Anyone who’s ever played baseball knows that, ideally, the ball should hit the bat’s “sweet spot”–its center of percussion. At least, that’s where you should hit the ball if you don’t want to hurt your hands.
Let’s call the point where you grip the bat the “pivot point”. If you hit the ball with the bat’s center of gravity, the entire bat will move with translational velocity. If you hit anywhere other than the center of gravity, the bat will have both translational and rotational velocity, which could potentially occur at the pivot point. The beautiful thing about the center of percussion is that if a horizontal impulse hits that point, there will be no reactive force at the pivot. The backward translational velocity of the bat and the forward rotational velocity will cancel out. In other words, no matter how hard the ball hits the bat (or the bat hits the ball, whichever you prefer), your hands at the pivot point won’t feel a painful backlash. No wonder my P.E. teachers kept going on about the sweet spot. For those of us who don’t play baseball, many swinging objects have centers of percussion. Including the hammer-like tails of glyptodonts.
Glyptodonts were giant armored mammals, equipped with bony armor all over their bodies, and heavy tail-clubs presumed to have spikes. During the Pleistocene Era, they migrated from South into North America, and survived until about 10,000 years ago. Car-sized armadillo ancestors that packed a punch, researchers thought that glyptodonts used their bony tails to defend themselves from predators, or maybe even to attack each other. Until recently, we weren’t sure exactly how this worked.
In Proceedings of the Royal Society B, Blanco, Jones, and Rinderknecht thought to look at the glyptodont tail the same way we look at a baseball bat. In some species, the latter section of the tail was made up of fused bone segments–in essence, a big bony bat with a hammer on the end. (The analysis would be a bit more complicated if the tail were very flexible).
The center of percussion on the glyptodont tail is markedly near the bony club–if the glyptodont were to hammer an enemy, the mobile joint at the base of the tail would not have been too damaged. Furthermore, most potential spike locations were very close to the observed center of percussion. The glyptodont would have lashed out laterally, striking with the part of the club closest to the center of percussion. Meanwhile, the spikes focused a large force over a very small surface area, causing major damage. The relative positioning of the spikes to the center of percussion suggests antagonistic behavior, the authors say. If the giant were to miss its sweet spot, there were still damage-dealing spikes in the general area of impact.
The authors posit that the design of the bony hammers would have favored static combat, more like ritual competition than self-defense. Only a high degree of accuracy could protect the caudal vertebrae from damage. Accuracy takes time–and there is precious little time when a highly mobile saber-tooth tiger wants to eat you.
And this is what inspires me–going backwards, using morphology to find function and behavior. From the physical properties of the glyptodont bone club, Blanco and his colleagues found out something about how it works, and something about glyptodont behavior in the process. Think about that next time you pick up your baseball bat/tennis racket/golf club.
For more on glyptodont tail structure and function, check the original article: Blanco, R., W.W. Jones, and A. Rinderknecht. 2009. The sweet spot of a biological hammer: the centre of percussion of glyptodont (Mammalia: Xenarthra) tail clubs. Proceedings of the Royal Society B. 276:3971-3978.
Evolutionary Thought 