HomePLANTS & ANIMALSPaleontology & FossilsMastodon tusk chemical analysis reveals the first evidence of one extinct animal's...

Mastodon tusk chemical analysis reveals the first evidence of one extinct animal’s annual migration

According to the first study to document the annual migration of an individual animal from an extinct species, a roving male mastodon died in a bloody mating-season battle with a rival in what is now northeast Indiana, nearly 100 miles from his home territory, around 13,200 years ago.

The Buesching mastodon, an 8-ton adult, was killed when an opponent punctured the right side of his skull with a tusk tip, a fatal wound revealed to researchers when the animal’s remains were discovered in a peat farm near Fort Wayne in 1998.

According to a paper scheduled for online publication on June 13 in Proceedings of the National Academy of Sciences, northeast Indiana was likely a preferred summer mating ground for this solitary rambler, who made the trek annually during the last three years of his life, venturing north from his cold-season home.

The research also suggests that the Buesching bull spent time in central and southern Michigan, which seems appropriate for a creature whose full-size fiberglass-cast skeleton is on display at the University of Michigan Museum of Natural History in Ann Arbor.

“The unique result of this study is that we’ve been able to document the annual overland migration of an individual from an extinct species for the first time,” said University of Cincinnati paleoecologist Joshua Miller, the study’s first author.

“We were able to show that large male mastodons like Buesching migrated to the mating grounds every year using new modeling techniques and a powerful geochemical toolkit.”

Daniel Fisher, a U-M paleontologist, and study co-leader, took part in the Buesching mastodon excavation 24 years ago. Later, he cut a thin, lengthwise slab from the center of the animal’s banana-shaped, 9.5-foot right tusk, which is longer and more completely preserved than the left.

This slab was used in the new isotopic and life-history analyses, which allowed scientists to reconstruct changing patterns of landscape use during two critical periods: adolescence and the final years of adulthood. According to the researchers, the Buesching mastodon died at the age of 34 in a battle over access to mates.

“You’ve got a whole life ahead of you in that tusk,” said Fisher, who has studied mastodons and mammoths for more than 40 years and assisted in the excavation of several dozens of the extinct elephant relatives.

“The animal’s growth and development, as well as its history of changing land use and changing behavior—all of that history is captured and recorded in the structure and composition of the tusk,” said Fisher, an earth and environmental sciences professor, ecology and evolutionary biology professor, and curator at the University of Michigan Museum of Paleontology.

According to the team’s findings, the Buesching mastodon’s original home range was most likely in central Indiana. The young male, like modern-day elephants, stayed close to home until he separated from the female-led herd as an adolescent.

Buesching travelled farther and more frequently as a single adult, often covering nearly 20 miles per month, according to the researchers. Furthermore, his landscape use changed with the seasons, with a dramatic northward expansion into a summer-only region that included parts of northeastern Indiana—the presumed mating grounds.

“The Buesching mastodon went to the same place every time it got warm—bam, bam, bam—repeatedly. The clarity of that signal was both unexpected and thrilling “Miller has used similar isotopic techniques to study caribou migration in Alaska and Canada.

Migration and other forms of seasonally patterned landscape use were likely critical for mastodon and other large mammals’ reproductive success in harsh Pleistocene climates. According to the new study, little is known about how their geographic ranges and mobility fluctuated seasonally or changed with sexual maturity

However, techniques for analyzing the ratios of various forms, or isotopes, of the elements strontium and oxygen in ancient tusks are assisting scientists in uncovering some of those secrets.

Mastodons, mammoths, and modern elephants are members of the proboscideans, a group of large, flexible-trunked mammals with elongated upper incisor teeth that emerge from their skulls as tusks. Each year of the animal’s life, new growth layers are deposited on top of the existing ones, in alternating light and dark bands.

The yearly growth layers in a tusk are similar to the annual rings in a tree, except that new tusk layers form near the centre, whereas new growth in trees occurs in a layer of cells next to the bark. A tusk’s growth layers resemble an inverted stack of ice cream cones, with the time of death recorded at the base and the time of birth recorded at the tip.

Mastodons were herbivores who ate trees and shrubs. Chemical elements in their food and water were incorporated into their body tissues as they grew, including the gracefully tapered, ever-growing tusks.

Strontium and oxygen isotopes in tusk growth layers allowed the researchers to reconstruct Buesching’s travels as an adolescent and as a reproductively active adult in the newly published study. Thirty-six samples were collected during the adolescent years (during and after separation from the matriarchal herd), and 30 samples were collected during the animal’s final years of life.

Under a microscope, a tiny drill bit was used to remove half a millimeter from the edge of individual growth layers, each of which lasted one to two months in the animal’s life. The milling powder was collected and chemically analyzed.

Strontium isotope ratios in the tusk provided geographic fingerprints that were mapped to specific locations to show how strontium changes across the landscape. The researchers were able to determine the time of year a specific tusk layer formed using oxygen isotope values, which show significant seasonal fluctuations.

Because the researchers collected both strontium and oxygen isotope samples from the same narrow growth layers, they were able to draw specific conclusions about where Buesching traveled at different times of year and how old he was when he made each trip.

The isotopic data from the tusks were then entered into Miller and his colleagues’ spatially explicit movement model. The model enabled the team to estimate how far the animal was moving and the probabilities of movement between candidate locations—something absent from previous studies of extinct-animal movements.

“Strontium isotope geochemistry is an exciting new field for paleontology, archaeology, historical ecology, and even forensic biology. It’s thriving “Miller stated. “However, we have only scratched the surface of what this data can tell us.”

The next step in Fisher and Miller’s mastodon research is to examine the tusks of another individual, either a male or a female.

Brooke Crowley and Bledar Konomi of the University of Cincinnati, as well as Ross Secord of the Nebraska State Museum and the University of Nebraska-Lincoln, are the other authors of the PNAS study.


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