News in Science

Cheetah Extinction

The cheetah is very fast. But it might not be able to outrun its own extinction.

Cheetahs of at least four different sub-species once roamed through North America, Asia, Europe and Africa. In fact, the name "cheetah" comes from the Hindi word meaning "the spotted one". But today the cheetah exists in sub-Saharan Africa as only one sub-species, with a tiny remnant population barely surviving in northern Iran.

We're not sure of the exact numbers, but we do know that cheetahs are endangered. In the mid-1950s, there were 20-40,000 cheetahs in the whole world. By the mid-1970s, their population had halved. How far they've dropped since then is anybody's guess.

The Sumerians, way back in 3,000 BC, were the first to use cheetahs as hunting companions. Since then, the pharaohs of Egypt, the kings of France, the princes of Persia, the Mongol emperors of India and the emperors of Austria have continued this tradition. When Marco Polo visited Kublai Khan at his summer residence in the Himalayan Mountains 700 years ago, he found that the mighty Khan kept 1,000 cheetahs to hunt deer and other slower animals.

Cheetahs are very inbred. They are so inbred, that genetically they are almost identical.

The current theory is that they became inbred when a "natural" disaster dropped their total world population down to less than seven individual cheetahs - probably about 10,000 years ago. They went through a "Genetic Bottleneck", and their genetic diversity plummeted. They survived only through brother-to-sister or parent-to-child mating.

If a species does not have much genetic diversity, it will not be able to adapt well to changes in their environment - such a climate change, or new bacteria or viruses. But if they do have a lot of genetic difference from one individual to the next, at least a few of them will be able to survive the changing times.

We think they are inbred because of tests involving enzymes, skin grafts and skull shape. The enzyme tests probably give the strongest evidence for inbreeding. The tests involving skin grafts and skull shape give "weaker" evidence.

Enzymes are medium-sized proteins which speed up chemical reactions (they are advertised in some washing powders). In your body, enzymes speed up the burning of food for energy by about one million times. (The average human life span is less than 1,000,000 hours, so if you didn't have enzymes in your gut, you couldn't eat your second meal because you would not have finished digesting your first meal!)

According to the enzymes, humans rate at about 70% identical. But laboratory rats and cheetahs rate at 97% identical. Laboratory rats have been inbred for at least 20 generations of brother-to-sister mating. So cheetahs are at least as inbred as laboratory rats.

In a skin graft, you transplant skin from one place to another. If you are burnt on your face, the surgeon may graft on some skin from your legs or your buttock. The operation will usually be a success, because your immune system won't reject your own skin. You will almost always reject a skin graft from another person - unless they are your identical twin. But cheetahs will accept skin grafts from each other, and not reject the graft, about 50% of the time. This means cheetahs must all be genetically similar to each other.

Their skulls are not symmetrical either, according to an examination of east African cheetah skulls currently in American museums. As an aside, many of these skulls were collected by the American President, Theodore Roosevelt. The left side of the skull is different from the right side, and is not a mirror image. We don't know why, but in general, the more inbred an animal is, the more asymmetrical the skull is.

So the evidence seems clear that they are very inbred. But why do some scientists think that cheetahs were reduced to a population of less than seven individuals, about 10,000 to 12,000 years ago.

They think less than seven individuals, because it has been shown that if a population is reduced to seven individuals and then expands quickly, the offspring still retain about 95% of their genetic variability. But cheetahs have almost zero genetic variability - there's hardly any difference between them.

They think about 10-12,000 years ago, because back then, there was a massive destruction of many different mammalian species, such as mammoths, sable tigers and cave bears. About 75% of all mammalian species died out in North and South America. So this was probably the "disaster" that knocked off most of the cheetahs. Perhaps this disaster was a severe climate change associated with the tailing-off of the last Ice Age.

Whatever the cause, some scientists think that the cheetahs got almost totally wiped out - perhaps more than once. Like true copycats, they then built up their numbers with generation after generation of brother-sister inbreeding.

Once again, cheetahs are very endangered, because we are stealing their land and killing them, so bringing them closer to the brink of extinction.

Now the Hindi name for the cheetah is "the spotted one" - and sure enough, if we keep killing them, you'd be very lucky if you spotted one.

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