Vibrating Skulls Help Snakes Hear
What's that? Despite having no external ears, ball pythons aren't as deaf as scientists once assumed.
Credit: Christian Christensen
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.
http://news.sciencemag.org/sciencenow/2011/12/vibrating-skulls-help-snakes-hear.html?ref=em&elq=0064d8179b214cf5ae154ef9099f9467
