Larger animals are more prone to cancer, but they possess stronger defense mechanisms against it.

The larger the organism, the greater the chances that its cells will start to mutate. The logic is simple: an elephant has thousands of times more cells than a mouse, which means the likelihood of oncological diseases should also be higher. However, decades of observations of animals have puzzled scientists. Why have whales, elephants, and giraffes—large creatures—not gone extinct from cancer if their size makes them an ideal target for it? The answer was sought in genes, immunity, and metabolism, but the data remained contradictory.

In 1977, British oncologist and epidemiologist Richard Peto (Richard Peto) proposed an explanation. He asserted that cancer incidence does not correlate with the number of cells in an organism; therefore, cancer is not more common in large animals than in small ones. This assertion became known as the “Peto's paradox”.

A team of British and American biologists led by George Butler (George Butler) claimed that their research contradicts Peto's idea. The scientists reported this in an article published in the journal Proceedings of the National Academy of Sciences.

The authors analyzed data from 263 species: 90 mammals, 79 birds, 63 reptiles, and 31 amphibians. The researchers examined autopsy protocols from animals in zoos and aquariums to determine which of them had cancer during their lifetimes.

The results showed that at the time of death, oncological diseases were more common in larger species than in smaller ones. Each 1% increase in body mass in mammals and birds raised the risk of cancer by an average of 0.1%. For reptiles and amphibians, where body length was considered instead of mass, a 1% increase in length raised the risk of oncological diseases by an average of 0.003%. However, these figures turned out to be significantly lower than expected.

“The increase in risk that we observed is very minor and not at all proportional to body size. If an elephant is a thousand times larger than a mouse, in theory, its cancer risk should be a thousand times higher. But this is not the case. Why? Perhaps larger species have developed more ways to protect themselves,” explained Vera Gorbunova from the University of Rochester (USA), one of the study's authors.

In their work, the biologists employed modern phylogenetic methods that account for the evolutionary relationships between species and discovered that those species of birds and mammals that increased in size more rapidly over a short evolutionary period are much better protected against cancer. For instance, the Asian elephant (Elephas maximus), whose ancestors grew particularly quickly, has a cancer risk 56% lower than expected for its mass. In contrast, smaller species, such as bats, display weaker protective mechanisms.

The authors of previous studies identified additional copies of the p53 gene in elephants, which halts the division of damaged cells. In whales, genes were found that accelerate DNA repair.

“Evolution is a kind of 'arms race.' To become larger, species had to improve their control over cell division. But this does not make them invulnerable—rather, they have developed better methods of protection,” explained Butler.

The authors of the scientific work noted that the key discovery in their research is not that large animals are more susceptible to cancer but rather how evolution “cheats” to reduce the risk of oncological diseases.

If specialists can understand how some species resist cancer and what unique biological mechanisms make them less susceptible to oncological diseases, it could lead to the development of new treatment methods for humans. Scientists may be able to create drugs that mimic the protective processes of these animals.