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. In other studies, the hearing range for the bottlenose dolphin has been measured in 75 to 150,000 Hz (0.075 to 150 kHz).
- The range of hearing of a young, healthy human is 15 to 20,000 Hz (0.015 to 20 kHz). Human speech falls within the frequency band of 100 to 10,000 Hz (0.1 to 10 kHz), with the main, useful voice frequencies within 300 to 3,400 Hz (0.3 to 3.4 kHz). This is well within a dolphin’s range of hearing.
- 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.
Ears, located just behind the eyes, are small inconspicuous openings, with no external pinnae (flaps).
Dolphins have acute vision both in and out of the water. A dolphin's eye is particularly adapted for seeing under water.
- Bottlenose dolphins have a double slit pupil allowing for similar visual acuity in air and water. Their eyes are adapted to mitigate varying light intensities. Studies show that the visual acuity of dolphins is similar or below the range of many terrestrial animals. There is currently no reference that measures distance of visual capability.
- 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.
Behavioral studies have suggested they might have some color vision. However, behavioral color vision studies are difficult due to the inability to accurately determine whether the animal is responding to color versus brightness.
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.
Dolphins are primarily monocular (using one eye to process visual stimuli), but also possess the capability for binocular vision (when both eyes are coordinated for vision).
Evidence suggest that bottlenose dolphins use their right eye predominantly for approaching and investigation when processing visual information.
The skin of bottlenose dolphins is sensitive to vibrations. Nerve endings are particularly concentrated around the dolphin’s eyes, blowhole, genital area, and rostrum, suggesting that these areas are more sensitive than the rest of the body.
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.
- 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.
- Three studies indicated that taste buds may be found within 5 to 8 pits at the back of the tongue. One study found them in young dolphins and not adults. Another study could not trace a nerve supply to the taste buds. Regardless, behavioral studies indicate bottlenose dolphins have some type of chemosensory capacity within the mouth.
Behavioral evidence suggests that bottlenose dolphins can detect 3 or 4 primary tastes (sweet, bitter, sour, salty), but the way they use their ability to “taste” is still unclear.
Olfactory lobes of the brain and olfactory nerves are absent in all toothed whales, indicating that they may not have a sense of smell.