When a rattlesnake shakes its tail, does it hear the rattling? Scientists have long struggled to understand how snakes, which lack external ears, sense sounds. Now, a new study shows that sound waves cause vibrations in a snake's skull that are then "heard" by the inner ear.
"There's been this enduring myth that snakes are deaf," says neurobiologist Bruce Young of the University of Massachusetts, Lowell, who was not involved in the new research. "Behavioral studies have suggested that snakes can in fact hear, and now this work has gone one step further and explained how."
In humans, sound waves traveling through the air hit the eardrum, causing the movement of tiny bones and vibrations of tiny hair cells in the inner ear. These vibrations are then translated into nerve impulses that travel to the brain. Snakes have fully formed inner ear structures but no eardrum. Instead, their inner ear is connected directly to their jawbone, which rests on the ground as they slither. Previous studies have shown that vibrations traveling through the ground—such as the footsteps of predators or prey—cause vibrations in a snake's jawbone, relaying a signal to the brain via that inner ear.
It was still unclear, however, whether snakes could hear sounds traveling through the air. So Biologist Christian Christensen of Aarhus University in Denmark took a closer look at one particular type of snake, the ball python (Python regius). Studying them wasn't easy. "You can't train snakes to respond to sounds with certain behaviors, like you might be able to do with mice," says Christensen. Instead, he and his colleagues used electrodes attached to the reptiles' heads to monitor the activity of neurons connecting the snakes' inner ears to their brains. Each time a sound was played through a speaker suspended above the snake's cage, the researchers measured whether the nerve relayed an electrical pulse (the snakes showed no outward response to the sounds). The nerve pulses were strongest, the researchers found, with frequencies between 80 and 160 hertz—around the frequency for the lowest notes of a cello, though not necessarily sounds that snakes encounter often in the wild.
The snakes don't seem to be responding to vibrations that these sounds cause in the ground, since these vibrations were too weak to cause nerve activity when they weren't accompanied by sound in the air, Christensen and his colleagues found. However, when the researchers attached a sensor to the snake's skull, they discovered that the sound waves were causing enough vibration in the bone—directly through the air—for the snakes to sense it. The results appear online today in The Journal of Experimental Biology.
Young calls the work "extremely nice," but he notes that the team studied only one species of snake. "Given that there are almost 3000 types of snakes, the next question would be how this differs between them." Some snakes, he notes, are known to be better at sensing vibrations through the ground, so their ability to sense sound waves in the air might be reduced. Since many sounds are too weak to cause ground-borne vibrations that snakes can sense, having both abilities helps them detect a wider range of noises. Some salamanders and frogs lack eardrums, too, he notes, and they may listen in the same way snakes do.
Young also says that there are probably other ways that snakes are sensing vibrations in the air and the ground. "We know snakes have some special sense organs in their skin and their head that likely react to vibrations. And we have some evidence that they detect vibration along the length of their body," he says. "This is unlikely to be the final word on how snakes sense sound and vibrations."