-- MAIN LINKS --

-- EVOLUTION & ADAPTATION -- BALEEN WHALE SPECIES --
-- CLASSIFICATION -- CHARACTERISTICS OF BALEEN WHALES --
-- TOOTHED WHALE SPECIES -- DOLPHINS -- SEALS --
-- ENDANGERED SPECIES -- THREATS -- GAMES --

DOLPHINS: Senses

--- Vision

Bottlenose dolphins can see equally well above and under water. A number of other species, including the killer whales, are known to have the same capability. This is not true for alldolphin and whale species, though.

The river dolphins, who live in very murky waters, have eyes that seem to be adapted for above
water vision only. There are some indications that these species may even be virtually blind.

The dolphin eye is optimised for underwater vision. In the human eye, most of the refraction is done by the cornea, while additional focussing is done by the lens. Underwater the human eye loses most
of its refractive power and the lens cannot compensate for that. Underwater we cannot get a clear picture: we are extremely far-sighted underwater. In the dolphin eye, the refractive power of the
lens has become greatly increased, because the lens is located further forward and is completely spherical.

The dolphin eye looks a lot like a fish eye. The dolphin pupil is rather special: instead of a round hole that narrows in bright light, there is a kind of "lid" (called operculum) that slides down, covering the centre of the pupil, leaving narrow slits on its edges. These narrow slits may give the dolphin more depth of vision above water and therefor better vision. Special adaptations in the edges of the lens may also improve above water vision.

The dolphin retina has both rods and cones like the human eye. The rods are the most sensitive to light and play a major role in vision under low light conditions. The cones are more sensitive to light and also play a role in colour vision.Colour vision in dolphins is probably poorly developed, though. There are indications that the dolphin eye is insensitive to red light.

The dolphin eye has a well developed reflective layer (tapetum lucidum) behind the retina. This indicates that the dolphin eye is adapted for vision in poor light conditions, not unlike the cat.

The dolphin cornea is organised differently than most mammal eyes: instead on one high-sensitivity area (or yellow spot), the dolphin eye has two such areas. One may be associated with forward vision and the other with lateral (sideways) vision.

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--- Hearing

Dolphins have tiny external ear opening, which are barely visible, just behind the eyes. These openings probably have no or only a limited function in hearing. The acoustic faculty in dolphins is well developed. The auditory systems in the brain are highly developed and much larger than for
instance in humans. This strong development of the auditory systems of the brain is at least in part an explanation for the large brains in dolphins. The auditory nerve has double the amount of nerve
fibres compared to the human auditory nerve. Bottlenose dolphins can hear sounds with frequencies between 75 Hz and 150 kHz (in humans the range is 10 Hz to 16-20 kHz). Dolphins are most
sensitive for sounds between 40 and 70 kHz.

An arrow pointing at dolphin's ear

In dolphins, the sound is conducted to the middle ear mainly via the blubber, which is an excellent sound conductor, and the lower jaw. The lower jaw of the dolphin is filled with a fatty tissue, which
conducts sound quite well. This tissue extends from a thin area of the lower jaw to the inner ears. Experiments in which a sound-absorbing hood of neoprene was placed over the lower jaw showed that dolphins with the hood in place had considerable difficulty hearing.

The middle ear cavities of dolphins are independently suspended and surrounded by air-filled spaces. This reduces the contact with the surrounding bone and can probably help the dolphin in directional hearing. The middle ear in dolphins serves 2 functions: one is to stiffen the sound transmission system, optimising it for high frequencies. The other is to balance the pressure between the inner ear and the external environment. The pressure of a given sound in water is about
60 times as high as the same sound intensity in air.

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--- Touch

Dolphins are very tactile animals. Their skin is very sensitive to touch. It is unclear if there are also pain and temperature receptors in their skin.

The areas that are most sensitive to touch are around the blowhole and the eyes and the upper and lower lips, near the corner of the mouth. The snout, melon and lower jaw are somewhat less sensitive while the skin on the back and tail stock are the least sensitive areas of the body.

Neonate dolphins have small "whiskers" which may have a function as a touch sensor. Some river dolphin species have whiskers even as adults. This may help them locate prey at close range.

--- Taste

Partly because dolphins swallow their food whole, many people assumed they have no sense of taste. It is unclear if bottlenose dolphins have tastebuds in their tongue. Tastebuds have been found
in other dolphin species, though. Individual dolphins have developed preferences for specific fish species. If that is related to taste or to consistency of the fish is unclear.

Dolphins seem to be able to detect certain chemicals in the water. There may be some form of chemical communication in dolphins: chemicals excreted by one dolphin can be tasted in the water by other.

--- Smell

None of the toothed whales have a sense of smell. It is possible that some baleen whales still have a rudimentary sense of smell.

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