Researchers retraced a woolly mammoth’s steps 17,000 years after it died
Woolly mammoths roamed over wide open spaces, tusk analysis suggests.
A woolly mammoth that perished above the Arctic Circle spent his life as a vagabond, scientists reported this week in the journal Science.
The researchers examined chemicals deposited over time in one of the mammoth’s well-preserved tusks in order to map his movements and geographic range. They discovered that the intrepid animal roamed over most of the state of Alaska before eventually starving in the frigid north. If this wanderlust was shared by other members of his species, the findings could help shed light on why woolly mammoths died out.
“There are many questions about woolly mammoth extinction,” says Yue Wang, a paleontologist at Georgia Tech who was not involved in the research. One of the most crucial considerations is how much ground the animals covered during their lifetimes, which can help scientists understand how woolly mammoths were impacted by a changing climate and human activities.
“This research kind of solves this question,” Wang says. “This gives researchers like me some indications that the lifetime range of woolly mammoths can be very huge.”
Present-day large herbivores such as elephants and caribou regularly cover large distances, and scientists have suspected that woolly mammoths behaved similarly. But it wasn’t clear when or how far they traveled, says Clement Bataille, a geologist at the University of Ottawa in Ontario and a coauthor of the findings.
To find out, he and his colleagues analyzed a 1.7-meter-long (5.6 footlong) tusk from a male woolly mammoth that died around the age of 28 (some researchers have estimated the animals lived for about 60 years). The remains date to about 17,100 years old, during the last ice age.
When animals eat or drink, traces of elements such as strontium and oxygen are integrated into their tissues. Certain versions, or isotopes, of these elements are more common in some locations than others. The ratios of different strontium isotopes in plants and soil reflect the geology of the underlying bedrock. The oxygen isotopes found in water vary depending on climatic conditions such as distance from the coast, temperature, and elevation, Bataille says.
“[In] the mammoth, because he has a continuously growing tusk, the strontium is progressively incorporated as he’s moving across the landscape into the tusk…like kind of a GPS record of what the animal is doing,” he says.
Bataille and his colleagues had previously used hundreds of rodent teeth from museum collections to map how strontium isotopes varied in ecosystems all over Alaska. Unlike mammoths, rodents don’t migrate very far, so they can provide hyper-local data. The researchers also examined oxygen isotopes from mammoth and mastodon teeth collected across the landscape.
The team then compared the isotopic signatures found in the woolly mammoth tusk with these maps. They developed an algorithm to reconstruct the mammoth’s movements over time, constrained by barriers such as cliffs and glaciers and the likely distance he could travel in a week, which they estimated based on their presumed metabolism, size, and how far present-day species, such as caribou, can travel.
The simulated pathways all suggested that the mammoth covered a “staggering” amount of territory in his lifetime. “He was wandering across a huge, huge range and across almost the entire Alaska landscape,” Bataille says.
The isotopes found within the growth lines of the tusk’s first few centimeters suggest that the mammoth spent its infancy in the lower Yukon River basin. As a juvenile, he explored a larger area in the lowlands of interior Alaska, likely as part of a herd.
Then, after reaching the age of about 16 years, the mammoth began undertaking much longer journeys that sometimes spanned hundreds of miles. This shift may have represented the point at which the mammoth was kicked out of his herd, as often happens with male elephants when they reach maturity.
Finally, the mammoth didn’t return from one of his trips northward. He spent the last year and a half of his life north of the Brooks Range, within the Arctic Circle. During this time, nitrogen isotopes in the tusk sharply increased, a phenomenon often seen when starving animals must break down their own proteins to produce energy. The mammoth may have become injured or too weak to make the voyage south toward more hospitable conditions.
The findings likely don’t reveal the woolly mammoth’s precise range, Bataille cautions. For one thing, the rodent teeth he and his team examined reflect Alaska’s recent landscape, which has changed since the time of the mammoth.
There are also limits to what can be determined from a single mammoth’s tusks, Matthew Wooller, director of the Alaska Stable Isotope Facility at the University of Alaska Fairbanks and a coauthor of the findings, said in an email. He and his colleagues plan to investigate other mammoths, including females, under differing time periods and climates.
Around 12,000 years ago, a warming climate allowed boreal forests to move north, which may have restricted the ability of woolly mammoths to travel and graze across the tundra.
“An implication of our work is that if mammoths continued to move around on this scale, it may have imparted extra stress on mammoths,” Wooller said. “As the environment changed after the end of the last ice age things might have become pretty tough for mammoths trying to move around this much.”
A vast geographic range may also have contributed to the woolly mammoth’s extinction by bringing the animals more frequently into the path of humans.
The findings also highlight the vulnerability of modern-day animals in the Arctic and how they might have to change their behaviors and movements in response to climate change, Wooller added.
“The Arctic is changing very quickly today, and this is kind of similar to what happened at the last interglacial climate change,” Bataille says. “Trying to understand the link between climate and mobility of a species is really critical to understand how resilient they can be to global [climate] change.”