A decades-old belief about cancer in the animal kingdom has been overturned after a new study found that larger species like elephants, giraffes, and pythons have higher rates of cancer than smaller ones like mice, bats, and frogs.
Researchers from the University of Reading, University College London, and the Johns Hopkins University School of Medicine analyzed cancer data from 263 species across four major animal groups—mammals, birds, reptiles, and amphibians.
The results challenge “Peto’s paradox,” a 45-year-old belief based on observations from 1977 that suggested there was no link between an animal’s size and its risk of cancer.
The new study revealed that bigger animals consistently had more benign and malignant tumors. In addition, species that rapidly evolved to be larger, like elephants, developed better natural defenses against cancer. The researchers examined cancer records of 90 mammals, 79 birds, 63 reptiles, and 31 amphibians.
They used statistical methods to analyze how body size correlated to cancer rates. Birds and mammals stop growing at a specific size, while reptiles and amphibians can continue growing throughout their lives. Therefore, they were observed as two separate groups.
Even though there were differences in their growth patterns, both groups demonstrated the same trend—larger species had higher rates of cancer.
Species that rapidly evolved over a short period of time developed mechanisms to control cell growth and prevent tumors. For instance, an elephant has about the same cancer risk as a tiger, which is one-tenth its size.
“When species need to grow larger, they also evolved remarkable defenses against cancer. Elephants shouldn’t fear their size—they developed sophisticated biological tools to keep cancer in check. It’s a beautiful example of how evolution finds solutions to complex challenges,” said Dr. Joanna Baker, a co-author of the study from the University of Reading.
Overall, larger species were found to get more cancer than smaller species. The researchers also identified a subset of species that get more or less cancer than expected based on their body size.
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The common budgie, a colorful parrot native to Australia, had rates of cancer over 40 times higher than expected for its body size. On the other hand, the mole rat showed almost no cancer.
The findings support the idea that mechanisms of cellular defenses evolved in larger species at certain points in time.
This paves the way for new approaches to fight cancer and can provide insight into how humans fit the size-cancer pattern.
“Finding which animals are naturally better at fighting off cancer opens exciting new paths for research. By studying these successful species, we can better understand how cancers develop and potentially discover new ways to fight the disease. This could lead to breakthrough treatments in the future,” said Dr. George Butler, the lead author of the study.
The details of the full study were published in Proceedings of the National Academy of Sciences.
