Kategorie: Angličtina (celkem: 879 referátů a seminárek)

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  • Přidal/a: anonymous
  • Datum přidání: 23. července 2006
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Příbuzná témata


Whale, any of the 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.

Evidence indicates that whales 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.

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