Biology Reference
In-Depth Information
Even this description, however, does a grave injustice to the astonishing complexity of a bat's echo-
location system. Imagine a jet fighter plane—a marvel of human ingenuity and technology—patrolling
the skies through the impenetrable gloom of night. Constantly searching for a possible enemy, its radar
sends forth radio waves at a relatively low frequency that return somewhat limited information but can
“see” objects at long range.
Suddenly this lower-frequency radar strikes what might be an approaching enemy. As the distance
between the planes decreases, the pilot shifts to radio waves of much higher frequency. Although these
lack the long range of the earlier signal, they provide a much more detailed picture of the potential en-
emy.
An amazing human achievement? Substitute sound waves for radio waves, and insects for an enemy
plane, and bats have been doing this for countless millions of years! In fact, bats that hunt by echoloca-
tion use
three
different systems, depending on the species of bat.
One system uses
constant frequency,
usually referred to as CF. A CF bat, as such bats are known,
utilizes brief, intermittent sound bursts at a given frequency. A typical frequency for a CF bat might be
115 kilohertz. This, incidentally, is far beyond the range of human hearing, which extends only to about
20 kHz. So-called CF bats are found in Europe, Asia, and Australia.
Our North American bats use different systems, however. Some, like the southwestern myotis
(Myot-
is auriculos)
use frequency modulation and are known as FM bats. Bats using this method of echo-
location emit bursts of sound that sweep through a wide range of frequencies in an astonishingly brief
period. A typical signal from an FM bat might sweep from 100 kHz to 50 kHz in just two
thousandths
of a second! The FM signals that these bats use are the sound-wave equivalent of radio FM signals.
Other North American bats use a mix of CF and FM sounds, very much in the manner described
in the jet fighter analogy. These bats are called CF-FM bats. When the longer-range CF sounds of a
CF-FM bat, such as the western red bat of Arizona
(Lasiurus blossevillii),
detect potential prey, the bat
zeroes in on its target with FM sweeps that give it a much more detailed idea of the prey's texture.
Because most bat sounds are far above our range of hearing, we fail to realize the extraordinary in-
tensity of their calls. For example, a common species such as the big brown bat
(Eptesicus fuscus)
gen-
erates some 110 decibels of sound at a distance of four inches. This is as loud as the alarm on a smoke
detector at the same distance; we just can't hear it! Animals with higher-range hearing than ours, such
as domestic dogs and their wild kin, can undoubtedly hear some of the lower-pitched sounds emitted
by bats; whether they can hear bat sounds in the upper range of 100 kHz and beyond is highly question-
able, however.
To further add to the amazing variety and complexity of bats' echolocation, consider that some bats
which snatch their prey from the ground or from foliage use
low-intensity
sounds that avoid alerting
their prey until it's too late to escape. Such bats, represented by the pallid bat
(Antrozous pallidus),
are
said to “whisper.” And as an added complication, even bats of a given species may vary their calls ac-
cording to local conditions or the prey which they're hunting.
