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.

The first cetaceans on Earth were not much like the whales, dolphins, and porpoises of today. They appear in the fossil record in the early Eocene (about 55 million years ago). Cetaceans evolved from land animals that made a gradual transition to water. The earliest cetaceans were semi-aquatic, living along the shores of the ancient Tethys Sea. Using all four legs, they probably half-swam, half-crawled through the water like hippos. By the mid-Eocene (about 45 million years ago), cetaceans became more seaworthy, with long noses, blowholes, webbed toes, strong tails, and flexible backbones for swimming. Front legs gradually evolved into flippers, and hind legs got smaller. It wasn’t until the Oligocene (about 35 million years ago) that hind legs disappeared into the body as remnant (vestigial) bones. Cetaceans were fully aquatic by the late Eocene and eventually developed the big brains and thick blubber that we see in today’s whales.