Though baleen whales include some of the largest animals on earth, they eat only tiny organisms. Instead of teeth, baleen whales have hundreds of plates of fringe (baleen) hanging down from the roof of their mouths. Made of keratin (fingernail material), the fringe serves as a strainer to capture plankton and other small prey. Baleen whales do not swim fast in pursuit of prey. Rather, they take large mouthfuls of seawater that gets filtered as it passes through the baleen. The size of the baleen plates varies, to as long as 4 meters (13 feet) in bowhead whales. Plate size, along with texture of the fringe, is an adaptation to the whale's diet. Long, fine fringe works for capturing tiny plankton, while wider fringe is used for larger prey, such as crustaceans. Baleen wears away during use, but continues to grow throughout a whale's life.