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martes, julio 01, 2008

THE FACE OF A WHITE TIGER


The face of a White Tiger, originalmente cargada por karin widemann.

White Tiger (Panthera tigris) is a tiger with a genetic condition that nearly eliminates pigment in the normally orange fur although they still have dark stripes. This occurs when a tiger inherits two copies of the recessive gene for the paler coloration: pink nose, grey-mottled skin, ice-blue eyes, and white to cream-coloured fur with black, grey, or chocolate-coloured stripes. (Another genetic condition also makes the stripes of the tiger very pale; white tigers of this type are called snow-white.)

White tigers do not constitute a separate subspecies of their own and can breed with orange ones, although all of the resulting offspring will be heterozygous for the recessive white gene, and their fur will be orange. The only exception would be if the orange parent was itself already a heterozygous tiger, which would give each cub a 50% chance of being either double-recessive white or heterozygous orange.

Compared to orange tigers without the white gene, white tigers tend to be larger both at birth and at full adult size. This may have given them an advantage in the wild despite their unusual coloration. Heterozygous orange tigers also tend to be larger than other orange tigers. Kailash Sankhala, the director of the New Delhi Zoo in the 1960s, suggested that "one of the functions of the white gene may have been to keep a size gene in the population, in case it's ever needed."

Dark-striped white individuals are well-documented in the Bengal Tiger subspecies (Panthera tigris tigris or P. t. bengalensis), may also have occurred in captive Siberian Tigers (Panthera tigris altaica), and may have been reported historically in several other subspecies. White pelage is most closely associated with the Bengal, or Indian subspecies. Currently, several hundred white tigers are in captivity worldwide with about 100 of them in India, and their numbers are on the increase. The modern population includes both pure Bengals and hybrid Bengal–Siberians, but it is unclear whether the recessive gene for white came only from Bengals, or from any of the Siberian ancestors as well.

The unusual coloration of white tigers has made them popular in zoos and entertainment that showcases exotic animals. The magicians Siegfried & Roy are famous for having bred and trained white tigers for their performances, referring to them as "royal white tigers" perhaps from the white tiger's association with the Maharaja of Rewa.

It is a myth that white tigers did not thrive in the wild, where small groups had bred white for generations. India once planned to reintroduce them to the wild. A.A. Dunbar Brander wrote in "Wild Animals In Central India" (1923): "White tigers occasionally occur. There is a regular breed of these animals in the neighborhood of Amarkantak at the junction of the Rewa state and the Mandla and Bilaspur districts. When I was last in Mandla in 1919, a white tigress and two three parts grown white cubs existed. In 1915 a male was trapped by the Rewa state and kept in confinement. An excellent description of this animal by Mr. Scott of the Indian police, has been published in Vol. XXVII, No. 47, of the Bombay Natural History Society's journal."

However, most white tigers are now bred in captivity, often by inbreeding parents and cubs to ensure the presence of the recessive gene. Such inbreeding often also leads to birth defects.

An additional genetic condition can remove most of the striping of a white tiger, making the animal almost pure white. One such specimen was exhibited at Exeter Change in England in 1820 and described by Georges Cuvier as "A white variety of Tiger is sometimes seen, with the stripes very opaque, and not to be observed except in certain angles of light.". Naturalist Richard Lydekker said that, "a white tiger, in which the fur was of a creamy tint, with the usual stripes faintly visible in certain parts, was exhibited at the old menagerie at Exeter Change about the year 1820." Hamilton Smith said, "A wholly white tiger, with the stripe-pattern visible only under reflected light, like the pattern of a white tabby cat, was exhibited in the Exeter Change Menagerie in 1820.", and John George Wood stated that, "a creamy white, with the ordinary tigerine stripes so faintly marked that they were only visible in certain lights." Edwin Henry Landseer also drew this tigress in 1824.

The modern strain of snow white tigers came from repeated brother–sister matings of Bhim and Sumita at Cincinnati zoo. The gene involved possibly came from the Siberian tiger, via their part-Siberian ancestor Tony. Continued inbreeding appears to have caused a recessive gene for stripelessness to show up. About one fourth of Bhim and Sumita's offspring were stripeless. Their striped white offspring, which have been sold to zoos around the world, may also carry the stripeless gene.

Because Tony is present in many white tiger pedigrees, the gene may also be present in other captive white tigers. As a result, stripeless whites have occurred in zoos as far afield as the Czech Republic, Spain and Mexico. Stage magicians Siegfried & Roy were the first to attempt to breed selectively for stripelessness; they own snow white Bengal tigers taken from Cincinnati Zoo (Tsumura, Mantra, Mirage and Akbar-Kabul) and Guadalajara, Mexico (Vishnu and Jahan), and a stripeless Siberian tiger called Apollo.

In 2004, a blue-eyed, stripeless white tiger was born at a wildlife refuge in Alicante, Spain. Its parents are normal orange Bengals. The cub was named Artico ("Arctic"). Stripeless white tigers were thought to be sterile until Siegfried & Roy's stripeless white tigress Sitarra, a daughter of Bhim and Sumita, gave birth. Another variation which came out of the white strains are unusually light orange tigers called golden tabby tigers. These may be orange tigers which carry the stripeless white gene as a recessive. Some white tigers in India have been very dark nearly reverting to the orange colour.

http://en.wikipedia.org/wiki/White_tiger


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