A. Hearing.

1. 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.

2. 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.

3. 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).

e. The specialized anatomy of the dolphin ear probably allows it to localize sounds under water effectively, a task that is difficult for humans.

f. 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.

g. 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.

1. Dolphins have acute vision both in and out of the water. A dolphin's eye is particularly adapted for seeing in water.

2. 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).

3. 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).

4. 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.

5. 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.

2. Bottlenose dolphins do have taste buds, although they haven't been extensively studied (Ridgway, 1990).

3. 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.