Comparative Hearing
Julia Nussbacher
Melissa Walker
Katherine Cummings
Kyle Rosenblad
Reptiles
-Julia Nussbacher
Reptiles!!!
Reptiles can either have an inner ear or an outer ear, though not in the sense that humans have
ears. The external ear of a reptile has a visible tympanic membrane (in humans it is known as the
eardrum), which is basically a thin membrane that acts as a barrier between the external portion
of the ear and the middle ear. This membrane is either directly next to the skin of the reptile or
located deeper in the head of the reptile. The function of the tympanic membrane is to cover the
middle ear cavity. On the side of the middle ear cavity adjacent to the inner ear are two distinct
openings. One of the openings is circular and is covered by a thin membrane. A membrane does
not cover the second opening, which is shaped like an oval and is located closer to the neck of the
organism[1].
Reptile auditory organ systems also include stapes, which are bones shaped like stirrups located
in the middle ear (in humans it is the smallest bone in the body!!!). The stapes of a reptile cross
the middle ear cavity and the inner end is situated in the oval opening while the outer end has a
cartilage cap that is adjacent to the tympanic membrane[1]. This particular cartilage cap is also
known as the extrastapes and is attached to the quadrate, which provides the majority of the
support for the lower jaw.
If we look beyond the middle ear we come to the inner ear cavity, which contains the organs,
which enable the organism to maintain balance, and also contains the organs associated with
reptilian hearing, more specifically, the cochlear duct. The inner ear contains a fluid known as the
perilymphatic fluid in which is suspended the saccule and the cochlear duct[1]. There is also
perilymphatic fluid within the cochlear duct itself. The duct has two distinct regions, which are
called the papilla basilaris and the macula lagenae. These regions are composed of multiple
sensory cells with cilia-covered membranes. The cells eventually connect to the auditory nerve.
So now that we know about the parts of the reptilian hearing system, lets look at how they work
together to allow reptiles to hear!
When a sound is made, it produces vibrations that travel through the air and eventually come into
contact with the tympanic membrane. Reptiles also have a bone in their body called the quadrate,
which can also detect vibrations in the surface, which they are on such as the ground. When the
sound vibrations contact the tympanic membrane or the quadrate it causes them to vibrate as well.
Vibration of the tympanic membrane causes the extrastapes and therefore the stapes to vibrate as
well. The result of this is the movement of vibrations through the middle ear cavity, through the
openings and into the inner ear cavity where the cochlear duct is located. The vibrations cause the
sensory cells of the cochlear duct to stimulate the auditory nerve facilitate the transmission of the
information from the external world to the brain.
It is interesting to note that many reptiles lack the tympanic membrane and so instead ‘hear’ with
the quadrate. Furthermore, the ability to hear with the tympanic membrane varies among species
of reptiles which is the result in variations of membrane thickness, the depth at which the organs
are located in the head, and also the relative sizes of the structures which make up the auditory
system.
Now lets look at the auditory system of some specific categories of reptiles: crocodilians, snakes,
and lizards
Crocodilians: Relatively speaking, crocodilians are most responsive to lower frequencies of
sound. They hear best within the range of 50-1500 Hz[1]. What makes crocodilians unique is that
they can hear sound by using organs other than those that are strictly sensory (the ears and jaw
bone). Crocodilians have what are called apical pits, which can be found on the scales that cover
their bodies. These apical pits can detect vibrations due to sound while the animal is submerged
in water.
Snakes: As it turns out, snakes do not have external ears like many other reptiles. The skin of
snake bellies has tiny mechanisms called mechanoreceptors, which are sensitive to vibrations in
the ground. The mechanoreceptors then transmit the vibrations to the quadrate by way of the
spinal nerves. The quadrate is a bone located on the side of the head which aides in transmission
of sound to the cochlea.
After the vibrations pass along the spinal nerves they make their way ultimately to the inner ear.
Snakes are most sensitive to sound frequencies around 200-300 Hz[1].
It is interesting to consider here the idea of snake charming. It turns out that the snakes are
actually mesmerized by the sight of the flute rather than the sound. That is not to say, however,
that the snakes are not aware of the music. They may not hear the music the way we do, but they
can sense the vibrations.
Lizards: Lizards are capable of hearing sounds ranging from 500 to 4000 Hz[1]. However, they
hear best at a frequency of 700 Hz. The papillae (a protrusion at the base of a hair follicle) of
lizards’ ears have two distinct types of hair cells. One type has larger diameters at the base, is
more numerous, and contains larger afferent (brining towards an organ) nerve cells and efferent
(conducting away from an organ) innervations. The other type is smaller in basal diameter, its
afferents are of a smaller size and a lesser number, and they are completely lacking in efferent
innervations. In addition, the afferent nerve fibers are much more frequency-selective[2].
