Marine mammals in the cetacean family include whales, dolphins and porpoises. These animals are completely aquatic, meaning they spend all of their time in the ocean and cannot survive on land. Whales, dolphins and porpoises have two front flippers, and their tails are uniquely shaped into two horizontal extensions, called flukes, that provide tremendous swimming power. Over time, their ancestors adapted to survive solely in the ocean environment. Their front legs turned into paddle-shaped flippers, they lost their back legs, their tails grew larger and widened to form flukes, and they developed a thick layer of fat, called blubber, to keep warm in the ocean. Also, their skulls elongated and the nostrils shifted to the top of their heads (blowholes) to aid in breathing at the ocean's surface