Big animals have higher cancer risk – but also evolved better defences

Bigger animals live longer and have more cells that could go awry, so we would expect them to have a greater risk of developing cancer. A comprehensive analysis of 263 species suggests this is indeed the case, but also finds that some large animals have evolved ways to curb the risk.

“We provide the first empirical evidence to show that there’s an association between body size and cancer prevalence, meaning that bigger species get more cancer than smaller species,” says George Butler at University College London.

The results stand in contrast to prior studies that have found no link between body mass and cancer rates. But many of these involved just a few dozen species, says Butler.

To gain a broader view, Butler and his colleagues analysed data on the size and cancer rates of 79 species of bird, 90 mammals, 63 reptiles and 31 amphibians. This data came from previous work by other researchers, who had sifted through autopsy records that logged whether captive animals – kept in places like zoos and aquariums – had cancer when they died.

The team found that larger animals were slightly more likely to have cancer at the time of their death compared with smaller ones. Across birds and mammals, every 1 per cent increase in body mass was linked to a 0.1 per cent increase in cancer rate, on average. Body mass data wasn’t available for reptiles and amphibians, so the team used body length, finding that every 1 per cent increase was linked to an average rise in cancer rate of 0.003 per cent.

Butler and his team say their findings challenge a long-standing idea known as Peto’s paradox, which points out that cancer rates should correlate with body size but don’t. On the other hand, Vera Gorbunova at the University of Rochester in New York state says the weakness of the correlation still demands explanation.

“The increase in risk they see is very, very minor, and it’s just not proportional at all to body size,” she says. “If you take a small animal like a mouse, and a human is maybe 100 times bigger, or an elephant is 1000 times bigger, the difference in cancer rate is not 100 times higher in humans, or 1000 times higher in the elephant.”

That suggests larger species have evolved more ways to protect themselves, says Gorbunova.

Indeed, by using evolutionary trees to infer animals’ rates of body size evolution, the team found that bird and mammal species of similar size had better defences against cancer if they had experienced a more rapid increase in size during their evolution.

Previous studies have pinpointed genetic adaptations in elephants and whales that may protect against cancer by improving DNA repair or stopping faulty cells from dividing.

A deeper understanding of how some animals resist cancer could lead to new therapies for people, says Gorbunova. “If you find out that, in these cancer-resistant animals, there are particular biological pathways that are tweaked differently, we could also design, for example, small molecules that would target these pathways and then either kill cancer cells more efficiently, or maybe even prevent cancer from occurring,” she says.

“These would be likely to be very promising drugs because, in the course of evolution, those mechanisms have been tested over millions of years,” she says.