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.

Cetaceans evolved from land mammals about 50 million years ago, making the transition to water through a variety of adaptations. A modern cetacean is so streamlined for swimming that it looks like a fish, despite its terrestrial origins. Its head is connected smoothly to its body, with no neck or shoulders showing. Nothing sticks out of a cetacean's body other than fins and a tail. External ears have been replaced by holes leading to inner ears. Male reproductive organs and female teats are housed behind slits. Cetaceans have body hair during development, but lose it as adults, even though they live in extremely cold water. Instead of insulating hair, they rely on a suit of body fat (blubber) just under their skin. Cetaceans include the largest mammal ever to have lived on earth (the blue whale), thanks to the support that water provides. Air, less dense than water, could never support a land animal the size of a whale.