Over 13,000 years ago, an American mastodon roamed what is today the American Midwest. Year after year, he returned to an area in northeast Indiana — believed to be a mating ground. It was there that he died in battle.
Where the mastodon spent his life and how he died were all recovered by studying chemical signatures recorded in his tusk, scientists reported Monday in the Proceedings of the National Academy of Sciences. Their techniques offer new insight into one of several ancient elephant relatives that roamed North America before going extinct.
Scientists studied the Buesching mastodon, named for the family farm where it was found in 1998, and now on display at the Indiana State Museum. Also known as Fred, his tusks, like those of modern elephants, record an animal’s entire life history and enable scientists to glean information from specific days, weeks or years. Thus, the scientists could specifically sample areas within its tusk from its adolescence and adulthood and determine how its migration changed over time.
This migratory detective work focused on strontium and oxygen isotopes in the tusks. Joshua Miller, a paleoecologist from the University of Cincinnati and an author of the study, described strontium isotopes as leaving signals all over the landscape.
Strontium isotopes leach from rocks into surrounding soil and water. As plants absorb those nutrients, they incorporate “those isotopic signatures,” he explained. Our hungry mastodon would come along and eat those plants, stamping that geographic fingerprint into his tusks.
Interpreting these geographical references and matching them onto the landscape takes one more step: a map of how strontium isotopes change across terrain. The authors built upon the work of other scientists, including Brooke E. Crowley, also of Cincinnati and one of the study’s co-authors, who had created such a map.
Oxygen isotopes helped to uncover the seasons in which Fred migrated. Each time it rained, atmospheric isotopes recording the season were incorporated 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 drastically changed for this mastodon from his 29th through his 32nd years. Suddenly, he was moving over great distances with signs of repeated injury. But he kept returning to northeast Indiana every year — a location, the authors noted, that he never explored in his adolescent years. There, in late spring and early summer, he suffered injuries, an important clue that it might have been a mating ground.
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 injuries leave behind. Those injuries leave an internal mark as well.
“It turns out that those pits form in places where the tusk, at some point in its growth history, was jammed into the back of its bony socket,” Dr. Fisher said. When male proboscideans thrust their tusks at opponents, the tusk jams back into the socket where it grows out of the skull. This affects internal growth within the tusk, leaving signs of which season the injury occurred in.
That these injuries consistently reoccurred in spring and summer within an adult male mastodon 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 a frequent occurrence.
The mortal craniofacial injury he sustained took place during that same season at that same mating ground.
“The methods that they’re using are part of a broader trend in Quaternary vertebrate paleontology to add a lot more detail to the behavior and the ecology of these animals,” said Chris Widga, a vertebrate paleontologist and head curator at the Gray Fossil Site in Tennessee, who was not involved in the research. “And it’s the first time that we have had 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 the migration patterns of female mastodons differ? Were there separate mating grounds for the various proboscideans that coexisted at that time? Or, Dr. Miller pondered, “Did they go to the same place, and this is just a crazed 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 that we can start to grasp at some of these foundational aspects of the biology of an individual,” he said, “I think it’s just so deeply, deeply exciting.”