Analysis of mastodon tusk reveals first evidence of seasonal migration

Using new isotopic and life-history analyses technology, scientists were able to reconstruct he landscape where the animal roamed during his adolescence and final years of his life.

 University of Michigan paleontologist and study co-leader Daniel Fisher participated in the Buesching mastodon excavation 24 years ago. He later used a bandsaw to cut a thin, lengthwise slab from the center of the animal’s banana-shaped, 9½-foot right tusk. (photo credit: COURTESY OF DANIEL FISHER)
University of Michigan paleontologist and study co-leader Daniel Fisher participated in the Buesching mastodon excavation 24 years ago. He later used a bandsaw to cut a thin, lengthwise slab from the center of the animal’s banana-shaped, 9½-foot right tusk.
(photo credit: COURTESY OF DANIEL FISHER)

Some 13,200 summers ago, when giants still roamed the Earth, a mortal battle took place between two male mastodons in the northeast of what is today the state of Indiana in the United States.

The final blow of the fierce battle came when one of the combatants punctured the right side of the skull of his opponent with a mighty tusk, leaving him lethally wounded and dying. The victor of the battle has long been left to anonymity, the only hint to his existence the puncture wound. But the other participant in the battle, an eight-ton adult who has become known as the Buesching mastodon – often just affectionately called “Buesching” for short – is now helping scientists better understand the migration patterns of his extinct species.

Buesching’s almost complete skeleton was recovered in 1998 from the peat farm of Kent and Janne Buesching near Fort Wayne, Indiana, and his fatal wound was noted then.

At the ripe old age of 34, Buesching had traveled nearly 150 km. (100 miles) from his home territory for the chance to mate. Instead, he met his end there, according to the first study to document the annual migration of an individual animal from an extinct species.

The results of the joint study were recently published online in Proceedings of the National Academy of Sciences.

 A mounted skeleton of the Buesching mastodon, based on casts of individual bones produced in fiberglass, on public display at the University of Michigan Museum of Natural History in Ann Arbor. The Buesching mastodon is a nearly complete skeleton of the adult male recovered in 1998. (credit: ERIC BRONSON/MICHIGAN PHOTOGRAPHY) A mounted skeleton of the Buesching mastodon, based on casts of individual bones produced in fiberglass, on public display at the University of Michigan Museum of Natural History in Ann Arbor. The Buesching mastodon is a nearly complete skeleton of the adult male recovered in 1998. (credit: ERIC BRONSON/MICHIGAN PHOTOGRAPHY)

“For the first time we’ve been able to document the annual overland migration of an individual from an extinct species,” said University of Cincinnati paleoecologist Joshua Miller, the study’s first author, in a press release. “Using new modeling techniques and a powerful geochemical toolkit, we’ve been able to show that large male mastodons like Buesching migrated every year to the mating grounds.”

The new study led by Miller together with University of Michigan paleontologist and study co-leader Daniel Fisher used oxygen and strontium isotopes from the mastodon’s right tusk to reconstruct changing patterns of landscape use during its lifetime. Fisher participated in the Buesching mastodon excavation 24 years ago.

Brooke Crowley and Bledar Konomi of the University of Cincinnati, and Ross Secord of the Nebraska State Museum and the University of Nebraska-Lincoln, co-authored the report.

Tusks: more than meets the eye

While fossil remains usually reveal little about the lifetime landscape of the animal other than its place of birth, the continuously growing tusks of the American mastodons maintain a complete record of this very important aspect of paleobiology, said the researchers in their paper.

Fisher used a band saw to cut a thin, lengthwise slab from the center of Buesching’s, 9.5-foot right tusk, chosen for the study because it is longer and more completely preserved than the left.

USING NEW isotopic and life-history analyses technology enabled the scientists to reconstruct the changing patterns of the landscape where the animal roamed during two key periods of his life: his adolescence and his final years of adulthood.

“You’ve got a whole life spread out before you in that tusk,” said Fisher in the press release. He has studied mastodons and mammoths for more than 40 years and helped excavate several dozen of the extinct animals.

Mammoths, mastodons and elephants

Both mammoths and mastodons are distant relatives of modern elephants, with fossil evidence showing that mastodons were slightly smaller with shorter legs and lower, flatter heads.

Mastodon upper tusks were straighter, while the mammoth’s curved, sometimes even crossing in front of each other, and some mastodons grew a small chin tusk on their lower jaw.

It is believed that mastodons came into existence first about 27 million to 30 million years ago, living primarily in North and Central America. Mammoths came long a bit later around 5.1 million years ago in Africa, migrating throughout Eurasia and North America.

