Glyptodonts Were Savvy Batters

26 August 2009 (All day)

Andrés Rinderknecht

Batter up. A new study finds that glyptodonts, shown here in a reconstruction, hit with the sweet spot of their clublike tails.

What do ancient armored mammals have in common with Babe Ruth? They both took advantage of the "sweet spot." New research suggests that some species of giant mammals called glyptodonts swung their hefty tails like baseball bats, landing powerful blows with the spot on their tails that minimizes potentially harmful vibrations for the slugger.

On a baseball bat, the sweet spot is a place where the vibrations generated by an impact cancel each other out. When a ball connects with this spot, the batter feels almost none of the tremors generated by the collision. Hitting a ball with the bat’s sweet spot helps batters avoid injury.

Relatives of modern-day armadillos, glyptodonts arose in South America some 20 million years ago and lived until about 10,000 years ago. They sported a turtlelike shell and heavy armor on their heads and tails. The largest species were massive. "We are talking of an animal the size of a small car," says Rudemar Ernesto Blanco, a physicist who studies paleobiomechanics at the University of the Republic in Montevideo, Uruguay, and co-author of the new paper.

Some scientists have suggested that glyptodonts with clublike tails used these structures like moose use their antlers, to battle for mates. Blanco and his colleagues reasoned if glyptodont tail clubs evolved to strike thick glyptodont armor, the clubs would have to land some pretty powerful blows. To prevent injuring the joint that connects the tail to the body, the animal would likely land those blows with the sweet spot on its tail.

To determine whether glyptodont tails were designed to strike with their sweet spots, Blanco and his colleagues gathered tail club fossils from five species with clublike tails. To find each tail's sweet spot, or center of percussion as it's known to physicists, the researchers took detailed measurements of the fossil tails, which ranged from 34 to 105 centimeters in length. Those measurements, along with a few calculations, enabled them to reconstruct the shape of the tail and locate the sweet spot.

For the three largest genera, Doedicurus, Castellanosia, and Panochthus, the researchers found that the sweet spot was near the elliptical depressions that many paleontologists believe once held horny spikes. That location suggests that glyptodonts indeed evolved to land strong blows with their sweet spots. "The design is much more precise than we thought previously," Blanco says. The research, which appears today online in the Proceedings of the Royal Society B, suggests that these species "used the tail for ritualized combat, probably between the males of the same species," he says.

Had the tails been used for another purpose--for example, to combat predators--hitting near the sweet spot would not have been so important. Predators living at that time were much smaller than the glyptodonts and comparatively fast, the researchers point out. Such a large, heavy tail club with a limited range of motion would not have been a very effective weapon.

Instead, the tail clubs seem well suited to deliver heavy blows against a slow-moving rival glyptodont. "But that does not imply that the tail cannot be used for defense also," Blanco says. In fact, in the smaller species, the spikes did not match the sweet spot. These animals may have used their smaller, more mobile tails in a different way, perhaps to defend themselves, he says.

"This is an excellent example of the application of mathematical modeling to increase our understanding of the behavior of extinct species," says Nick Milne, a functional anatomist with the University of Western Australia in Crawley. "It provides more evidence to support earlier suggestions" that these tail clubs evolved to hit the rounded shell of another glyptodont.