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WhalesThe freezing waters off Antarctica are the summer home for many whale species. While there, the whales feed on the rich supply of krill (creatures resembling tiny shrimp). When the Antarctic winter starts to set in, the whales migrate North to warmer waters for the Austral (Southern) winter.
Whales comprise about eighty species of large, exclusively aquatic placental mammals, members of the order Cetacea, which also includes dolphins and porpoises. The term whale is ambiguous: it can refer to all cetaceans, just the largest ones, or only to members of particular families within the order Cetacea which leads to difficulties, as the Killer Whale and the Pilot Whale are members of the family Delphinidae and technically dolphins. The cetaceans are divided into two suborders: Mysticeti - the baleen whales - and Odontoceti - the toothed whales. Many species of whales were hunted nearly to extinction, for their meat, fat (used to make lamp oil), oil and ambergris (from Sperm Whales), a perfume ingredient. International treaties now sharply restrict whaling. Canada, Iceland, Japan and Norway and other countries hunt not endangered whales, while small aboriginal groups in the United States and Pacific island nations hunt endangered whales on a small scale. EvolutionLike all members of the order, whales evolved from land mammals which returned to the sea undergoing aquatic adaptation, probably in the Eocene, between 55 and 34 million years ago. The precise ancestry of whales is still obscure, as there is no commonly agreed succession, but they are thought to have evolved from a group of carnivorous artiodactyla (even-toed hoofed animals). In 2001, two important 47-million-year-old partial fossils, named Rodhocetus Balochistanensis and Artiocetus clavis, were discovered in Balochistan, Pakistan. These fossils represent intermediate forms between land-living ungulates and whales and are evidence that the whales' closest relatives on land might be hippos, which had been previously suggested by DNA studies.CharacteristicsLike all mammals, whales breathe air into lungs, are warm-blooded (to be precise, endothermic), breast-feed their young, and have some (very little) hair. Whales have two blowholes which they use for breathing. When breathing out after a dive, a "V" shaped spout can be seen from the right perspective. Whale submerge underwater for vast amount of time. Some whales, such as the Sperm Whale, can stay underwater for up to two hours in a single breath.Whales have a unique respiratory system that lets them stay underwater for long periods of time without taking any oxygen. Because of their environment, whales are conscious breathers: They have to decide when to breath. So how do they sleep? All mammals sleep, and so do whales, but they can't afford to fall in unsconscious state of sleep for too long periods of time, since they need to be conscious in order to breath. The solution is that only the other hemisphere of the brains of the whale sleeps at the time, so whales are never completely asleep, but still get the rest they need. Whales "sleep" around 8 hours a day. Whales have a four-chambered heart. Baleen, the sieve-like structures which baleen whales use to filter food out of the water, is made from keratin. Whales are broadly classed as predators, but their food ranges from microscopic plankton to very large fish. The male is called a bull; the female, a cow; and the young, a calf. Especially noteworthy is the Blue Whale, the largest animal that has ever lived. It may be up to 30 meters long and weigh 180 tons. SonarEnvironmentalists have long argued that some cetaceans including whales are endangered by sonar and especially by the very powerful sonar used by the US defense department. British scientists have recently suggested (in the journal Nature) that the sonar is connected to whale beachings and to signs that the beached whales have experienced decompression sickness (see a BBC report about the Nature article or the Nature article itself (requires subscription)). Mass whale beachings do occur naturally amongst many species and in fact the frequency and size of beachings around the world, recorded over the last 1000 years in religious tracts and more recently in scientific surveys, has been used to estimate the changing population size of various whale species, under that assumption that the proportion of the total whale population beaching in any one year is constant. Despite the concerns raised about sonar as mentioned above which may invalidate this assumption, this population estimate technique is still popular today.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Whale" and from http://www.white-on.com
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