Dolphins

Dolphins are marine mammals closely related to whales and porpoises. There are forty species. There are almost forty species of dolphin in 17 genus. They vary in size from 1.2 m and 40 kg, up to 9.5 m and 10 tonnes. They are found worldwide, mostly in the shallower seas of the continental shelves. Dolphins are carnivores, eating mostly fish and squid. Dolphins breathe through a blowhole on top of their head.

Dolphins have a streamlined body, adapted for fast swimming. The tail fin, called the fluke, is used for propulsion, while the pectoral fins, together with the entire tail section, provide directional control. The dorsal fin, in those species that have one, provides stability while swimming. Dolphins frequently leap above the water surface, and sometimes perform acrobatic figures. Scientists are not certain about the purposes of the acrobatics.

The dolphin brain is large and highly complex, and is different in structure from most of land mammals. Most dolphins have acute eyesight, both in and out of the water. They have a small ear opening on each side of their head. They can hear frequencies ten times or more above the upper limit of adult human hearing. Hearing is also used for echolocation, which all dolphins have. The dolphin's sense of taste and touch are also well-developed. However, dolphins are believed to have no sense of smell. Dolphins are capable of making a broad range of sounds using nasal airsacs located just below the blowhole. Categories of sounds that can be identified: frequency modulated whistles, burst-pulsed sounds and clicks.

Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats. Earlier sleep stages can occur simultaneously in both hemispheres. In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external stimuli. In this case, respiration is automatic; a tail kick reflex keeps the blowhole above the water if necessary.