Whales are marine mammals in the order Cetacea. They are unique among all mammals in that they carry out their complete life history, from birth to death, in water. The term cetacean is used to embrace all known species (about 79) of whales, dolphins, and porpoises.
   Whales are descended from a four-legged land animal, perhaps a primitive ungulate (hoofed mammal) that may also have given rise to modern ungulates. The earliest known whale fossils are 52 million years old, but many scientists estimate that whales date from 60 million years ago. Fossilized cetacean skeletons dating back to the Eocene epoch (56.5 million to 35.4 million years ago) were recently discovered in Pakistan. These fossils indicate that early whales swam by undulating their vertebral column, thus forcing their feet up and down in a way similar to modern otters.
      Whales are divided into two suborders: toothed whales and baleen whales. Most smaller whales, and all the dolphins and porpoises, belong to the toothed whale suborder. Those more than 4 to 5 m (13 to 16 ft) long are generally referred to as whales, whereas smaller species are known as dolphins or porpoises. Toothed whales have teeth that are uniform in size and shape although they vary considerably in number, and they feed on fish and invertebrates such as squid and crustaceans; one species, the killer whale, has a more varied diet that includes seabirds and marine mammals. A few species are commercially valuable as exhibits in aquariums and oceanariums, and some of the smaller whales are hunted to a limited extent. One toothed whale, the sperm whale, sometimes known as a cachalot, is quite large: the male grows to a length of 18.3 m (60 ft), and the female grows to a length of 12.2 m (40 ft). It was heavily hunted in the past.
      The rest of the larger whales belong to the baleen whale suborder. In this group of ten species-all of which have been or are currently being hunted-teeth have been replaced with large structures, known as baleen plates, that hang like vertical Venetian blinds from the upper jaw. The plates number 160 to 395 on each side, are frayed into bristles on their inner edges, and are used to capture the plankton or krill on which the animals subsist. When feeding, a baleen whale swims with its mouth open in order to engulf plankton and seawater by the ton. Then, shutting its cavernous mouth and pressing its tongue against the back of the baleen bristles, the whale forces the water out of its mouth, trapping the plankton on a mat of overlapping baleen plates.
      Probably the largest animal ever to have lived is a baleen whale, the blue whale, which has been measured up to 30.5 m (100 ft) in length, with a weight of about 190 tonnes. Baleen whales tend to spend the summer in polar seas, where plankton blooms provide abundant food. After months of heavy feeding they migrate to temperate or tropical zones, often fasting there over the winter.

     The dramatic streamlining of whales in the course of their evolution resulted in an animal that appears remarkably fish-like. Thus, the front limbs became modified as paddle-shaped flippers, the bones of which are still reminiscent of jointed limbs and digits, but the hind limbs were lost. The broad horizontal tail flukes that provide the main propulsive thrust bear no anatomical connection to the lost hind limbs, but are a separate and distinct development. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous and elastic tissue. The body is enveloped in a thick layer of blubber that aids in buoyancy, helps to preserve body heat, and is a source of stored energy. A whale's skin is free from sweat glands, oil glands, has almost no hair, and feels much like smooth, wet rubber to the touch.
      Whales, like other mammals, have lungs. They breathe air through a single nostril, or pair of nostrils, situated on the top of the head (the blowhole); but contrary to a popular image, they do not spout water when they exhale. The visible "spout", the size and shape of which is unique to many species, is simply water vapour in the lungs and a small amount of water present in the depression around the blowhole, which is blown into the air as the whale comes to the surface of water (breaches) and exhales.
      A number of physiological adaptations enable whales to perform deep dives. First, they have a larger blood volume than land mammals of comparable size and weight, and they also have a greatly increased capacity to store oxygen in their blood and muscle tissue. Second, each breath provides an 80 to 90 per cent renewal of air in a whale's lung, compared with only 10 to 20 per cent in most land mammals. Third, whales have a resistance to the metabolic by-product carbon dioxide, the build up of which in the tissues, rather than a lack of oxygen, triggers the involuntary breathing response of most mammals. Baleen whales can hold their breath for up to 50 minutes when diving, and sperm whales for up to 60 minutes; sperm whales dive to depths of 1,000 m (3,300 ft) in search of their prey, the giant squid. Finally, whales are able to restrict blood flow to vital organs during a deep dive, so that essential organs such as the heart and the brain do not suffer injurious oxygen deprivation.

Showing the characteristic white patches above the eyes and under the jaws, a male and female killer whale, Orcinus orca, swim protectively on either side of their baby. Killer whales maintain close ties to the social structure of their natal pods, or groups, for life. To prevent inbreeding, however, the whales typically seek mates outside of their original pod.

     Sexually mature animals go through a courtship period followed by copulation, which in the whale takes place belly to belly in the water. (Whether mated pairs remain together after this time is not known.) The pregnant female carries her unborn young for 9 to 16 months, depending on the species, and usually a single large, well-developed calf is born underwater.
      A healthy calf can swim from the instant it is born, rising to the surface for its first breath. In some species it is helped to the surface by its mother. Soon after, it begins to suckle from either of two mammary teats enclosed in slits located on each side of its mother's genital opening. Whale milk is very rich, and calves grow rapidly. For example, a blue whale calf that is 7 m (23 ft) long and weighs 1.8 tonnes at birth doubles its weight in the first week; at seven months the infant is 17 m (56 ft) long and weighs 22 tonnes. Juvenile whales are probably weaned off their mothers' milk somewhere between eight months and two years after birth; the age at which a young whale leaves its mother, however, is not known for many species. In some species, such as the killer whale, young animals appear to remain permanently in the family pod, which usually numbers 5 to 12 animals.
      A whale reaches sexual maturity at 6 to 13 years of age. The lifespan varies: for small-toothed whales such as the beluga it is thought to be about 30 years; for larger-toothed cetaceans such as the sperm whale it is up to 50 years or more; and baleen whales probably live for as long as 80 years. As with all other animals, not all whales survive to old age. Disease, injury, and predation by killer whales, sharks, and whalers all take their toll.

Sperm whales, found in most oceans, derive their name from the oily fat-called spermaceti-contained in their large heads. Diving deep in search of squid, sperm whales may remain submerged for as long as an hour.

This humpback is a magnificent sight as it leaps entirely free of the water before falling back down in a cascade of foam. It is an activity that can be witnessed throughout the year.

     The Southern Right Whale is a large baleen whale that occurs south of the Tropic of Capricorn. Southern Right Whales can be recognized by the lack of a dorsal fin, the strongly arched mouth, and the large callosities that are present on their heads and chins. When seen from a distance, they may also be recognized by the distinctive V-shaped pattern of water blown from the blowhole. Southern right whales grow to 15 m (50 ft) in length, and can weigh up to 60 tonnes.Despite their massive bulk, they are incredibly active cetaceans, with breaching, lob-tailling and flipper-slapping all relatively common. A particular favourite is 'sailing', where the whale hangs vertically upside-down in the water, 'standing' on its head, with its tail flukes in the air. They communicate through 'moans' and 'burping' noises.

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