A longstanding scientific belief about a link between cancer prevalence and animal body size has tested for the first time in our new study ranging across hundreds of animal species.
If larger animals have more cells, and cancer comes from cells going rogue, then the largest animals on Earth—like elephants and whales—should be riddled with tumours. Yet, for decades, there has been little evidence to support this idea.
Many species seem to defy this expectation entirely. For example, budgies are notorious among pet owners for being prone to renal cancer despite weighing only 35 g. Yet cancer only accounts for around 2 percent of mortality among roe deer (up to 35 kg).
Peto’s paradox is that bigger, longer-lived species should have higher cancer prevalence, yet they don’t seem to. Back in 1977, Professor Sir Richard Peto noted that, on a cell-by-cell basis, mice seem to have much higher susceptibility to cancer than humans. This has led to speculation that larger species must have evolved natural cancer defenses.
Several examples of these cancer defenses have since been identified. For example, Asian elephants, a species with notably low cancer prevalence, have over 20 copies of a tumour suppressor gene (TP53) compared to our own lone copy. However, scientists are yet to find broader evidence across a range of animal species.
Our new study challenges Peto’s paradox. We used a recently compiled dataset of cancer prevalence in over 260 species of amphibians, birds, mammals, and reptiles from wildlife institutions. Then, using powerful modern statistical techniques, we compared cancer prevalence between the animals.
Credit: Jo Baker and George Butler Large species have a much greater risk of getting cancer (solid line), but faster evolution rates reduce that risk (dashed line).
We found that larger species do, in fact, have more cancer compared to smaller ones. This holds across all four major vertebrate groups, meaning that the traditional interpretation of Peto’s paradox doesn’t hold up. But the story doesn’t end there.
At first look, our findings seemed to be at odds with another long-standing scientific idea. Cope’s rule is that evolution has repeatedly favored larger body sizes, because of advantages like improved predation and resilience. But why would natural selection drive species toward a trait that carries an inherent risk of cancer?
