With all the attention heaped on moray eel pharyngeal jaws last month, I’m surprised that no one’s commented on the equally interesting analysis of the alligator death roll that appeared in the August 15th issue of the Journal of Experimental Biology. They’re similar evolutionary stories: moray eels and alligators are both impressive predators with limited feeding abilities that they work around with a nifty adaptation. Moray eels can’t suck in their prey, but they’ve evolved a protrusable pharyngeal jaw that pulls prey down their throat. Alligators can’t bite off chunks of their prey, but they’ve evolved a behavior that’s just as effective at tearing it to shreds.
Alligators (and other crocodilians) are ambush predators. They mainly eat small animals like fish and turtles, though they can tackle a large mammal if the opportunity arises. But then they have a problem. Fish can be swallowed whole, but something the size of a deer needs to be broken into smaller chunks before it can go down a gator’s gullet. And alligators have cone-shaped teeth that are good at grabbing prey, but terrible at cutting it into manageable pieces.
So when an alligator grabs hold of something that’s too big for it to just shake apart or swallow, it starts to roll. The effects are alarming. A rolling alligator can rip off a limb in seconds. The video below will show you what the roll looks like, but if you want to see the other video of an alligator handler’s arm getting ripped off, you’ll have to click here. I have my limits.
But although it’s common knowledge that alligators spin to rip their prey apart, no one had ever looked at how they spin. Frank Fish, a biology professor at West Chester University, decided to change that.
Fish is no stranger to aquatic locomotion. He’s studied how animals swim for more than twenty years, racking up papers on swimming muskrats, platypus, ducks, dolphins, beetles, seals, alligators, and -- of course -- fish. So when he looked at a rolling alligator, he didn’t just see a terrifying predator moving at great speed. He also noticed something unusual. Other animals roll in the water by using their legs (or flippers) to push themselves around. Alligators are different. They don’t use their legs when they roll. They tuck their legs close to their bodies instead.
How does an alligator roll without using its legs for that first push? And how do they keep the spin going? To answer these questions, Fish got alligators to roll in front of the high-speed cameras in his lab. He used baby alligators, presumably because they’re much more manageable. But when they’re handed a chunk of meat to bite, they roll just as ferociously as a ten-foot-long adult.
Fish found that as soon as mouth meets meat,
an alligator tucks in its legs, bends its tail to one side, and starts to roll. The tail
movement is crucial – alligators will not spin if their tails are restrained. By moving the tail away from the center of its body, the alligator changes its moment of inertia, which sets it spinning. Tucking its legs close to its body lets it spin faster, the same way spinning figure skaters speed up by crossing their arms across their chests.
These miniature death rolls produced a shear force of around 0.015 Newtons at the baby alligators’ snouts. Small potatoes, force-wise, but the force produced by the roll turns out to be very sensitive to the size of the alligator. Each time an alligator grows a little longer, it can produce a considerably larger shear force, so where a 300 mm long alligator rotating at 2 turns a second produces 0.015 Newtons of shear force, a 3 meter long alligator spinning at the same rate produces shears closer to 500 Newtons. Spin faster, and the forces get even larger.
Rolling works because muscles and tendons are strong in tension, but weak in torsion. Pull on flesh, and it resists that tensile force. Twist it, and it tears much sooner. So when an alligator latches on to a deer and rolls, it’s using the deer’s own mass against it. The roll may produce a huge force, but a small animal just rolls along with the gator. A larger animal, weighed down by its own tissues, resists the torque and gets torn apart.
Source and image:
Fish, F. E. et al. 2007. Death roll of the alligator: mechanics of twist feeding in water. J. exp. Biol. 210: 2811-2818.
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