A woolly mammoth’s tusk is a story written in ivory. It sprouts from beneath the mammoth’s gums, cells dividing continually, even daily.
“The tip of the tusk is the young mammoth,” says Matthew Wooller, an ecologist at the University of Alaska Fairbanks. “The base of the tusk is the old mammoth. Everything between is the mammoth’s lifetime.” The tusk is a record of its owner’s travels, diet, and even its death. The concept is simple, says Wooller. What’s tricky is accessing a tusk’s chemical composition and interpreting it.
In a new study Wooller led a group of researchers doing just that. Examining the tusk of a woolly mammoth that lived about 17,000 years ago, they uncovered details about its activities from birth to death. They also retraced its footsteps across Ice Age Alaska over 28 years, marking the first time scientists have been able to reconstruct a mammoth’s life history in such fine detail.
The research, published today in Science, relied on cutting-edge tools and techniques to provide clues about how woolly mammoths lived, including their possible interactions with humans. The work may also inform studies about why these iconic animals ultimately went extinct and about how today’s big mammals might react to a steadily warming world. (Find out how scientists think they could resurrect a woolly mammoth.)
The researchers came as close as possible to reconstructing a mammoth’s life, short of “going back in time and putting a GPS collar on a woolly mammoth,” says Vanderbilt University paleontologist Larisa DeSantis, who wasn’t involved in the study.
“Just reading the paper, I felt like Jane Goodall observing these animals.”
Carving into a mammoth tusk
I met Wooller in the summer of 2018, at the university office where he leads the Alaska Isotopic Research Laboratory. Isotopes are atoms of an element that, because of missing or extra neutrons, have atomic weights slightly different from one another. In recent years scientists have used isotopes to reconstruct ancient human diets, solve cold case murders, and identify drug smuggling routes.
“It’s a real boom industry,” Wooller says.
As he later points out in an email, his name bears a striking resemblance to the subject of his latest research. (“We call it the Wooller mammoth,” jokes Florida State paleontologist Gregory Erickson, a co-author on the paper.) But that’s where the similarities end; Wooller’s face and head are clean-shaven, his glasses trim, his accent crisply British.
From his office, he leads the way down the hall to his lab. Gas cylinders and bits of bone and horn fringe the lab’s walls. In the center of the room, resting on a black counter, sits the mammoth tusk—five-and-a-half-feet long, thicker than my arm, with a slow corkscrew curve from one end to the other. It’s split lengthwise down the middle like a long, spiraling hotdog bun.
Tusks grow in distinct internal layers that look like a stack of ice-cream sugar cones, Wooller explains, but the divisions between layers become irregular on the outside of the tusk. To capture the entire chemical record of the tusk, the team needed samples from its center. Maneuvering the curved, 50-pound tusk through a bandsaw was the project’s first challenge. “We broke a couple blades,” he says.
After splitting the tusk, the team took wedge-shaped cuts from its center, each five centimeters long, so they could feed the pieces into a machine that occupies one side of the lab. Called a Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometer, the machine uses a laser to vaporize—or ablate—tiny bits of material. It then analyzes the resulting atomic mass of the chemicals.
The machine can take many isotopic readings per inch, providing a level of detail unavailable by other methods. Slowly, its laser scrawling a pointillism track across the ivory, it teased out the chemical clues held in the tusk fragments. (Also find out how scientists extracted million-year-old DNA from mammoth teeth.)
"Like a chemical GPS"
Strontium isotopes make up the heart of the study. Animals acquire strontium through the plants they eat, which absorb it from the underlying rock. Different geological regions have different strontium isotopic signatures. While the climate of Alaska has changed dramatically since the Pleistocene epoch, its geological arrangement hasn’t. As the mammoth ate its way across the landscape, the strontium being sequestered in its tusks became a record of its travels. The tusk is “like a chemical GPS,” Wooller says—one with 340,000 entries.
“It’s super-high-resolution data, using the laser to look at, like, daily strontium isotopes across the mammoth tusk,” says study co-leader Clément Bataille, a geochemist at the University of Ottawa.
To tie the strontium data points to actual locations on the map, Wooller’s team used modern Alaskan voles. The University of Alaska’s Museum of the North has an extensive collection of the small, herbivorous rodents. They tend not to travel very far, which means that the strontium in their teeth is a good representation of the geological region where they live. “They’re like citizen scientists,” Wooller says of the voles. “They’re sampling the local conditions.”
Bataille and Juliette Funck, then a Ph.D. student at Wooller’s lab, used the vole data to create a strontium map of Alaska. Bataille then built a model that correlated the data points from the tusk to that map. The model took into account certain physical realities—a mammoth can’t fly or scale cliffs, Wooller noted, “so we can rule out certain parts of the landscape.” Tracing its footprints from the location of its death, Bataille followed the mammoth’s path back to the beginning.
While Wooller and his colleagues worked to track the mammoth’s movements, Beth Shapiro and Katie Moon at the University of California, Santa Cruz, extracted fragments of ancient DNA from the mammoth fossil to learn its sex. They also pinpointed the century of its birth using radiocarbon dating.
At Florida State, Erickson analyzed tiny striations in the tusk’s dentine, caused by seasonal changes and even the daily cycle of activity and sleep to mark out the years, months, and days of the mammoth’s life. Joined together, these details transformed the chemicals contained in the tusk into an Ice Age biography of unprecedented richness, a story of family, migration, and an untimely death.
