Cross-eyed white tigers

Almost all white tigers have crossed eyes.

Корзун Андрей, CC BY-SA 3.0, via Wikimedia Commons

The right sides of our bodies are mostly controlled by the left side of our brains. The left sides of our bodies are mostly controlled by the right side of our brains. Why is an evolutionary mystery. The so-called contralateral brain – that cross-over between left and right – is common to every single animal with a backbone. And, incredibly, it doesn’t exist in animals without a backbone.

(Why is that incredible? Well, it suggests that this crossover came about a long time ago in evolutionary history – perhaps half a billion years – and it has stayed stable ever since. There are a few theories about why this happens, and the current front runner seems to be something called the axial twist hypothesis: essentially a sort of rotation very early in embryonic development.)

One notable example of this crossover in practice is in our eyes. The nerves from our left eye go to our right brain. The nerves from our right eye go to our left brain. And these two lines cross over in an area known as the optic chiasm.

In some vertebrates’ optic chiasm those nerves cross over without much interaction. In our heads – and in the heads of other mammals, birds, amphibians, and some reptiles – there is some intermingling between the nerves. Some optic nerves go to the same side of the brain as the eye – right eye to right brain, for example. It’s thought that this improves binocular vision. But, sometimes, that intermingling is a problem.

Most white tigers bred in captivity have serious problems. The inbreeding necessary to produce white colouration comes with a host of genetic weaknesses: twisted necks and bad kidneys, for example. But the gene that produces the white coat also increases the number of nerves in the optic chiasm that go to the same side of the brain.

Because the optic nerves are scrambled, white tigers go cross-eyed to compensate. Sometimes this only comes out when they’re stressed, or only manifests in one eye. But this visual disruption is a hallmark of white tigers (and Siamese cats too).

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