While mastodons look a lot like modern elephants, they are not closely related, according to the American Museum of Natural History. Mastodons branched off from the ancestors of modern elephants about 25 million years ago, while the ancestors of mammoths diverged about five million years ago.

Both species began to disappear between 12,000 and 10,000 years ago, though a small pocket of woolly mammoth numbering at about 300 members persisted on Wrangel Island north of the Siberian mainland for thousands of years after the others had disappeared, finally dying out around 3,700 years ago.

In a 2012 study, British scientists reported that analysis of bones found earlier on the Greek Island of Crete actually belonged to a previously unknown island “dwarf mammoth.”

Some scientists believe hunting by early humans may have contributed to the extinction of both mammoths and mastodons, but there is also evidence climate change and variations in habitat which affected their food sources also contributed to their demise.

Mastodons were herbivores that foraged on trees and shrubs. As they grew, chemical elements present in their food and drinking water were incorporated into their body tissues, including the continuously growing tusks. The variants of these chemical elements are called isotopes and they vary according to geographical location, leaving a specific geographic fingerprint of sorts on organic material such as teeth – or tusks. Now, using advanced technology, these variations can be measured and compared to other data, actually enabling researchers to pinpoint geographical locations and time periods the animal traveled.

“The field of strontium isotope geochemistry is a real up-and-coming tool for paleontology, archaeology, historical ecology, and even forensic biology. It’s flourishing,”

 Prof. Joshua Miller, University of Cincinnati paleoecologist

In each year of the animal’s life, new growth layers are deposited upon those already present, laid down in alternating light and dark bands, noted the press release. Thus, the growth layers in a tusk resemble an inverted stack of ice cream cones, with the time of death recorded at the base and the time of birth at the tip.

“The growth and development of the animal, 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, a professor of earth and environmental sciences, a professor of ecology and evolutionary biology, and a curator at the University of Michigan Museum of Paleontology.

ANALYSIS OF Buesching’s tusk revealed a structural record of battles for at least eight years prior to his death, and always during spring/early summer, observed the researchers in their paper. His battles, believed to have been prompted by periods of hormonally heightened aggression as in modern elephants, intensified late in life, leading to tusk fractures and ultimately climaxing with the four-to-five-centimeter-diameter puncture in the side of his skull that killed him, they said.

“Under harsh Pleistocene climates, migration and other forms of seasonally patterned landscape use were likely critical for reproductive success of mastodons and other megafauna,” the researchers wrote in their paper. “However, little is known about how their geographic ranges and mobility fluctuated seasonally or changed with sexual maturity.”

Buesching's exploits

The team’s analyses of the strontium and oxygen isotopes from the two different time periods revealed that the Buesching mastodon’s original home range was likely in central Indiana.

The researchers noted in their report that like modern-day elephants, the young male mastodon stayed close to home until he separated from the female-led herd as an adolescent. In Buesching’s case, as he left his maternal herd at around age 12, his mobility increased and his landscape use expanded away from his native home range, they said. Growing into adulthood, his monthly movement increased, often covering nearly 20 miles per month, and landscape use became seasonally structured, with some areas such as northeast Indiana used only during the assumed mastodon spring/summer mating season.

Buesching made this solo trek annually during the last three years of his life, according to the research paper.

“Every time you get to the warm season, the Buesching mastodon was going to the same place – bam, bam, bam – repeatedly. The clarity of that signal was unexpected and really exciting,” said Miller in the press release. He has used similar isotopic techniques to study the migration of caribou in Alaska and Canada.

Methods and future implications

A tiny drill bit, operated under a microscope, was used to grind half a millimeter from the edge of individual growth layers, each of which covered a period of one to two months in the animal’s life, according to the press release. The powder produced during this milling process was collected and chemically analyzed.

Ratios of strontium isotopes in the tusk provided geographic fingerprints that were matched to specific locations on maps showing how strontium changes across the landscape. Oxygen isotope values, which show pronounced seasonal fluctuations, helped the researchers determine the time of year a specific tusk layer formed.

Then, isotopic data from the tusks were entered into a spatially explicit movement model developed by Miller and his colleagues, noted the press release. The model enabled the team to estimate how far the animal was moving and the probabilities of movement between possible locations – something absent from previous studies of extinct-animal movements, noted the press release.

In order to really understand the regional significance of the northern Indiana mating ground, more sampling of additional male and female mastodon tusks from northern and central Indiana and surrounding states will be necessary, the researchers said in their paper.

“The field of strontium isotope geochemistry is a real up-and-coming tool for paleontology, archaeology, historical ecology, and even forensic biology. It’s flourishing,” Miller said in the press release. “But, really, we have just scratched the surface of what this information can tell us.”