II.

Physical Description

Dolphins range in size from the small tucuxi dolphin, which grows to about 1.2 m (4 ft) in length and weighs about 50 kg (110 lb), to the killer whale, which can grow to 9.8 m (32 ft) in length and weigh over 5,500 kg (12,100 lb). In most dolphins males grow slightly larger than females.

Dolphins are commonly black, brown, or gray in color and are often marked with patterns of white or light colors. The dolphin body is streamlined and adapted to a life underwater. Two finlike flippers near the front of the body help stabilize and steer the animal as it swims. The flippers contain bones that are similar in arrangement to the bones in the human hand. Horizontal tail flukes, composed of tough connective tissue, move up and down to propel the animal through the water.

Many dolphins have a dorsal fin made of connective tissue in the middle of the back, which helps the animal maintain balance. The dorsal fin of each dolphin has a unique shape, enabling scientists to use this feature to identify individual dolphins in the wild. The dorsal fin of some ocean dolphins is hooked rather than triangular in shape. In some species the fin is tall—the dorsal fin of a male killer whale can reach a height of 1.8 m (5.9 ft). The dorsal fin of river dolphins is small, sometimes barely forming a ridge on the back.

Most dolphins have a prominent snout and, in some species, the jaws contain as many as 250 conical teeth. Some river dolphins have molar teeth in addition to conical teeth. Dolphin teeth are adapted for ripping and tearing prey but not for chewing. Scientists use growth layers in the teeth to determine a dolphin’s age.

The eyes of a dolphin contain elastic lenses that expand and contract, enabling the animal to focus its vision both above and below water. Special glands protect the dolphin’s eyes from stinging salt water. Dolphins often use echolocation (use of reflected sound to perceive surroundings) to navigate and forage for food. They have a bulging forehead, called the melon, which contains a fatty substance that aids in echolocation. Clicks made by the dolphin project from the melon as a sonar beam. The beam bounces off nearby objects, and the dolphin analyzes the reflected sound waves to locate prey and avoid obstacles.

Dolphins breathe through a blowhole located on top of the head. A muscular plug closes the blowhole to keep water out of the animal’s lungs during a dive. When dolphins come to the water’s surface to breathe, they make a short, explosive exhalation through the blowhole, then take a longer inhalation before submerging again. In this process they may empty and refill their lungs in less than one-fifth of a second.

Dolphins have a layer of blubber (body fat) that lies just beneath the skin. This layer of fat insulates the animal, enabling it to maintain a constant body temperature even in the coldest waters. The thickness of blubber varies among species according to their need for insulation. For instance, hourglass dolphins living in the frigid waters around Antarctica have a much thicker layer of blubber than do rough-toothed dolphins inhabiting the warm waters in the Gulf of Mexico.