SA Forum is an invited essay from experts on topical issues in science and technology.
Crowds love black-and-white animals. Perhaps the sharp contrast of light and dark conjures long-lost memories of how the world looks to people in their first weeks of life. Whatever the explanation, we’re often transfixed by the sight of zebras, orcas, giant pandas—and especially by the presence of white tigers. Just ask the Las Vegas entertainers Siegfried and Roy.
Sometimes the attraction turns fatal, as it did a couple of weeks ago for a young man who toppled into a Delhi zoo’s tiger enclosure. But long before that deadly incident animal welfare advocates began disputing the wisdom of raising white tigers. More than 30 years ago William Conway, director of the then New York Zoological Society (now known as the Wildlife Conservation Society), became convinced that the rare cats were merely the victims of a hereditary defect that was propagated because the animals were kept and deliberately bred as glorified sideshow exhibits. “White tigers are freaks,” he declared more than 30 years ago. “It’s not the role of a zoo to show two-headed calves and white tigers.” Three years ago the Association of Zoos and Aquariums endorsed that opinion, instructing all AZA members to cease any breeding (pdf) of captive white tigers.
The truth is that white tigers are the product of a rare but naturally occurring genetic variant within the wild Bengal population. Even so, the experts’ confusion about the subject has been understandable, given the previous lack of precise information on the white tiger’s genetic roots. It was only last year that our team published the workcracking the mystery at last.
Using state-of-the-art genetic tools we scanned the entire genome of a family of Bengal tigers that included orange and white individuals alike and validated our findings against data from 130 unrelated members of the same species. The result was crystal clear: The white tiger’s distinguishing characteristic arises from a single mutation, the substitution of one amino acid for another—valine for alanine—in the “solute carrier” protein geneticists call SLC45A2. Its job is basically to transfer specific molecules across cellular barriers.
Similar variations in SLC45A2 have been observed in other vertebrate species ranging from humans to chickens. With rare exceptions the swap’s only effect on the animal is a decrease of external pigmentation. That’s what makes the white tiger white. And until trophy-hunting humans came along the mutation made little difference to the animals’ ability to survive and reproduce—most of its prey species are color blind.
Records dating back at least four centuries indicate that wild white tigers once prowled freely in the forests of India. Some were shot, others were captured and sent to royal menageries and still others remained in the jungles to perpetuate their lineage. The last known specimen in the wild was shot dead in 1958, leaving behind only the captive breeding population. Trophy hunting, habitat loss and habitat fragmentation drove the rest to extinction.
Almost all of the white Bengals alive today are descended from a solitary male cub that was captured in 1951. Deliberate inbreeding has maintained the animals’ recessive coloration but it also has led inevitably to a whole range of health problems that helped inspire William Conway’s “two-headed calves” overstatement. In fact, observations of 52 white tigers born in the U.S. at the Cincinnati Zoo detected no significant heritable defects other than some weakness in the animals’ eyesight
In any case, we now know how to reduce or eliminate the problems that have arisen from inbreeding among white tigers. Now that the crucial mutation has been identified, it will be possible to identify and crossbreed pairs of Bengal tigers, each one possessing a single copy of the recessive gene. Basic Mendelian rules give a 25 percent probability that any given pregnancy will produce white-tiger offspring while significantly expanding the gene pool of healthy animals.
And recognizing that white tigers are part of the natural genetic diversity of their species, we humans should consider saving them. Well-managed captive populations of wild animals have proved to be valuable assets in education, research and fund-raising—and they can serve as genetic reservoirs for the planet’s dwindling wild species.
No one knows how many centuries—quite possibly millennia—white tigers lived freely in their natural habitat before human hunters eradicated them. Doesn’t our species now have a responsibility to maintain at least a few white Bengals in good genetic health?
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