In many ways life in the ocean is very different from life on land. One of the most important differences is in visibility. On a clear day on land, it is possible to see 10 miles or more. Under water, even in rare instances when light and water conditions are ideal, visibility may be only a few hundred feet. Another difference is in the way sound travels. Sound carries better in water, traveling much farther and about five times faster than in air.

Because of these differences, many marine animals have adapted to life under water in ways that emphasize sound and their sense of hearing, while de-emphasizing sight. Dolphins and whales, or cetaceans as these animals are known collectively, specialize probably more than any other type of marine animal in their use of sound. Not only do they hear very well -- some species hear more than 50 percent better than humans do -- but many have also evolved vocalizations specialized for communication, navigation, and locating food.

Communication is probably the most important use of sound among cetaceans. Using clicks, whistles, and songs, often in combination, whales and dolphins keep tabs on members of their family or social group. This is especially important during long-distance migrations, in the middle of chaotic feeding frenzies, and more regularly simply to reassure members of the group that all are present and accounted for.

Many marine mammals also use sound to navigate through their environment and to find food. In a manner similar to bats, some whales and dolphins use what scientists call echolocation to learn about the size, shape, and distance of objects around them that they are unable to see. They do this by emitting high-frequency clicks and whistles and then listening for those sounds to echo, or bounce off the objects. To a whale or dolphin, vocalizations bouncing off of structures on the ocean floor sound much different from those bouncing off of a school of fish or squid. Many species possess such highly attuned echolocation abilities that they are able to find and catch prey in complete darkness.