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Why do so few elephants get cancer?

Elephants play in the Amboseli National Park, southern Kenya, Oct. 9 2013.

Photo: EPA

In 1977, a University of Oxford statistician named Richard Peto pointed out a simple yet puzzling biological fact: We humans should have a lot more cancer than mice, but we don’t.

Dr. Peto’s argument was beguilingly simple. Every time a cell divides, there’s a small chance it will gain a mutation that speeds its growth. Cells that accumulate several of these mutations may become cancerous. The bigger an animal is, the more cells it has, and the longer an animal lives, the more times its cells divide. We humans undergo about 10,000 times as many cell divisions as mice — and thus should be far more likely to get cancer.

Yet humans and mice have roughly the same lifetime risk of cancer, a circumstance that has come to be known as Peto’s paradox.

A number of scientists have speculated that large, long-lived animals must evolve extra cancer-fighting weapons. And if that’s true, they reason, then the biggest, longest-lived animals should have an especially big arsenal. Otherwise, these species would become extinct.

Dr. Joshua Schiffman, a pediatric oncologist at the Huntsman Cancer Institute at the University of Utah and his colleagues have found that elephants have a remarkably low rate of cancer. They reviewed zoo records on the deaths of 644 elephants and found that less than 5 percent died of cancer. By contrast, 11 percent to 25 percent of humans die of cancer — despite the fact that elephants can weigh a hundred times as much as we do.

Dr. Schiffman is now investigating how to translate the new findings on elephants into cancer treatments for people. But he said it would be useful to look at other big or long-lived animals as well. Naked mole rats, for example, live up to 30 years without ever getting cancer. Dr. Schiffman speculates that parrots, tortoises and whales may all have special longevity tactics of their own.

“The war on cancer was going on long before there were humans,” he said. “So let’s look at nature’s strategies.”

This article is an edited version of a piece that originally appeared in the New York Times

(Carl Zimmer, New York Times)










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