Bull mastodons in deadly combat; sound and fury from silent bones

October 16, 2003

ANN ARBOR, Mich.---The American mastodon, a massive, tusk-bearing relative of elephants, inhabited much of North America until its extinction just 10,000 years ago. Strictly plant-eaters, mastodons are often portrayed browsing peacefully on vegetation or lumbering around in small family groups. But mastodons may have had an aggressive side as well.

New studies of bone damage on fossil remains of mature mastodon males---aided by 3-D computer graphics---indicate that some died of wounds inflicted by the tusks of other males. University of Michigan paleontologist Daniel Fisher will discuss the results at a news conference Oct. 16 during the annual meeting of the Society of Vertebrate Paleontology in St. Paul, Minn.

The most telltale evidence for mortal combat among male mastodons consists of areas of crushed skull bone, typically on one side only, behind the eye and under the cheek region. Inflicting bone damage in this location would have required a tusk to penetrate tough hide and nearly 20 inches of muscle, causing extensive blood loss and trauma to the muscles used for chewing. The same skeletons bear other signs of damage, such as crushed vertebrae, suggesting paralyzing blows to the back.

Weighing six tons or more, an adult male mastodon would have been no pushover. "Their skeletal structure suggests even greater strength than we see in elephants today," said Fisher, "and their tusks could have inflicted enormous damage."

In addition to impact damage on skulls and vertebrae, clues that mastodons used their tusks as weapons can been seen in tusk sockets. In life, the animal's tusks were held in place by ligament fibers embedded in the inner wall of the socket. These fibers decomposed long ago, but pits in the socket wall still show where they attached and how large they were. In mature males, the upper edge of the socket shows signs of greatly enlarged fibers, forming an effective shock absorber exactly in the position needed to deal with impact on the upturned tusk tip.

"A shock absorber in this position would not have been needed for ramming straight ahead with the tusks or for tusk use during feeding," Fisher said, "but it would have helped if the animal was sweeping its head forcefully upward, thrusting its tusk tip into an opponent."

One problem in figuring out how bull mastodons fought is that their tusks are spirally curved and usually found separate from skulls. This makes it difficult to visualize accurately the three-dimensional movements of tusks and skulls of both opponents. Fisher used 3-D digital models of mastodons, manipulated on a computer, to "try out" various fighting styles.

"This is one place where computer graphics gives us more than just a fancy display," said Fisher. "The consistency of placement of the damage on skulls suggested some stereotyped fighting behavior, but before working with the models, it wasn't clear how you could get a tusk tip into the right position to produce the damage we saw."

Understanding the behavior of male mastodons helps in assembling a broad picture of the ecology of these animals. Scientists still debate the causes of mastodon extinction, along with that of mammoths (another relative of elephants) and many other large mammals at the end of the Ice Age. "Climate change, disease and hunting by humans are all possible factors," Fisher said, "but conflict between adult males needs to be recognized as part of the ongoing background with which other causes of mortality should be compared."
For more information:
Society of Vertebrate Paleontology---http://www.vertpaleo.org/
Daniel Fisher---http://www.geo.lsa.umich.edu/dept/faculty/fisher/index.html

University of Michigan

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