Whales, dolphins and porpoises are members of the order Cetacea. They are totally adapted to a life at sea but, as air-breathing mammals, they must surface to breathe. Modifications to the standard mammal design involve a hairless fishshape 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 suborders: the Odontoceti (toothed whales with a single blowhole) and the Mysticeti (whalebone or baleen whales with a double blowhole).

In diving, the blowholes are firmly closed and the heart rate is slowed down. Whales are tolerant of a high concentration of carbon dioxide in the blood with which they are plentifully supplied; the result is that they are able to hold their breath for periods that would drown land animals. The breathing passages are separated from the gullet so that they are able to feed underwater without choking.

Killer whale The Arctic by Tarpan ShutterstockKiller whales are cosmopolitan and common, found in coastal areas throughout the seven seas © Tarpan, Shutterstock

Working in murky water and at great depths, toothed whales find their prey by echo-location, using ultrasonic pulses which are inaudible to human ears. They also 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 – 30m long and weighing 150 tonnes.

The three truly Arctic whales – bowhead, narwhal and beluga – do not have dorsal fins, an adaptation that fits them for working under ice (and of course they are well supplied with heat-retaining blubber). But it is possible to see several other species of whale in Arctic waters, especially in the summer. Fin, humpback, minke and pilot whales have been recorded commonly, and even the blue whale is sometimes seen in the coastal waters of the Denmark Strait in summer. There is always the chance of killers.

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