The biggest animals should have the highest risks of developing tumors, but they don’t.
In 2012, on a whim, Vincent Lynch decided to search the genome of the African elephant to see if it had extra anti-cancer genes. Cancers happen when cells build up mutations in their DNA that allow them to grow and divide uncontrollably. Bigger animals, whose bodies comprise more cells, should therefore have a higher risk of cancer. This is true within species: On average, taller humans are more likely to develop tumors than shorter ones, and bigger dogs have a higher cancer risk than smaller ones.
But this trend breaks down when you look across species. Elephants are no more susceptible to tumors than Chihuahuas, and whales are no more likely to develop cancers than humans—if anything, their risk is lower. That’s especially strange because big animals also tend to have longer life spans, giving more opportunities for each of their already abundant cells to become cancerous. They ought to be walking (or swimming) masses of tumors—but clearly they aren’t. For the vast majority of mammals that have been studied, the odds of dying from cancer range from 1 to 10 percent, whether you’re talking about a 50-gram grass mouse or a 5,000-kilogram African elephant.