It
was an Italian, Lazzaro Spallanzani, who in 1793 first discovered that
bats were disoriented when they could not hear but that blinded bats could
still avoid obstacles. I the late 1930s the invention of a microphone sensitive
to high frequencies enabled Donald Griffin in the US to discover in 1938
that bats produce ultra-sonic sound. the term "ultra-sonic" means
sounds of higher frequency than is audible to humans. So, humans can perceive
sounds from 20 Hz to 20.000 Hz, while bats' sensitivity ranges from less
than 100 Hz to 200.000 Hz.
Probably all microchiropterans use ultrasounds which they are produce
with their larynxes. A single species, such as the high, fast-flying
Noctule bat in Europe, produces different emissions while migrating, cruising
looking for food, chasing and catching food, and when flying or feeding
in close company with other bats. During high migrating flight loud low-frequency
pulses at one-second intervals are used, presumably to keep it in contact
with the ground. If prey is detected pulses lasting under 5 milliseconds
(msec) are produced sweeping down through a frequency range of over 40
kHz and up to 200 per second in the terminal phase of the chase. Individuals
flying in a group alter their frequencies slightly so that they can more
easily detect their own echo.
Sounds are emitted through the open mouth or nostrils depending on species.
Those bats with elaborate noses, including horseshoes, leaf-nosed, slit-faced,
false vampires and spear-nosed bats, and some common bats like the long-eared
Plecotus,
emit sounds through the nose. The nose-leaf, acting as a tranducer, may
modify, direct and focus the sound, producing a more concentated beam.
In order to scan an area the head is moved from side to side. the shape
of the nose-leaf is constantly modified to accomodate the changing needs.
Members of only one megachiropteran genus, Rousettus,
produce echolocation sounds. Rousette bats usually roost in caves
and their echolocation enables them to navigate out of the caves, and thereafter
they rely on sight. Unlike other echolocation bats, rousettes produce sound
pulses with their tongues. The sounds are audible to humans as
metallic clicks.
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