1. Kaplan, Melissa. (2002). Reptile Hearing. Herp Care Collection. < http://www.anapsid.org/
reptilehearing.html>
2. Manley, G. A. (2000) Proc. Natl. Acad. Sci. USA 97, 11736-11743.
Fish
-Melissa Walker
If you walk along a stream, the term ‘babbling brook’ will certainly make a great
deal of sense. Turbulent water like this can be quite loud when perceived by our ears and
by those of other creatures as we walk by on land. Imagine, though, how the many
species of fish living within the stream itself must deal with the constant noise of running
water. Even areas underneath grand waterfalls are home to an abundance of fish, all of
whom share similar ear structures for hearing. These ears, however, are quite different
from our own due to the environment that shapes them.
Sound for fish, of course, does not travel in waves through the air as it does for us.
Instead it (quite literally) travels in waves through the water. Since air density is much
less than that of water density, the mechanics of these waves change. Sound actually
moves much faster through water than through air, and this leads to its wavelength also
being exceptionally longer. This means that noises at certain frequencies may have longer
wavelengths than the fish’s body itself! Typically, this constrains fish to hearing a range
of low frequency noises roughly between 40-1,000 Hz. As far as perceiving these waves
is concerned, density plays a large role. For fish, whose flesh density is roughly
equivalent to the water itself (lest they sink or float), these sound waves would pass
through them without disturbing any tissue. Their auditory structures, appropriately then,
are designed to be different densities than the surrounding environment.
Fish boast a great diversity of hearing organs and hearing methods among
themselves, but are basically made of the same components. Two inner ear assemblies,
made of bone, contain structures such as the otolith, saculus, lagena, and cranial nerve.
The otolith itself is covered in more than 100,000 hair cells, which are shifted by the
sound wave. This directionally specific change displaces the otolith, since it has a greater
density than water. The cranial nerve perceives this shift, and the message then travels to
the brain.
Since fish live in such diverse environments, some have more advanced auditory
methods than others. Fish in streams, for instance, benefit from simpler ears, since the
background noise they experience overpowers quiet signals. Fish in the deep ocean, on
the other hand, have extremely complex systems, since their environment is so quiet and
sensitive to the smallest noise. Lack of other sensory, since there is no light to see by,
also puts strain on having good ears at such depths. In general, this divides fish into two
broad groups: hearing specialists or hearing generalists. Hearing generalists typically live
in loud stream environments that necessitate a high threshold for noise. Hearing
specialists- the fish that live in quieter waters- often use other organs to enhance their
hearing sensitivity, such as their swim bladders. These gas filled sacs are sensitive to
sound vibrations because they are much lighter in density than the fish’s flesh or the
water surrounding. By connecting this bladder to the inner ear, most successfully through
Weberian ossicles, these vibrations can be transmitted and perceived. Fish with this
adaptation can hear a much greater range of frequencies than those with simpler systems.
In general, fish can be just as sensitive to auditory stimulation as other
vertebrates. They can also distinguish between a sound’s intensity, frequency, and
direction, as well as pick out certain tones among white noise. Since these capabilities are
so similar to that of humans, it is thought that fish may have laid the evolutionary
precursor to our own ears.
Sources:
http://www.parmly.luc.edu/parmly/fish_aud_psych.html
http://jn.physiology.org/cgi/content/full/77/6/3060
http://www.life.umd.edu/biology/popperlab/research/deepsea.htm
http://www2.biology.ualberta.ca/jackson.hp/IWR/Content/Anatomy/Inner_Ear/index.php
Dolphins
-Katherine Cummings
Sound travels five times faster in water than air, making a dolphin’s sense of sound its most
important sense. Without it, a dolphin would be unable to communicate or locate objects
in its environment. Dolphins use echolocation, a process so precise it could be referred to
as “seeing with sound.”
First, a dolphin generates a clicking noise with its nasal sacs, located behind its
forehead (or “melon.”) The melon consists of fatty tissue and fluid and acts as an acoustic
lens, focusing each click into a narrower, more direct path. When the sound reaches an
object, some of the energy bounces back to the dolphin in the form of an echo. The echo
reaches the panbone of the lower jaw, and is transmitted to the middle ear by the fatty
tissue behind the panbone. From the middle ear, the sound travels to the brain.
