Toothed whales have as few as one pair to more than 100 pairs of teeth for eating fish, squid, or other animals. Most toothed whales have good eyesight, but they also use sonar to catch prey. By emitting pulses of sound and interpreting their echoes (echolocation), they get a picture of what's around them. Toothed whales have no vocal cords; sounds are made in nasal passages under their blowhole. A slit-like opening is slapped shut to make a clicking sound, after which a fatty organ on the top of their head (the melon) focuses the sound. Oil-filled cavities in the lower jaw receive the echoed sound and send it to the ears. Because water transmits sound waves better than air, toothed whales need special adaptations to pinpoint the direction of a sound. Their middle-ear bones are detached from the skull and buffered with foam to better isolate sounds that may alert them to a nearby fish.

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 External ears have been replaced by holes leading to inner ears.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.