The disembodied tail

When things are bad, and I mean really bad, horribly you-are-in-the-jaws-of-death bad, sometimes you have to let go of something.

Like a tail.

This leopard gecko (Eublepharis macularius) can, when hassled, have its tail fall off. Losing a limb (autotomy) is not a particularly unusual trick for this species. Lots of animals can drop legs and tails if necessary. But this one is noteworthy because if it does so, the tail doesn’t just come off, but it will continue to twist and writhe for up to several minutes after the tail has been separated from the rest of the body.

We’re not just talking about simple twitching here. We’re not talking about something regular, like a horror-show heart that beats after removal from the body. Higham and Russell show that that the tail is doing at least two things. One is a slow, rhythmic swinging, and occasionally, much faster contortions that made the tail flip or jump around. The flips tend to fade out faster than the slower swinging, though.

When we think about vertebrates movements, we normally think that the brain is involved somehow. But here, the tail has been completely severed from the brain, so how are these movements generated and controlled?

Taking the information from the muscle recordings they made from the tail, Higham and Russell think that the slower rhythm is generated by neurons left in the spinal cord of the tail. We’ve known for a long time, probably since the 1970s, that the spinal cord in vertebrates holds a lot of the neural circuitry needed to generate basic locomotor motions.

Higham and Russell argue that the gecko’s tail the faster movements are more interesting. They think that these flips are not controlled by a set of neurons in the spinal cord that out a rhythm. They marshal a few pieces of evidence for their hypothesis. First, they note that the flips only occur for a couple of minutes after the tail’s been removed, whereas the slow movements continue for up to half an hour. Second, the flips are extremely variable compared to the slow movements, even after you take into account the fact that they’re shorter. Third, when a flip occurs, the muscles along the tail are active simultaneously, compared to the slow movements, where the muscles along the tail are activated one after another.

They don’t know yet what the mechanism of these fast flips might be. Higham and Russell note that working on the neural basis of this behaviour has an advantage: you can do neurophysiological experiments on the spinal cord without having to kill the animal. I’m sure that the gecko appreciates this, but I still bet it will miss that beautiful tale it used to have. They never grow back as nice as the original one.

Reference

Higham T, Russell A. 2012. Time-varying motor control of autotomized leopard gecko tails: multiple inputs and behavioral modulation The Journal of Experimental Biology 215(3): 435-441. DOI: 10.1242/jeb.054460

Photo by A. Jaszlics on Flickr; used under a Creative Commons license.

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