A dolphin can tell how far away the object is by emitting more clicks, evaluating the
length of time between each click and echo to determine the distance. Depending on
which side of the panbone receives the echo, a dolphin can also determine the exact
direction of the object. This system is so precise, a dolphin is capable of locating a pingpong ball- sized object that is a football-field away. Dolphins can hear 7.5 times more
accurately than humans; their range is from 0-150 kHz, and humans can only detect sound
from 0.2 kHz to 20 kHz.
They can echolocate on distant and proximate objects at the same time in a noisy
area, while simultaneously whistling to communicate with other dolphins.
DOLPHIN COMMUNICATION
Every dolphin has its own unique whistle, which aids in identification and efficient
communication. These whistles, known as signature whistles, can be thought of as
dolphins’ names. Dolphins emit their whistles when looking for prey, in danger, or trying
to locate members of their family. They’re social creatures, acting together to find prey or
protect each other from danger. These whistles allow dolphins to find one another in bad
visual conditions over long distances. Without their fine-tuned sense of sound, dolphins
would be solitary and wholly unable to navigate the sea.
http://www.dolphins.org/
http://www.inkokomo.com/dolphin/echolocation.html
http://www.dolphinsplus.com/dolphin-information.htm#echolocation
Bats
-Kyle Rosenblad
Echolocation in Bats
Basics: Echolocation is a sensory system used by a variety of animals—such as
bats, porpoises, toothed whales, and a few species of birds and shrews—in which they
emit sounds and listen to the returning sound waves that bounce off of objects in their
environments to locate those objects. The brain of an echolocator is able to process fine
details of the returning sound waves to determine the distances, shapes, and relative
directions of surrounding objects (“echolocation”). All known bats of the suborder
Microchiroptera, or small bats, are echolocators, whereas no known bats of the suborder
Megachiroptera, or large bats, except those of the genus Rousettus use echolocation
(“sound reception”).
Ear Structure: Bat ears are adapted for echolocation. Their large pinnae make
optimal sound collectors, and they are able to very precisely manipulate their ears in
order to focus on specific auditory stimuli. Bats are thus adept at “funneling” desired
sounds toward the inner ear (“sound reception”).
Frequencies: The cries of most echolocating bats range from 80,000 to 30,000
hertz (“sound reception”). They use such high frequencies because in order for a bat to
gain an accurate “auditory image” of its surroundings, the sound waves it emits must be
small in relation to the objects off of which they will bounce and return. This helps the
bat better discern differences in shape, distance, and direction of surrounding objects. The
finer the sound waves it receives, the more detailed the spatial information it can glean
(“sound reception”). In addition, animals with smaller heads must use higher frequencies
because the distance between ears is an important factor in distinguishing details in
echolocation. When the ears are close together, the differences between the waves
returning to one ear and those hitting the other are less pronounced, so the bat must
“counter” by emitting cries of a high frequency (Heffner et al).
Predation and Coevolution: Many bat species use echolocation for hunting
(“bat”). Often, natural selection engenders an auditory/vocal “arms race” between
echolocating, predatory bats and their prey. For instance, some species of echolocating
bats in Canada and Côte d'Ivoire prey primarily on moths. Many of these bats use
frequencies between 20,000 and 40,000 hertz for echolocation. Unfortunately for these
bats, natural selection has thus favored moths that are able to hear well in this frequency
range and thus anticipate bat predation. However, natural selection has given some bat
species another leg up—these species have developed echolocation strategies that use
lower frequencies that are much more difficult for moths to detect (Fenton and Fullard).
In this way, natural selection shapes the strategies that predator and prey use to achieve
their respective goals of eating and escaping. Those who manage to eat or escape are
given the best chance at surviving to reproduce and pass their advantageous traits on to
subsequent generations.
References
"bat." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 22 Oct. 2006
<http://search.eb.com/eb/article-252422>.
"echolocation." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online.
22 Oct. 2006 <http://search.eb.com/eb/article-9031903>.
Fenton, M. Brock and James H. Fullard. “The influence of moth hearing on bat
echolocation strategies.” Journal of Comparative Physiology A: Neuroethology,
Sensory, Neural, and Behavioral Physiology. Volume 132, Number 1 / March,
1979. Pages77-86.
Heffner, H.E., R.S. Heffner, and G. Koay. “Hearing in large (Eidolon helvum) and small
(Cynopterus brachyotis) non-echolocating fruit bats.” Hearing Research
Volume 221, Issues 1-2 , November 2006, Pages 17-25.
"sound reception." Encyclopædia Britannica. 2006. Encyclopædia Britannica Online. 22
Oct. 2006 <http://search.eb.com/eb/article-64827>.