Whales are about 80 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.
A complete up-to-date taxonomical listing of all cetacean species, including all whales is maintained at the Cetacean article.
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.
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2 Characteristics 3 Sonar 4 Miscellanea 5 See also 6 References and external links |
Evolution
Like all members of the order, whales evolved from land mammals which returned to the sea, 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.
Characteristics
Like 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 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.
Sonar
Environmentalists 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. [1].
Miscellanea
The hunting of whales is the subject of one of the classics of the English language literary canon, Herman Melville's Moby Dick.
See also
References and external links
