Researchers have reconstructed the geographical movements of a single woolly mammoth (Mammuthus primigenius) using chemical ‘GPS tags’ preserved in one of its tusks. The findings show that the animal travelled so widely across what is now Alaska that it could have circled Earth almost twice — a discovery that offers tantalizing clues about why the species went extinct.
Although researchers know a fair amount about the diet, genetics and ecology of woolly mammoths, insights into the life histories of individual animals are scarce.
“We can’t go back and watch these things like a modern ecologist might, but we can use chemistry to come up with good proxies,” says Chris Widga, a palaeontologist at East Tennessee State University in Johnson City. Every place on Earth has a distinct chemical signature based on differences in its geology. The ratios of various isotopes of elements such as strontium and oxygen in the bedrock and water create a unique profile specific to that location that remains consistent over millennia, and is incorporated into soil and plants. As mammoths grazed on the Arctic plains, these isotopic signatures were integrated into their ever-growing tusks, creating a permanent record of the animals’ whereabouts with almost daily resolution.
Until now, no one had analysed these chemical GPS tags across the full length of a tusk, which reflects the mammoth’s entire life. “This is by far the largest and most comprehensive study of its kind,” says Matthew Wooller, a palaeoecologist at the University of Alaska Fairbanks, who co-led the study with geoscientist Clement Bataille at the University of Ottawa in Canada and colleagues.
Their findings — published on 12 August in Science1 — provide a glimpse of the life and death of a single woolly mammoth during the last ice age.
Mammoth soap opera
Previous analysis of the 1.7-metre-long tusk had showed that it belonged to a male mammoth that died around 17,100 years ago, when it was at least 28 years old. The researchers split the tusk down the middle to reveal the layers of growth, which look like a curving stack of ice cream cones. The base of this stack “is the day that it died and the tip is the day that it was born”, says Wooller. “Everything in between is the lifespan of the mammoth.”
The researchers used lasers to sample the tusk’s chemical composition at approximately 340,000 points along the full length of the cone tips. They then compared the isotopic profiles at each of these data points with a geological map of Alaska and northwest Canada, and used a computer algorithm to map out the most probable routes for the mammoth to have travelled, backtracking from where its remains were found.
“It’s a total soap opera, all the way up to the day it died,” Wooller says.
The bull spent much of its early life in the Yukon River basin and wider Alaskan interior, where it made repeated, long-distance journeys between smaller territories. The migratory behaviour is similar to those of modern elephant groups, which suggests that the young mammoth was moving with a herd.
At about 16 years old, the isotopic pattern in the tusk becomes more variable. The mammoth probably wandered longer distances in less regular patterns than during its juvenile years. This could indicate that it left its herd to roam freely, as has been observed in mature males from living elephant species. For around a decade, it travelled widely across its range, sometimes visiting areas in which other mammoth remains have been found.
In the last year and a half of the animal’s life, its stamping grounds shrank to a single region near the northern coast of Alaska within the Arctic Circle. A distinctive isotope pattern recorded at the base of the tusk showed the “telltale hallmark of starvation in mammals”, which was probably what caused its death, says Wooller.
“The fact that this study presents an ‘iso-biography’ for a single individual is part of what makes it so exciting,” says Kate Britton, an archaeological scientist at the University of Aberdeen, UK. “We are gaining individual insight into the behaviour of an animal that roamed Alaska more than 17,000 years ago, and the strontium isotopes allow us to follow in its footsteps.”
Splitting more tusks
Scientists still don’t know exactly what caused woolly mammoths to go extinct, but many agree that the changing climate could have had a role.
Their extinction coincided with a period when the planet was warming and much of the mammoth’s Arctic range was becoming hotter, wetter and more forested. If mammoths regularly migrated as widely as this bull, that could explain why they were so negatively affected by habitat loss, Wooller says.
Britton cautions against using one bull’s movements to attribute behaviours to a whole species across time. But she’s excited by how isotope profiles in tusks can offer insights across the full lifespan of extinct creatures, and hopes that the technique will be applied more widely.
Wooller and his colleagues want to analyse other mammoth tusks to compile a more robust collection of life histories. But that would mean splitting open — and thus partially destroying — precious museum specimens.
Christine Garcia, collections manager for geology at the California Academy of Sciences in San Francisco, is intrigued by the idea, but cautious. Although it’s important to be mindful about the use of destructive sampling techniques, she says, this research “speaks to the potential value of a lot of these collections and how they can be useful”. Now she wonders what other life stories are hidden among the isotopes of the mammoth tusks — and other specimens — in her own care.