The killer whale is technically a dolphin – the largest of the species © Tory Kallman, Shutterstock
Whales, dolphins and porpoises are members of the order Cetacea. They are totally adapted to a life at sea but, as mammals, they must surface to breathe. Modifications to the standard mammal design involve a hairless fish-shape encased in a thick layer of insulating blubber, the nose on top of the head, forefeet becoming paddles, effective loss of hind feet and the tail becoming a horizontal fluke. Supported by water, they are free to grow to a great size and weight. They are divided into two broad sub-orders, the Odontoceti (toothed whales) and the Mysticeti (whalebone or baleen whales). Sperm and killer whales (which are large dolphins) and the other dolphins belong to the toothed division, and are beasts of prey, whereas the whalebone whales feed by filtering plankton through a series of baleen plates which hang from the position normally occupied by the upper teeth.
Working in murky water at great depths, toothed whales find their prey by echo-location, using ultrasonic pulses which are inaudible to human ears. They communicate within their group with trills, whistles, grunts and groans which are perfectly audible above water.
Baleen whales have a profoundly different method of feeding. In relatively shallow water, they plough through the concentrations of plankton (possibly finding them by taste), gulping great quantities of water, expelling it through filter-plates of whalebone by contracting the ventral grooves of the throat and pressing the large tongue against the roof of the mouth, then swallowing the catch of uncountable numbers of small shrimps and larval fish. Not needing the agility and manoeuvrability of the hunting whales, they enjoy the advantages of greater size: the blue whale is the largest animal ever to live on earth.