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Giant animal beneath the sea: whale
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IT weighs as much as 20 elephants but lives beneath the sea.
The blue whale (蓝鲸) is the earth's largest animal. Larger than the largest of ancient dinosaurs, blue whales can grow to be more than 30 metres long and weigh nearly 150 tonnes. Not all whales are so large. The much smaller pilot whale (领航鲸，又名圆头鲸) grows to about 8.5 metres in length.
Although whales spend their lives in the sea, they are, like humans, warm-blooded mammals. After a baby whale is born, it nurses on its mother's milk, just like the young of land mammals.
Whales live in all of the open seas of the world. Some species, such as the white whale (白鲸), or beluga, may travel upstream in large rivers. Some species migrate with the seasons; others remain year-round in the same habitats.
The skin of whales is usually black, gray, black and white, or all white. Some, such as the blue whale, have skin that is bluish-gray. The surface of the skin is smooth, but like other mammals, whales have hair. In adult whales, hair is confined primarily to a few bristles (刚毛) in the head region and is largely absent over most of the body. Whales that live in polar regions are insulated (隔离) from the extreme cold by a layer of blubber (鲸脂), or fat, enveloping their bodies.
Whales have lungs, not gills, so they must come to the surface of the ocean to breathe. The smaller whales can remain underwater for several minutes, and some of the larger species can dive for an hour or longer. Sperm whales (抹香鲸) are believed to be able to dive as deep as 1,100 metres in search of food.
Whales sometimes swim into very shallow water and become stranded on the beach. This phenomenon is called beaching (搁浅), or stranding. When a whale is onshore it is helpless. Its heartbeat accelerates and its body becomes overheated very rapidly. Without the support of water a whale cannot move and its lungs may be crushed by the weight of its body. Pilot whales are the species of whale most likely to exhibit beaching behaviour.
The explanation for beaching behaviour has not been resolved by scientists. There are several possible reasons for the strandings. The most probable explanation is a malfunctioning or disturbance of the whale's echolocation (回声定位机能) process. This hypothesis is supported by the fact that baleen whales (长须鲸), which do not use echolocation, seldom become stranded.
Loud, upsetting sounds such as underwater explosions have also been blamed for mass strandings.
Being seasonal breeders, whales usually become pregnant between spring and fall. The period of gestation (pregnancy) is nine to 12 months for most species. Most whales give birth to a single offspring, though twins are sometimes born.
(SD-Agencies)
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