Embattled lives: mastodon tusk 'scars' record a history of a combat
(NC&T/UM) "American mastodons were not just docile herbivores that whiled away their time in forests and meadows," said University of Michigan paleontologist Daniel Fisher. "The males, at least, spent a significant portion of each year of their adult lives promoting their own access to females by fighting with other males. They were very aggressive animals."
This new perspective builds on previous work in which Fisher determined—by analyzing bone damage on mastodon skulls and vertebrae and using 3-D computer graphics to "try out" fighting styles that could have produced such damage—that some mature male mastodons died of wounds inflicted by the tusks of other males. Fisher realized that a much longer record of mastodon behavior might be recovered when he looked more closely at one of the animals studied earlier, now exhibited by the Paleontological Research Institution in Ithaca, NY. Along the outer curve of its tusks, he noticed regularly spaced rows of shallow pits. Microscopic studies of the tusk interior showed that internal structures also were deformed along these rows of pits.
Combining knowledge of how mastodon tusks grew with his earlier evidence for how mastodons fought, Fisher developed an explanation for how the pits formed. In combat, male mastodons used their tusks in several ways, but the most damaging blows came when one of the bulls delivered something like a boxer's "upper cut," dipping its head and then swinging it upward, thrusting a tusk tip into the neck or skull of its opponent.
"When you ram a tusk tip into an opponent, the equal and opposite reaction force jams the tusk tip downward," said Fisher. "This causes the lower part of the growing margin, on the outer curve of the tusk base, to rotate backward, crunching it against the back wall of the tusk socket."
Even though the tusk continues growing by adding layers to its base, inside and out, the location of the original damage is marked permanently because the cells responsible for producing new layers of tusk material are damaged when they are pinched between the tusk margin and the socket wall. A series of fights thus leads to a series of pitted scars along the outside curve of the tusk base, one for each year in which the fights occurred.
Fisher also knew from previous research that growth in mastodon tusks follows seasonal patterns. The location of the rows of pits within these patterns told him that the pits always formed at a certain time of year—mid-spring to early summer—and that they formed every year beginning in the animal's late teens or early twenties.
In addition to obtaining a detailed picture of this mastodon's life, Fisher has found evidence of seasonal fighting in tusks from other mastodons. The work also bolsters Fisher's contention that humans hunted mastodons as well as harvesting meat from animals felled in combat. The cases in which he believes hunting occurred involve mastodons that died in autumn.
"We've wondered before if some of these males could have just died in musth battles, but this now seems unlikely. Year after year, musth battles seem only to happen in spring."
Additional data on mastodon musth behavior will help to clarify the animals' response to environmental changes and detect declines in their abundance (resulting in fewer battles) prior to their extinction.
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