Bat Echolocation Essay
Bat Echolocation Essay
Bat Echolocation Essay
Lucinda Smart
frequency at 2040 kHz (Waters & Jones, 1994). Microchiropteran bats on the
other hand possess possibly the most sophisticated echolocation systems of all
mammals, well adapted to their niche. Through the medium of laryngeal sound
emission, they use a range of different call types and wavelengths, as well as 3
different phases of call depending on which stage of the hunt they are in. The
wavelengths, pitch, frequency and duration of call all have a considerable range
and vary of from species to species; some bats calls have durations from as short
as 1 ms up to over 50 ms (Neuweiler, 1983). Some microchiropterans make use
of FM (frequency modulated) calls whilst others use CF (constant frequency)
and some use a mixture of the two. FM is the best method for determining target
distance and properties, as waves are emitted and received in a wider range
(however, the quality is reduced because of this). CF bats emit waves in a very
narrow range, and are therefore better for long range detection more energy at
a single frequency will be carried further. CF is also better at detecting how the
target is moving, but to avoid pulse/echo interference, the bats also have to
exhibit Doppler-shift compensation. (Tomasi, 2009) This is done by changing the
frequency of the call so that the frequency of the echo is constant, meaning that
the bat does not have to process an echo of multiple frequencies. As a result,
information is processed much more quickly and so the bat can react more
quickly to capture its prey. Shrews (Sorex and Blarina) and Madagascan tenrecs
also echolocate using the larynx. Shrew sounds are frequency modulated,
broadband, and multiharmonic; but unlike bats they use low-amplitude
wavelengths (Tomasi, 1979). Their calls contain no short echolocation clicks
with reverberations and for this reason appear only to be used for orientation
purposes (Siemers et al., 2009), possibly to aid them in navigating their way
around their burrows given they have poor eyesight. It is understood that they
do not use echolocation in order to find or catch prey.
A further method of echolocation employed by five families of
microchiroptera (Rhinolophidae, Hipposideridae, Phyllostomidae, Nycteridae,
Megadermatidae) is nasal echolocation. Nasal echolocation varies from laryngeal
echolocation in terms of the shape of the skull laryngeal echolocators have a
skull shape similar to that of most terrestrial mammals, whereas in contrast
nasal echolocators have the rostral part of the skull rotated ventrally beneath
the skull of the braincase. The nasal cavity is aligned in the direction of flight, as
opposed to the mouth. (University of Arizona, 2011) Nasal echolocation has its
advantages in that it allows for simultaneous calling and chewing. (University of
Cambridge, 2008) Toothed whales also echolocate nasally, although via a slightly
different mechanism. They produce short clicks, which result in a broadband
sound. They force air out through their phonic lips in the skull, and the lips close
resulting in the production of vibrations. The vibrations are then transmitted
through fluid-filled nasal sacs which surround the lips, reflect off of the cranium,
and are then sent towards the melon which is filled with lipids of different
densities. The melon acts as an acoustic lens, focusing the sound waves into a
forwards direction. The returning echo sound waves are received by the lower
jaw, and these are transmitted via the oil-filled sinus in the dentary to the
auditory bullae and in towards the middle ear. (University of Arizona, 2011)
This example of differing skull features in order to echolocate via a similar
mechanism is an example of how different echolocating mammals have different
morphologies, regardless of whether they are from the same family or not. Many
Lucinda Smart
species of microchiroptera have specially adapted pinnas, which are capable of
amplifying the required frequency. The larger the pinna, the better it is at
transmitting lower frequencies. Furthermore, the ear and nose leaf also vary in
shape in order to focus the sound so it is received more clearly by the ears.
There are some problems with echolocation which the mammals making
use of it have to overcome. The first problem encountered by bats is that they
must pay attention to the returning echoes rather than the outgoing sounds.
There are several solutions to the problem and the first is that they can employ
self deafening. The bats coordinate their wing flaps with the emission of the call
in order not to deafen themselves. There are also adaptations within their ear
they have evolved sound dampening muscles which contract or relax depending
on the type of sound required. (University of Arizona, 2011). It is not usually
necessary for the other mammals in water to self deafen as they emit at a lower
frequency. Tenrecs and shrews also do not require self deafening as the sounds
they emit are low amplitude. It is also possible in bats for nerve impulses from
the cochlea to be reduced in lower auditory relay nuclei, in the lateral lemniscus
in the brain. (University of Arizona, 2011). This is called neural attenuation and
combined with self defeaning, it can reduce the perceived outgoing sound by
around 40%. Further solutions are low duty cycles and Doppler shift
compensation.
Lucinda Smart
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