A careful measurement of isotope ratios in animals’ teeth could offer a new way to closely track their movements, according to a recent study that showed how the approach would work in Mongolian sheep and goat herds.
Tooth enamel is a record of sorts, revealing where an animal has gone and eaten. That’s because the geology of a landscape determines how much of the stable isotopes strontium-87 and strontium-86 end up in the region’s plants and, ultimately, in the animals that eat those plants. The moment a tooth mineralizes, it has the ratio of strontium-87 to strontium-86 from that area. Since a tooth grows from top to bottom, the oldest strontium is at the top of the crown. As the tooth grows, incremental layers similarly capture the ratio of the locales where mineralization happened. The tooth, then, offers a kind of map showing where the organism roamed and ate.
“The ratio is guided by the ratio found in food, which itself is connected to the geological substrate where the food grows,” says Antoine Zazzo, a bioarchaeologist at the French National Centre for Scientific Research. “There’s a direct link between what you eat and where you are.”
Forensic scientists and archaeologists, among other researchers, have used strontium isotope analyses of teeth enamel for decades, but mostly at large scales of time and space—for example, to distinguish local animals from migrants. In the recent work on Mongolian animal herds, published in Scientific Reports, Zazzo and his colleagues describe a precise way to analyze the enamel, using laser ablation, in order to reconstruct a high-resolution time series of the animals’ travels—revealing where they roamed month-by-month. With further refinement, Zazzo says, the technique could be used not only on modern herds but also for sophisticated archaeological investigations, such as identifying the origins and movements of animals found at burial sites.
“They have really fine-tuned the process and done something that we haven’t been able to do before,” says paleontologist Chris Widga, curator at the Gray Fossil Site and Museum in eastern Tennessee, who has used strontium isotopes to study movements of bison and mammoths. “They’re able to track these goats in real time, and I can’t do that with teeth from the fossil record, like with mammoths or bison.” The new work “makes excellent advances in our understanding of how strontium tracks movement,” says paleoecologist Brooke Crowley at the University of Cincinnati, who has used isotope analyses to track modern and ancient mammals.
Zazzo and his colleagues began by creating an “isoscape” of the study area—essentially a map showing the distribution of strontium-87 to strontium-86 across the terrain. To construct it, they analyzed the isotopes found in leaf, stem, seed, and fruit samples collected from plants in more than 150 sites on the eastern edge of the Altai mountain range in far western Mongolia. Those data revealed how isotope availability changed throughout the region.
Then they studied the animals. Working with nomadic herders in the area who move among different campsites about 10 times per year, they attached GPS-equipped collars to four goats and four sheep that roamed the area. The collars recorded and transmitted the animal’s location every few hours, and over a period of 29 months the researchers watched where the animals went. “Day after day, we were able to follow the movement of the animals, from France,” says Zazzo.
The next step was comparing the two datasets, noting the availability of strontium ratios in the places where the goats and sheep wandered. That allowed them to predict what ratios they should expect to find within the tooth enamel. “They got these tiny snapshots of where the animal was on the landscape as the tooth formed,” says Widga.
Finally, to test the model, Zazzo’s group used laser ablation, paired with mass spectroscopy, to precisely measure the isotope ratios within the animals’ molars. (They collected the teeth after the herders slaughtered the animals for food.) The laser allowed them to study tiny samples, which helped them better identify an animal’s location when that layer mineralized. Their measurements showed that the isotope ratios matched those predicted by the model, confirming that the new approach could be used to track movement between two places, about 45 days apart.
The degree to which Zazzo’s work will be applied to other locales isn’t yet known. Extensive strontium analysis and laser ablation can be expensive and time-consuming. Plus, the researchers found considerable variability in the isotopes across the region but didn’t describe the local geology or why it was so varied, says Crowley. “Without this context, it may be challenging to apply their approach to other settings,” says Crowley.
Widga notes that data specific to western Mongolia may not be directly useful in other places, but the methodology that Zazzo described could inform similar studies elsewhere. “We can’t take the isoscape and apply it to Tennessee,” Widga says of the Mongolia-specific data, “but we can take what they’ve learned and apply it somewhere else.”
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