Cetaceans

Cetologists have been able to infer a good deal about the sensory powers of whales and dolphins. Their vision is good, but is limited to about 45 ft (13.7 m) or so, even in the clearest water; the depths of the ocean are quite dark, and vision is of no use. Their sense of hearing is much more important, in part because water is such an excellent conductor of sound. In addition, many cetaceans navigate and find food using echolocation, or sonar.

An echolocating animal perceives objects in its path by listening for the reflected echoes of pulsed sounds that it produces. In keeping with this practice, the cetacean hearing range is much greater than ours; some species can hear sounds up to 180,000 Hz. The sound is produced in a complex chamber in the airway atop the head, and is conducted out through the melon, a waxy, lens-shaped structure in the forehead. The melon functions to focus the beam of sound the way a magnifying glass focuses a beam of light. We know that around a dozen species of toothed whales and dolphins use sound to find food items; for instance, blindfolded bottlenose dolphins can find fish swimming in their tanks. Baleen whales lack the sophisticated structures for true echolocation, but may use echoes from lower-frequency sounds for a more rudimentary echonavigation.

Scientists once believed that cetaceans had no sense of taste or smell. More recently, bottlenose dolphins have been shown to distinguish the four basic taste stimuli (sour, sweet, salty, and bitter). In addition, beluga whales (Delphinapteras leucas) have been observed to show alarm when swimming through areas where other belugas have been killed and quantities of blood are present in the water. Several cetacean species are sensitive to substances found in mammalian urine and feces, which could provide information on the identity or status of other individuals. Most modern cetologists agree that taste and smell are important to many cetaceans.

The cetacean sense of touch is very keen, as becomes obvious to anyone who watches two familiar animals interacting: whales and dolphins large and small, rub up against each other, stroking and petting one another with flukes or flippers. Such touching clearly feels good to the recipient, and captive dolphins have been trained to do various tricks using touch alone as the positive reinforcement.

Scientists have suggested the existence of a cetacean magnetic sense; this would help to explain their remarkable navigational powers during long migrations in the otherwise featureless marine environment. In support of this possibility, the mineral magnetite has been found in the brains of some species (including common dolphins, Dall's porpoise, humpback whales and beaked whales). A magnetic sense could help explain the bizarre phenomenon of live stranding; although rare, stranding is generally fatal to the whale, which is ultimately crushed by its own weight out of the supporting water. A magnetic sense might not be fool-proof, and could be upset by various disturbances from on shore, leading the animals to beach themselves. Others have proposed that mass strandings of cetaceans are the result of the intense social bonds that form among members of some species. Perhaps the urge to avoid the dangers of separation is stronger than that to avoid the fatal risk of stranding along with a sick or injured comrade seeking shallow water.