August 1, 2016

Thirst may have doomed the last mammoths on a tiny Alaskan island

by Julia Rosen

Woolly mammoths flourished during the last ice age, when they tromped across North America and Eurasia grazing on tundra plants. These massive animals disappeared from both continents between 14,000 and 13,200 years age, unable to withstand the combined pressures of a fast-warming climate and hunting by prehistoric humans. But a few small populations persisted on remote islands in the Arctic — safe from spears and buffered from warming — for thousands of years after that.

St. Paul Island is an old volcano on what used to be the Bering Land Bridge — a broad isthmus that connected Siberia and Alaska during the last glacial period — where researchers have found mammoth remains dating back 6,500 years. The animals got stranded there as rising sea levels first isolated the Alaskan island, then caused it to shrink from roughly 1,600 square kilometers to less than a tenth of that. However, researchers didn’t know if the youngest mammoth remains they’ve found on St. Paul were the last individuals to inhabit the island, or simply the last to be preserved. So a group of scientists went looking for clues, which they found in ancient lake sediments.

As sediment accumulates, it entombs everything from windblown pollen to tiny aquatic critters — all of which can be used to reconstruct what the environment was like in the past. Scientists can also determine the age of buried organic matter using radiocarbon dating, which allows them to reconstruct a history through time as they dig deeper into the mud. But how could scientists use these sediments to tell whether mammoths were living on St. Paul?

This model of a woolly mammoth is on display at the Royal BC Museum in Victoria, Canada. (Credit: Flying Puffin via Wikimedia Commons)

One approach involved searching for traces of mammoth DNA, which cropped up in the sediment until about 5,600 years ago. Scientists also looked for indirect signs of mammoths. In particular, they looked for the spores of certain kinds of fungi that live on the feces of large animals. These are known as coprophilous — or “dung-loving” — fungi, and they too disappeared from the sediment at the same time as the DNA. (They have reappeared in the last century, likely due to the introduction of reindeer).

“It’s amazing that everything turned out so precisely with dating of extinction at 5,600 plus or minus 100 years,” said Russell Graham, a geologist at Penn State University, in a statement. The demise of St. Paul’s mammoths is now one of the most tightly constrained extinctions in prehistory, Graham and his coauthors wrote in the study, published this week in the Proceedings of the National Academy of Sciences.

Exactly what led to the extinction remains much less clear. The scientists say that it’s unlikely that volcanic activity, changing vegetation, or humans were the cause — Russian whalers were the first to set foot on the island in 1787. They also argue that the island’s size probably wasn’t to blame, since the most rapid land loss occurred before about 9,000 years ago, long before the mammoths bit the dust.

Between 21,000 years ago and the present, rising sea levels inundated the Bering Land Bridge between Siberia (on the left) and Alaska (on the right). In the process, they isolated what is now St. Paul Island, a tiny fleck of land north of the Aleutian arc. (Credit: NOAA)

Instead, the researchers suggest that a lack of freshwater might have done the beasts in. Elephants — their closest living relatives — require up to 100 liters of water a day, and mammoths might have needed even more to stay cool under their thick fur coats. But there’s not a lot of water on St. Paul Island; today, there are only a few freshwater lagoons and 3 lakes, the deepest of which a human adult could stand up in.

Evidence from the sediments suggests that there used to be more water. Various environmental markers suggest that the lakes got shallower and dirtier in the millennia leading up to the mammoths’ disappearance, perhaps due to regional climate variability. The animals themselves may have contributed to the problem too. As they walked around the shores, they would have torn up the landscape, increasing the amount of sediment that got washed into the lake. “It paints a dire picture of the situation for these mammoths,” said Matthew Wooller, a paleoclimatologist at the University of Fairbanks and a coauthor on the study, in another statement. “Freshwater resources look like the smoking gun for what pushed them into this untenable situation.”

The lesson to take from this, the researchers say, is that a scarcity of freshwater resources may be a powerful — and underappreciated — threat to island populations. The confluence of forces faced by the mammoths also remain relevant today, the authors write in the paper: “This study reinforces 21st-century concerns about the vulnerability of island populations, including humans, to future warming, freshwater availability, and sea level rise.”

LEARN MORE

Learn more about the Bering Land Bridge in our module on the hydrologic cycle.

Read about the possibility of bringing back mammoths, and other species that have gone extinct, at Scientific American.

Explore the elephant family tree at National Geographic news.

Julia Rosen

Written by

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.

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