Botos, also called Amazon river dolphins or pink river dolphins, live only in fresh water rivers in South America. They are the largest and most abundant of the river dolphins. Adult botos range in length from 6.6 to 8.5 feet (2 to 2.5 meters) and in weight from about 185 to 400 pounds (85 to 180 kilograms). Males are larger than females. Young animals are usually dark gray. As they mature, their color changes and they become pink. However, individuals that live in dark, muddy water tend to remain darker than those that live in clear water.
Botos have thick bodies and a large slender beak (snout) that contains about 140 teeth. Instead of a distinct dorsal (back) fin, they have a small triangular peaked ridge along their back. Their flippers are large and pointed. Botos are very flexible, allowing them to live in shallow, cluttered environments. One reason for their flexibility is that their cervical vertebrae, or neck bones, are not fused or joined, giving them the freedom to twist and turn their head easily.
Botos have good eyesight both above and under water. However, because they often live in dark, murky water, they usually rely on echolocation (eck-oh-loh-KAY-shun) to avoid objects and find food. The forehead of a dolphin is a lump of fatty tissue called the melon. Dolphins make sounds (scientists disagree about how this is done) that seem to be focused through the melon and skull. These sounds are then sent out into the environment. When the sounds bounce back, the echo is passed through special tissue in the lower jaw to the inner ear. From the time it takes to collect the echoes, their strength, and their direction, dolphins construct a "sound picture" of their environment. This system is extremely sensitive and allows the animal to locate objects very small objects.