WILD NEIGHBORS: Careful, the Snake Might Hear You

By Joe Eaton
Friday January 20, 2012 - 01:00:00 PM
A little cello music for the ball python?
Holger Krisp, via Wikimedia Commons
A little cello music for the ball python?

I’ve lived with a ball python named Shep for something like eight years, and all that time I’ve assumed he was effectively deaf, as snakes were supposed to be. He has never seemed to respond to music, even to bass lines (in contrast to Matt the cat, who leaves the room when fiddle music is in progress.) We all know that the Indian snake-charmer routine works because the cobra responds to the flute-player’s movements, not the sound of the flute. 

This is apparently not quite the case. A Danish biologist named Christian Christensen has demonstrated that ball pythons, at least, can detect airborne sounds. The ball python is not native to Denmark; Christensen probably chose to work with them because they’re small (for constrictors), easy-going, and available in the pet trade. 

“You can’t train snakes to respond to sounds with certain behaviors, like you might be able to do with mice,” Christensen told a reporter. In fact, it’s hard to train a snake to do anything. He and his colleagues attached electrodes to the pythons’ heads to monitor neurons that connected their inner ears with their brains. 

(Yes, pythons have inner ears, although they’re rudimentary compared with ours; instead of the human malleus, incus, and stapes, they have only one inner-ear bone, the columella auris.) 

The snakes were exposed to sound from a speaker suspended above their cage, eliminating the possibility that they would pick up groundborne vibrations. They made no overt response to the sounds, but the nerves fired at some frequencies. The strongest pulses occurred at frequencies between 80 and 160 hertz, comparable to the lowest notes of a cello. The biologists then attached tiny sensors—vibrometers--to the pythons’ skulls and found that the airborne sounds made the bone vibrate. 

Ball pythons rarely encounter cellos in their native West African savannas. It’s not clear how the snakes benefit from their limited hearing ability, or whether the cranial reception of sound waves is more than a vestigial function. It’s also undetermined whether other snakes have similar abilities, although some, including rattlesnakes, pick up sonic vibrations through their jawbones. 

Snakes are highly specialized lizards, in the same sense that butterflies are highly specialized moths. And many lizards, particularly geckos, are highly vocal. They call to attract mates and warn off rivals. One South African gecko species performs in choruses, like a frog. Although some lizards, such as the North American earless lizard, lack external ear openings, they’re still be able to detect sounds. 

So what happened to the ancestral snakes’ ears? Some herpetologists argue that snakes went through a burrowing phase during which their limbs, eyes, and ears degenerated. (Others support an aquatic ancestry; I believe the jury is still out on that one. It’s true that a number of sand-burrowing lizards have reduced or lost their limbs.) The idea is that sound travels well enough through sandy soil that fully functional ears became an evolutionary luxury. Mutant protosnakes with reduced hearing were able to survive and reproduce as well as their predecessors. 

An interesting sidebar: when ancient snakes lost their eyes, they apparently had to re-evolve them from scratch when they returned to the surface and needed a better way to detect prey or predators. Ivan Schwab, the UC Davis ophthalmologist who won the first (and I believe only) IgNobel prize in ornithology for his study on why woodpeckers don’t get headaches, points out that the eye of a typical snake is more like that of a fish in structure than that of a lizard, alligator, or turtle. Snake optical specializations include a large spherical lens, fused eyelids, and blood vessels that feed nutrients and oxygen to the inner retina. (Schwab’s new book, Evolution’s Witness: How Eyes Evolved, is a wonderful exploration of the many ways of making an eye—a good follow-up if you’ve read Dawkins’ Climbing Mount Improbable.) 

Yes, I’ve found myself lowering my voice around the python. But I haven’t played any cello concertos for him yet.