Over 13,000 years ago, an American mastodon roamed what is today the American Midwest. Year after year, he returns to an area in northeastern Indiana – believed to be a breeding ground. There he died in battle.
On Monday, scientists reported in the Proceedings of the National Academy of Sciences that where a mastodon spent its life and how it died has been recovered by studying the chemical signatures recorded in its fangs. Their techniques offer new insight into an ancient relative of elephants who roamed North America before moving on to genetics.
Scientists have studied Buesching’s mastodon, named after the family farm where it was found in 1998, and is now on display at the Indiana State Museum. Its tusks, also known as unique, like the tusks of modern elephants, tell the entire life history of the animal and scientists have been able to gather information from specific days, weeks or years. Thus, scientists can specifically sample areas within their canines from adolescence and adulthood and determine how their migration has changed over time.
This migratory investigative work focused on strontium and oxygen isotopes in canines. Joshua Miller, a paleobiologist from the University of Cincinnati and author of the study, described strontium isotopes as leaving signals throughout the landscape.
Strontium isotopes seep out of the rocks into the surrounding soil and water. He explained that while plants absorb those nutrients, they incorporate “those isotopic signatures.” Our hungry mastodons would come and eat those plants, stamping that geographical footprint in their tusks.
Interpreting these geographical references and matching them to the landscape takes a step further: a map of how strontium isotopes change across the terrain. The authors drew on the work of other scientists, including Brooke E. Crowley, also from Cincinnati and one of the study’s co-authors, who created such a map.
Oxygen isotopes helped reveal the seasons in which Fred migrated. Every time it rains, atmospheric isotopes that record the season have been incorporated into local bodies of water and eaten up when he drinks from nearby ponds and streams.
Combined with complex statistical modeling, the team was able to determine the movement of this animal.
From the age of twenty-nine to thirty-two, things changed drastically for this mastodon. Suddenly he moved long distances and showed signs of repeated injury. But he kept returning to northeastern Indiana every year—a location that, the authors note, was never explored in his teenage years. There, in late spring and early summer, it suffered injuries, an important clue that it may have been a breeding ground.
Daniel Fisher, a University of Michigan paleontologist and also an author of the study, explained that the craters on the surface of a mastodon’s tusks are just one trace of the injuries they leave behind. These injuries leave internal fingerprints, too.
“It turns out that these pits form in places where a canine, at some point in its development history, is jammed into the back of its bony cavity,” Dr. Fisher said. When the proboscis males push their canines toward their opponents, the canine sticks back into the cavity where it grows outside the skull. This affects the internal growth of the tubercle, leaving marks on the season in which the infection occurred.
The persistent recurrence of these infections in the spring and summer within an adult male mastodon led the team to suspect it was going through mastodons, an aggressive time associated with breeding in modern male elephants, where quarrels with other males are frequent.
The craniofacial injury to which he was exposed occurred during the same season in the same breeding ground.
said Chris Widga, a vertebrate paleontologist and curator of the Gray Fossil Site Museum in Tennessee, who was not involved in the research. “And this is the first time we’re getting this data, and it’s really, really good.”
Whether migration and injury patterns account for all male American mastodons is a question for future research. The team hopes to study more fossils of both male and female mastodons.
For now, the study opens the door to more questions: How do the migration patterns of female mastodons differ? Were there separate interbreeding grounds for the various hoses that coexisted at that time? or, d. Miller wondered, “Did they go to the same place, and that’s just a crazy area of hormonally charged hoses?”
Whatever the broader possibilities about mastodons as a species, Dr. Miller returned to the team’s findings about the Buesching sample.
“To be at a point in geochemistry and modeling and paleobiology in general that we can begin to understand some of these foundational aspects of an individual’s biology, I think is very exciting and profound,” he said.