The animal on the cover of Java Network Programming, Third Edition, is a North American river otter (Lutra canadensis ). These small carnivores are found in all major waterways of the United States and Canada, and in almost every habitat except the tundra and the hot, dry regions of the southwestern U.S. They weigh about 20 pounds and are approximately two and a half feet long, and females tend to be about a third smaller than males. Their diet consists mainly of aquatic animals like fish and frogs, but since they spend about two-thirds of their time on land, they also eat the occasional bird or rodent.Two layers of fur—a coarse outer coat and a thick, dense inner coat—protect a river otter from the cold, and, in fact, they seem to enjoy playing in snow and ice. When diving, a river otter's pulse rate slows to only 20 beats per minute from its normal 170, conserving oxygen and allowing the otter to stay underwater longer. These animals are sociable and domesticated easily, and in Europe, a related species was once trained to catch fish for people to eat.