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.

All cetaceans are aquatic, and they swim by pumping their tail up and down, using paddle-like flippers for steering. The tail fins (flukes) are flexible, with no bones, and attached to the body with cartilage. Huge back muscles power the tail, making them powerful swimmers. Cetaceans are also good divers, some able to stay underwater for hours and reach depths of hundreds of meters. As air breathers, they must hold their breath to dive, yet their lungs are small and compress to an even smaller size during a dive. The stored air is pushed into their windpipe, where a thickened lining keeps it out of contact with the rest of the body. Sealing off the air prevents a buildup of nitrogen (from air) in the bloodstream, which protects cetaceans from the "bends" that human divers are in danger of when they submerge. During diving, oxygen reserves stored in special molecules (myoglobin) supply oxygen to cetaceans' bodies.