dolphin, aquatic mammal: Bottle-nosed Dolphin

Bottle-nosed Dolphin

The bottle-nosed dolphin is blue-gray with a dorsal fin and white belly. Its average length is 9 ft (2.7 m) and its average weight 350 lb (160 kg). Its domed forehead, called the melon, contains an oily substance thought to protect the brain case and to act as an acoustic lens. With age the 200 or more teeth of the bottlenose wear down, hence the name truncatus. Members of this species live about 25 years. Bottle-nosed dolphins swim in large schools with a social organization and hierarchy, hunting the small fish, crustaceans, squid, and cuttlefish that make up their diet. They have been clocked swimming at 30 mi (48 km) per hour, although 20 to 24 mi (32–39 km) per hour is their usual speed. They can dive 70 ft (20 m) and remain underwater for 15 minutes. They sleep by night, just below the surface of the water, rising for air every three or four minutes.

Their aquatic natural enemies are sharks and killer whales; these they attempt to outswim, using complex evasive strategy, or batter to death, acting in a group. If one of their number is injured or sick they make every effort to rescue it, holding it above the water for air. Play behavior is highly developed in the bottlenose from infancy through old age, and in this connection it displays considerable tool-making, tool-using, and manipulative ability; for example, a dolphin has been observed to kill a fish, strip its skeleton, and use the bones, held in the mouth, to pry another fish out of a crevice. Sex play is frequent and is initiated by any individual toward any other, without regard to size, age, sex, relationship, or even species; approaches to human beings and to turtles are common.

Courtship and impregnation occur mainly in spring, when males vie for the attention of the females. A single calf, 31⁄2 ft (97 cm) long and weighing 30 lb (14 kg), is born tail first after a gestation of 12 months. The mother or a female assistant bites the umbilical cord in two and pushes the calf to the surface to breathe; it is nursed for one to two years. One female may watch over several calves while the mothers hunt, or during battle.

The senses of the bottlenose have been subjected to intensive investigation, as have their intelligence and their remarkable systems of echolocation and communication. In relation to body size, the brain of the adult bottlenose is comparable in size to that of humans; it is twice as convoluted and possesses 11⁄2 times as many cells. The bottlenose has partially stereoscopic vision that is keen both in water and in air; when the animal leaps from one medium to the other, its brain corrects for the difference in refractive index. The eye has a glowing layer for night vision and a brownish filter that is lowered over the iris in bright sunlight. The brain has no olfactory lobe and the sense of smell is presumably missing, but the taste buds are well developed and are used to detect underwater chemical traces, as when the dolphin tracks fish.

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