washington who was not involved in the work. “This research can and should serve as a model for those interested in other vertebrate extinctions during the past 50,000 years or so.”
The study’s import is not limited to the past. As we face climate change and rising sea levels today, the danger of freshwater sources becoming contaminated with will likely grow increasingly relevant. “We think of places like Florida and the South Pacific islands as being flooded by sea level rise, but what we haven’t discussed much is the impact of the saltwater wedge,” Graham says. “This study clearly indicates that’s a serious issue.”