Following in Kik’s footsteps
Kik, as the team nicknamed this mammoth, was a male born 17,100 years ago in what is now northeastern Alaska. At the time of his birth, the last glacial maximum of the Pleistocene was just beginning to wane. Between Alaska and Russia stretched a vast plain that was too dry for glaciers to form. Scientists debate exactly what this region looked like, but the plentiful fossils of bison, mammoths, caribou, horses, musk oxen, and lions suggest something like a cold-weather Serengeti. The landscape was clothed in a mix of grasses, rushes, and sagebrush that scientists now call the “mammoth steppe.”
Kik spent the first years of his life in a small part of Alaska’s interior, south of the Brooks Range, the mountain chain that cuts across the state’s upper third. He was weaned at two years old, a dietary shift recorded by carbon and oxygen isotopes. In the years that followed, his movements increased. He seems to have spent winters in the lowlands and summers in the foothills—perhaps to avoid biting insects, Wooller says.
He ranged back and forth, in a migratory pattern that mirrors the movements of modern African elephants, Wooller says. Scientists had long thought might be the case. “We use elephants as a bit of a mental image of what mammoths might be doing on the landscape,” he says. But until now, there was no proof.
It’s likely Kik’s experiences mirrored modern elephants in another way: “When it was a young mammoth, it was probably part of a herd,” Wooller says. The mammoth must have spent his early years with his mother, and probably with a larger group of other female and juvenile mammoths. In effect, tracking young Kik means tracking his herd too. (Also read about the similarities between mammoth tusks and elephant ivory—and why that’s a problem for combating smugglers.)
When Kik was 16, his movements abruptly changed. When male African and Asian elephants reach sexual maturity, they leave the herd and go wandering, either on their own or in small groups with other male elephants. It appears that Kik did the same. He began ranging farther, through the passes at the western edge of the Brooks Range and onto Alaska’s North Slope, a route that migrating caribou herds still follow today.
“They were once upon a time migrating alongside mammoths,” Wooller says. “I think that’s sort of a neat image.”
The final isotope the team studied tells how Kik’s story ends. In the summer of his 27th year, the nitrogen isotopes in his tusk began to change. Different foods produce different nitrogen isotope signatures, and that summer, Kik’s nitrogen isotopes began to resemble those of a carnivore. For a plant-eating mammoth, it could mean only one thing: Kik’s body was eating itself. He was starving.
Sometimes old elephants starve after their teeth are too worn down to use, says study co-author Daniel Mann, a Quaternary scientist at the University of Alaska Fairbanks. But Kik was still relatively young, and since the tusk was found next to pieces of the mammoth’s skull, the scientists could examine the teeth and see they were in good condition. “I would suspect that it had some kind of injury,” Mann says of the mammoth.
In the fall of his final year, Kik traveled from today’s Alaska’s Seward Peninsula to the northeastern flank of the Brooks Range. There he stayed, wandering through the winter in a region of sand dunes west of the Colville River. Then, in late winter or early spring, he arrived in the area where he would remain for most of the next 17,000 years, what is now the edge of a shallow canyon carved by one of the Colville’s tributaries, the Kikiakrorak River—Kik for short.
Ancient clues to future calamity
“It’s pretty wild to have that kind of data set for an individual animal,” says Yukon government paleontologist Grant Zazula, who was not involved in the study. “This is really going to open up a huge door of new projects in places like Alaska and Yukon, where we have a real wealth of fossils available for study.”
Eventually, isotopic studies could help answer one of the biggest lingering questions about woolly mammoths: why they went extinct. Many scientists think that human hunters were what drove woolly mammoths and scores of other large mammals to extinction in the late Pleistocene and early Holocene. Proponents of this “overkill” hypothesis argue that even a small number of people could have killed off the slow-breeding mammals. Other scientists argue that a changing climate played a bigger role in the extinctions.
While Kik’s story doesn’t settle the debate, Wooller says, it does point to how both human predation and climate change could have contributed.
The earliest unequivocal evidence of humans in the region is from around 14,000 years ago, thousands of years after Kik died. While archaeologists fiercely debate when the first people arrived in North America, mammoths didn’t go extinct on mainland Alaska until around 13,000 years ago, so it’s likely that mammoths and humans overlapped there for more than a thousand years.
Bison and elk seem to have been hunters’ preferred targets, says study co-author Joshua Reuther, an archaeologist at the Museum of the North. The fact that Kik followed a regular migration route suggests that woolly mammoths might have made an attractive target too. “Herd animals, they’re much more reliable than solitary animals like moose,” he says.
In the millennia after Kik’s life, the mammoth steppe slowly filled with trees, Wooller says, shrinking the mammoths’ preferred habitat, constricting their movements across the landscape, and perhaps pushing them even more into the path of this new and dangerous predator. It is a hint too, he says, of the pressures that modern animals may face as climate change causes sudden shifts in ecosystems. Already, scientists say, the changing climate in Alaska has caused shifts in the movements of caribou, the mammoth’s ancient migrational companions.
“We’re living in a world in which humans and climate change are both having impacts on animals,” DeSantis says. “If that was the deadly combination that led to the large animal extinctions that happened in the Pleistocene, we really need to be cautious.”