Marine mammals known as cetaceans are broken into two large groups: toothed whales (including dolphins and porpoises) and baleen whales. One group has a single blowhole to breathe air, the other has two. One group has tall dorsal fins to help maintain balance as they chase prey. The other scoops up water and prey in their large mouths as they swim slowly along—they don’t need large dorsal fins. One group uses echolocation, the other doesn’t. Learn more about what these animals have in common and how they are different in this latest installment of the Compare and Contrast Series.