More than 13,000 years ago, an American mastodon roamed what is now the American Midwest. Year after year, he returned to an area of northeastern Indiana—presumably a breeding ground. It was there that he died in battle.
Where the mastodon spent its life and how it died were all recovered by studying chemical signatures recorded in its tusk, scientists reported Monday in the Proceedings of the National Academy of Sciences. Their techniques offer new insight into one of the many ancient relatives of elephants that roamed North America before their extinction.
Scientists studied the Buesching mastodon, named for the family ranch where it was found in 1998 and is now on display at the Indiana State Museum. Also known as Fred, its tusks, like those of modern elephants, record an animal’s entire life history and allow scientists to gather information from specific days, weeks, or years. This allowed the scientists to specifically sample areas within its tusk from adolescence and adulthood and determine how migration changed over time.
This migratory sleuthing focused on strontium and oxygen isotopes in the tusks. Joshua Miller, a University of Cincinnati paleoecologist and author of the study, described strontium isotopes as signals left throughout the landscape.
Strontium isotopes leach from the rocks into the surrounding soil and water. As plants take in those nutrients, they take in “those isotopic signatures,” he explained. Our hungry mastodon came by and ate those plants and imprinted that geographic fingerprint into his tusks.
Interpreting these geographic references and aligning them with the landscape requires one more step: a map of how strontium isotopes change across the terrain. The authors built on the work of other scientists, including Brooke E. Crowley, also of Cincinnati and one of the study’s co-authors, who created such a map.
Oxygen isotopes helped uncover the seasons in which Fred migrated. Every time it rained, atmospheric isotopes that recorded the season were absorbed into local bodies of water and ingested when he drank from nearby ponds and streams.
Together with complex statistical modeling, the team was able to determine the movement of this animal.
Things changed drastically for this mastodon from age 29 to age 32. Suddenly he moved great distances with signs of repeated injury. But he kept returning every year to northeast Indiana — a location, the authors noted, that he’d never explored in his teens. There, in late spring and early summer, he suffered injuries, an important clue that it may have been a mating site.
Daniel Fisher, a paleontologist at the University of Michigan and also an author of the study, explained that pits on the surface of a mastodon’s tusk are just one trace injury left behind. Those injuries also leave an internal mark.
“It turns out that those pits form where the tusk, at some point in its growth history, was lodged in the back of its bony socket,” said Dr. Fisher. When male proboscideans push their tusks at opponents, the tusk locks back into the socket where it grows out of the skull. This affects the internal growth in the tusk, showing signs of what season the injury occurred.
That these injuries consistently recurred in an adult male mastodon over the spring and summer led the team to suspect he was going through musth, a time of aggression associated with reproduction seen in modern male elephants, where sparring with other males is common. prevents.
The fatal craniofacial injury he sustained occurred during that same season at the same mating site.
“The methods they use are part of a broader trend in Quaternary vertebrate paleontology to add much more detail to the behavior and ecology of these animals,” said Chris Widga, vertebrate paleontologist and chief curator. from the Gray Fossil Site. in Tennessee, who was not involved in the investigation. “And it’s the first time we have this data, which is really, really good.”
Whether the migration patterns and injuries are representative of all male American mastodons is a question for future research. The team hopes to study more male and female mastodon fossils.
For now, the study opens the door to more questions: How did female mastodons’ migration patterns differ? Were there separate mating grounds for the different proboscideans that coexisted at the time? Or, Dr. Miller pondered, “Did they go to the same place, and this is just some crazy region of hormonally charged proboscideans?”
Whatever the broader possibilities about mastodons as a species, Dr. Miller returned to the team’s discoveries about the Buesching specimen.
“To be at a point in geochemistry, modeling and paleobiology in general where we can start to understand some of these fundamental aspects of an individual’s biology,” he said, “I think it’s just so deeply, deeply exciting. “
