Woolly mammoths were champion walkers. In the space of his lifetime, one single mammoth who trundled through the ancient Arctic traveled so persistently that his accumulated mileage would have been enough to circumnavigate the planet—twice. The clues come from geochemical isotopes locked inside the Ice Age beast’s tusk, a toothy time capsule that acts like an ancient mammoth tracker.
The mammoth at the center of the new Science paper by University of Alaska Fairbanks researcher Matthew Wooller and colleagues lived to be about 28 years old, and roamed around ancient Alaska around 17,100 years ago. Based upon the single X chromosome found in the genetic analysis, the researchers identify the mammoth as a male. What’s special about the tusk, though, isn’t just how well-preserved the huge tooth is, but the isotopes preserved within.
During the frigid days of the Pleistocene, the woolly mammoth munched on a variety of Ice Age plants. These plants contained isotopes like strontium, which were taken into the mammoth’s body through digestion and became part of the mammal’s tusk tissues. Different locations had different strontium signatures, which became preserved in the mammoth’s tusks. Thanks to the way they grow, tusks create records of an individual mammoth’s life, with the younger years of the mammoth’s life preserved at the tip and the adult years at the base.
Paleontologists have used similar techniques to study the movements of ancient elephants before, but the origin of this particular study, Wooler says, was inspired by his former PhD student Sean Brennan—who passed away in a skiing accident and to whose memory the new paper is dedicated. “I remember him coming into my office for the first time and saying that he wanted to use strontium isotopes in fish ear bones to track their movement,” Wooller recalls. That inspired Wooller to think about applying the same logic to something bigger—a whole mammoth tusk—to investigate an aspect of mammoth ecology that isn’t well-understood. By looking at the Strontium values in certain parts of the mammoth tusk—such as those that formed when the mammoth was an infant, a juvenile and an adult—and comparing those values to an isotope map of the ancient Arctic, the researchers were able to outline the life of this ancient beast.
The resulting map covers a significant swath of ancient Alaska and northwestern Canada. When the mammoth was very young, the paleontologists propose, the mammoth lived in the interior of Alaska around the Yukon River basin. By the time the mammoth was two years old, though, he was moving north to spend more time between the Alaska and Brooks mountain ranges. And that’s what he did for the next 14 years, likely following the north and south movements of his herd.
But after his sixteenth birthday, something changed for the mammoth. “After this point, the mammoth’s range really picked up even more and included spending more time at higher elevations and more regularly north of the Brooks range of mountains and Arctic Circle,” Wooller says. This finding tracks with the identification of the mammoth as a male. In modern elephant species, adult males often leave the matriarchal herds they grew up in and either become solitary or hang out with small groups of other males. These mammoths would have continued to roam in search of food, opportunities to mate and, much like Alaska’s modern mammals, refuge from the swarms of biting insects that spring up in the warm months. “The results of this paper match what I would expect given the hypothesis that mammoth behavior is similar to elephant behavior,” says Georgia Southern University paleontologist Kathlyn Smith.
“Mammoth tusks are an amazing archive of biological information,” says Royal Alberta Museum paleontologist Christina Barron-Ortiz, and those details cover the entirety of a mammoth’s existence. The last four inches of the study mammoth’s tusk, at the tooth’s wide base, record the end of the mammoth’s life. The male didn’t move around as much. He seemed to hang out in an area north of the Brooks Range and no longer ventured far and wide over the ancient tundra. And he probably starved there. Other isotopes, primarily oxygen, indicate that the mammoth perished during the late winter or early spring, marked by little food and biting cold.
“Seeing the data emerge in almost real time was like watching an ongoing soap opera of a life’s movement and behavior before our very eyes,” Wooller says. But the story is greater than that of the individual mammoth.
If woolly mammoths typically roamed over large carpets of tundra, then the world’s shift to a warmer, wetter climate may have broken up the habitat they relied upon. That hypothesis dovetails with the fact that the very last mammoths, which lived on Wrangel Island in the Arctic Circle, were inbred and underwent a “genetic meltdown” prior to their extinction. The changing climate may have put woolly mammoth populations under incredible stress, which was only exacerbated by the arrival of humans in North America. It’s possible that the mammoth in the study may have even seen some of the continent’s earliest human inhabitants, as his route took him by some of the Arctic’s early archaeological sites.
The initial study looked at the life of a single male mammoth, but the same approach could be applied to dozens of other Ice Age fossils. If male mammoths really did leave their family herds while females remained together, Smith notes, then the lifetime movements of a female mammoth might be very different. “Would the range be smaller? Would the range increase in times of environmental stress? Would the range increase or decrease during pregnancy?” Smith wonders. More than that, by looking more broadly at isotopes paleontologists might find important clues as to why woolly mammoths—and many of their megafaunal neighbors—were extirpated or went extinct as the Pleistocene ended.
Paleontologists and archaeologists are already piecing together a much more detailed map of who lived where and when, which adds critical evidence to questions about why some Ice Age animals perished and others were able to persist. If paleontologists were to create geochemical isotope maps of where male and female mammoths roamed and compared those maps to archaeological sites and modeled environmental change, Barron-Ortiz notes, then perhaps scientists could replay what really happened at the end of the Pleistocene. Until then, the comings and goings of these ancient lives are wrapped up in tooth and bone, waiting to have their stories told.