Comparisons of mammal brains are described as the ratio of brain size relative to body size. Bottlenose dolphin brains are larger than many other mammals of the same body size. Scientists are still determining what aquatic adaptations require the large brain size. One likely theory is that a larger brain size in dolphins may be at least partially due to an increased size of the auditory region to facilitate sound processing.
Hypotheses that large brain size in dolphins indicates high intelligence are untested and disputed. The ability of an animal to process information is based upon its brain anatomy as well as the specific experiences the animal has. Rating the intelligence of different animals is misleading and extremely subjective. In fact, a reliable and consistent intelligence test for humans has yet to be developed. It would be improper to attempt to quantify or qualify the intelligence of animals using only human guidelines.
Dolphins have a well-developed, acute sense of hearing.
The auditory cortex of the brain is highly developed.
The dolphin's auditory nerve is about twice the diameter of the human eighth nerve (connecting the inner ear to the brainstem) leading to more rapid sound processing for dolphins. In addition, a dolphin's auditory nerve supply is about three times that of humans — possibly providing more ultrasonic information to a dolphin's central nervous system for echolocation.
Bottlenose dolphins hear tones with a frequency up to 160 kHz with the greatest sensitivity ranging from 40 to 100 kHz. The average hearing range for humans is about 0.02 to 20 kHz.
A dolphin's small external ear openings don't seem to be important in conducting sound. They lead to reduced ear canals that are not connected to the middle ears.
Soft tissue and bone conduct sound to a dolphin's middle and inner ears. In particular, fat lobes in a toothed whale's lower jaw appear to be an adaptation for conveying sound to the ears.
In dolphins, ears aren't attached to the skull. Ligaments hold each ear in a foam-filled cavity outside the skull. This separation of the ears allows a dolphin to localize sound, which is important for echolocation. Humans and most land mammals cannot effectively localize sounds under water.
Dolphins have acute vision both in and out of the water. A dolphin's eye is particularly adapted for seeing under water.
Dolphins have excellent eye sight both above and below the water's surface.
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.
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.) Researchers theorize that all modern cetaceans, including all toothed whales, lack S-cone cells and therefore aren't able to discriminate color in the blue wavelengths.
Dolphins' eyes have a well-developed tapetum lucidum, a light-reflecting layer that reflects light through the retina a second time, adapting their vision to low-light levels.
Studies of bottlenose dolphins suggest that the most sensitive areas on the dolphin's body are the blowhole region and areas around the eyes and mouth.
Little is known about a toothed whale's sense of taste. They do have taste buds at the base of the tongue, although they haven't been well studied.
Whales and dolphins have taste buds. One research study showed that bottlenose dolphins can distinguish chemicals such as citric acid. In zoological parks, whales and dolphins show strong preferences for specific food fishes.
Dolphins have taste buds at the base of their tongues, but little is known about their sense of taste.
Olfactory lobes of the brain and olfactory nerves are absent in all toothed whales, indicating that they may not have a sense of smell.