Have you ever heard someone say they're
"blind as a bat"?
Well that expression is a bit misguided.
During the day, some kinds of bats can
see almost as well as humans. But when
the sun goes down, nocturnal bats rely on
their ears more than their eyes to navigate.
This special navigation process
is called echolocation. In echolocation,
bats use a combination of the voice,
ears, and echoes to create a perceptual
map that allows them to "see" in the dark.
You might be wondering where an echo
comes from. An echo is the reflection of
sound reaching a listener. Sound is a
vibration that ripples away from its
source in waves. The rate at which this
vibration occurs is called its frequency.
Sound waves at high frequencies are
high-pitched. Humans can hear sounds at
frequencies between 20 and 20,000
waves per second and don't
perceive pitches outside that range.
Bats, on the other hand, can hear and
produce sound at frequencies over
100,000 waves per second- way beyond the
range of human hearing. Here, we've slowed
down a bat's chirp, so you can hear it!
This chrip is the first step in the
process of bat echolocation, the primary
sound that the bat will use to detect
information about its surroundings.
The bat listens as the chirp leaves its body
and bounces off its surroundings.
When the sound returns to the bat, the
cartilage of the outer ear funnels the
sound waves into the ear canal. The waves
then reach the eardrum, which vibrates in
response. These vibrations are
transferred further into the ear to the
basilar membrane, which runs the length
of the cochlea. If you uncoil the cochlea
and the basilar membrane, they would look
like this.
Each area of the membrane moves in
response to a particular frequency. Then,
the cochlea converts these vibrations
into nerve impulses for the brain to
interpret. The bad echolocate so fast
that it can track down a dinner of
insects moving around in complete
darkness. Special muscles in its throat
make it possible for the bat to chirp
160 to 190 times
per second! Because it receives so much
information at once, it has to
differentiate relevant information from
signal clutter. To do this, the bat emits
chirps of varying frequency patterns,
each of which echoes off objects
differently depending on the object's size,
shape and distance from the bat. Then, by
analyzing resulting echo intensities, it
creates a mental map of its surrounding
physical space. When looking for prey, the
bat first sends out low-frequency chirps
over a wide range. Llarger objects will
reflect more intense echoes, but it's
often the smaller objects, like insects
that the bat is targeting. Once a bat
receives low-intensity echoes from small
potential prey, it will remit a high
frequency chirp directed toward it. When
both high-and low-frequency echoes
return to the bat, it is able to ignore
the now-irrelevant, low-frequency ways
that reflect from the trees or rocks
that surround its meal. To gauge how far
away a given object is, the bat analyzes
the time delay between when it hears its
chirp and perceives the resulting echo.
Directional information comes from
comparing information the bat receives
in each ear. For example, let's imagine
that a bat's chirp echoes back from an
insect to this side of the bat. Since
this ear is closer to the insect, it
will receive the sound before the other ear.
By interpreting the difference
between when it receives the echo in
each ear, the bat can guide itself
towards the insect. Bat are only one
type of animal that use echolocation.
killer whales, dolphins, porpoises and
some birds echolocate as well. Scientists
who developed sonar and radar got their
ideas from the study of echolocation in
bats and whales. The U.S. Navy and other
research groups are especially
interested in echolocation because it
could lead to even more technology!
Maybe now you'll think twice the next time
someone says "blind as a bat!"