Dolpins have a well-developed, acute sense of
hearing.
a. The auditory cortex of the brain is highly developed.
b. The auditory nerve may have 67,900 or more cochlear fibers.
This is twice as many as in the human auditory nerve.
Hearing
range.
a. Bottlenose dolphins respond to tones within the frequency
range of 1 to 150 kHz. (The average hearing range for humans
is about
.02 to 17 kHz.) (Ridgeway, 1990).
b. Peak sensitivity (the range that dolphins hear beast) is
40 to 100 kHz (Ridgeway, 1990).
c. Bottlenose dolphins can detect sound frequencies of less
than 1 kHz, if they are loud enough.
Sound reception.
a. Most sound reception, or hearing, probably
takes place through the lower jaw. Studies show that the lower
jaw most effectively
receives sounds with frequencies above 20 kHz (Brill, et
al., 1988). A dolphin may also receive sound through soft
tissue and bone surrounding
the ear.
b. Unlike humans, a dolphin's inner ear is encased
in a separate bone, called the auditory bulla, which is
connected to
the skull with fibrous tissue. Thus, the bulla is essentially
isolated from the skull, and sound enters the ear most
efficiently through
the jaw and middle ear.
c. A fat-filled cavity in the lower
jawbone appears to conduct sound waves through the jaw to bones
in the middle
ears.
The lower jawbone of toothed whales broadens and is hollow
at the
base, where
it hinges with the skull. Within this very thin, hollow
bone is a fat deposit that extends back toward the auditory
bulla
(earbone
complex). Sounds are received and conducted through the
lower jaw to the middle ear, inner ear, and then to hearing
centers
in the
brain via the auditory nerve. (Brill, et al., 1988).
d. The specialized anatomy of the dolphin ear probably
allows it to localize sounds under water effectively, a task
that is
difficult for humans.
e. A dolphin's middle ear cavity is filled with
a highly vascularized (supplied with blood) tissue. When
a dolphin dives, this tissue
helps adjust pressure on the middle ear.
f. A dolphin has small external ear openings, a few inches
behind each eye. Each opening leads to a reduced ear canal
and an eardrum.
Some scientists believe that dolphins receive sound through
these openings. Research has shown that the external ear openings
may
receive sounds with lower frequencies, below 20 kHz (Brill,
et al., 1988). Other scientists believe that a dolphin's external
ear openings are nonfunctional (Bryden and Harrison, 1986).
B.
Eyesight.
Dolphins have acute vision both in and out
of the water. A dolphin's eye is particularly adapted for seeing
in water.
In air, certain features of the lens and cornea
correct for the refraction of light caused
by the transition from aquatic
to aerial vision. Without this adaptation,
a dolphin would be nearsighted in air (van der Pol, Worst,
and van Andel, 1995).
The retinas of odontocetes
have two central areas that receive images (human eyes have
only one) (van
der Pol, Worst, and van
Andel, 1995). Due to this feature of the
retina, bottlenose dolphins have binocular vision in
air, and may have both binocular and
monocular vision under water (Mass and
Supin, 1995).
A dolphin's retinas contain both
rod cells and cone cells, indicating that they may have
the ability
to see in both dim
and bright light. (Rod cells respond
to lower light levels than cone cells do.) (Ridgway,
1990) The
presence of cone cells suggests
that dolphins may be able to see color,
although studies on bottlenose dolphins haven't documented
color vision.
Dolphins' eyes have a well-developed
tapetum lacidum, a light-reflecting layer that reflects
light through
the retina a second time, giving
them enhanced vision in dim light.
C. Tactile.
Anatomical studies and observations of behavior indicate
that a bottlenose dolphin's sense of touch is well developed.
A bottlenose dolphin's skin appears to be sensitive
to a broad
range of tactile sensations (Herman, 1980).
D. Taste.
1. Little is known about a dolphin's sense of taste.
Features of the brain and cranial nerves suggest they may
have some
sort of a taste sensation.
Bottlenose dolphins
do have taste buds, although they haven't been extensively studied
(Ridgway,
1990).
Dolphins show strong preferences for
certain species of food fishes (Barros and Odell, 1990).
E. Smell.
Olfactory lobes of the brain and olfactory nerves are absent
in all toothed whales, indicating that they
have have a limited sense of smell.
