Large Animals Have Higher Cancer Rates Than Small Ones, Challenging Decades-Old Theory

A new study has found that large animals, such as elephants, giraffes, and pythons, have higher cancer rates than smaller animals like mice, bats, and frogs, overturning a 45-year-old understanding of cancer in animals.

Conducted by researchers from the University of Reading, University College London, and the Johns Hopkins University School of Medicine, the study analyzed cancer data from 263 species across four major animal groups—amphibians, birds, mammals, and reptiles. The findings challenge Peto's Paradox, a theory proposed in 1977, which suggests that an animal's body size does not correlate with its cancer risk.

Published in the Proceedings of the National Academy of Sciences (PNAS), the study revealed that both benign and malignant tumor prevalence is higher in larger animals. However, researchers also found that some species, such as elephants, have evolved natural defenses against cancer.

Using veterinary autopsy data from 31 amphibian species, 79 bird species, 90 mammal species, and 63 reptile species, the study employed advanced statistical methods to assess the relationship between cancer rates and body size while accounting for evolutionary factors. Despite differences in growth patterns—birds and mammals typically stop growing at a certain size, while amphibians and reptiles continue growing throughout life—the results consistently showed that larger body size is associated with higher cancer risk. Interestingly, species that evolved rapidly to a larger size, such as elephants, developed more effective cellular growth control and tumor prevention mechanisms.

The study also identified anomalies in cancer prevalence among certain species. For example, budgerigars (parakeets), which weigh less than 30 grams, had cancer rates 40 times higher than expected, while naked mole rats were found to be almost entirely cancer-resistant. These insights open new avenues for cancer research, shedding light on potential mechanisms for cancer resistance in nature.