The bodies of whales and dolphins are streamlined for efficient movement through the water. The few external parts they do have include a strong tail with horizontally flattened flukes which propels them forward, pectoral flippers for steering and a dorsal fin for stabilising. Cetaceans are kept warm by a thick insulating layer of fat, known as blubber, which in the bowhead whale can be as thick as 50 cm. They range in size from the smallest dolphins and porpoises, which can be as small as 1.2 m in length, to the largest baleen whales, which can grow to more than 30 m in length. Some species are blandly coloured in drab greys and browns while others are made up in stripes, spots and other intricate and striking markings.
Some species are robust and short, others are slender and long.
Even within species there are often many differences between males and females, animals of different ages and among populations in different parts of the world. Many of these, however, are deep sea animals which are rarely seen from shore.