White tigers, despite the name, aren’t all white. These variants of the Bengal tiger are white with black or dark brown stripes, and they also have blue eyes, pink paw pads and a pink nose. They’re not true albinos, but they’re rare. Really rare. As in, none have been seen in the wild since 1958. All the known white tigers in the world live in captivity, usually zoos.
What strips the orange stripes out of these tigers? A team of researchers led by Xiao Xu of Peking University in Beijing, reporting this week in Current Biology, examined tiger genes and found that a single amino acid change — a simple switch of one chemical piece of a protein — resulting from a mutation in the gene SLC45A2 is responsible for the white color. When a tiger has two copies of that mutated gene, it can’t produce the pigment pheomelanin that gives the cat its orange fur.
Alterations to that gene underlie similar color changes in cream-colored horses and silver chickens. The same mutation in the gene has even been found once in a human, in a German who had pale skin and dark blonde hair.
The researchers think that the mutation, which they unsurprisingly named white, evolved just once in the wild Bengal tiger population. That mutation slowly spread through the tiger’s descendents and created the occasional white tiger. The animals were spotted infrequently in India from the 1500s until 1958, when the last wild white tiger was shot. But these variants have survived in modern menageries.
White tigers are mythical, romantic creatures, which might account for why they’ve been heavily bred in some zoos, notably the Cincinnati Zoo in Ohio, in recent decades. But some have argued that white tigers should be allowed to die out. Here’s what Slate had to say in December:
On the face of it, being a white object in the Bengal tigers’ tropical habitat of India and Southeast Asia can’t be good for a predator that needs to be camouflaged. Other, more subtle problems that go along with the white coat would also prevent white tigers from ever becoming established as a wild population. The mutation (which is not albinism—white tigers can still produce melanin) also causes serious defects. White tigers in captivity tend to have problems with the way that their brains control their eyes and process visual stimulation. The animals are often cross-eyed in one or both eyes, bump into objects, and have trouble understanding spatial relationships when they are young. Animals with defects like these couldn’t survive for long in the wild, even though they have long lives in captivity. Other disorders, such as kidney problems, club feet, and shortened tendons, come from the severe inbreeding required to keep this recessive gene around.
But the new study knocks down several of these arguments. Over the years, the researchers write, many of the white tigers that were captured or killed were mature adults, so lacking the orange-and-black camouflage couldn’t have been too much of a problem. And, inbreeding — not the white mutation itself — accounts for many of the health problems that modern white tigers face, including deformities, stillbirths and premature deaths, they say.
“Despite its low frequency, this polymorphism has persisted for at least several hundred years and should be considered a part of the genetic diversity that is worth conserving,” the scientists argue in their paper.
There’s still a question, however, of whether devoting scarce conservation resources to preserving a rare variant of a creature is worth it when the entire species is threatened with extinction. There may be as few as 3,000 of these cats left in the wild. Their habitat is shrinking. And they’re being devastated by the wildlife trade. With tigers facing threats like those, perhaps a dedicated program to preserving the white mutation is little more than a luxury, even if it could be done more responsibly, with less inbreeding.
After reading the new study, I wouldn’t call white tigers “freaks,” as William Conway, former director of the New York Zoological Association, once did. And I’ll watch in amazement should I encounter one in a zoo. But I won’t be sad if the current (in)breeding programs go away and the money and resources get redirected to other conservation efforts. Because all I want is the tiger, any tiger — white, orange, Siberian, Bengal, Sumatran, et cetera — to continue to survive.
Image credit: Chimelong Safari Park, via EurekAlert