Elephants, giraffes, pythons and other large species have higher cancer rates than smaller ones like mice, bats, and frogs, a new study has shown, overturning a 45-year-old belief about cancer in the animal kingdom.

The research, conducted by researchers from the University of Reading, University College London and The Johns Hopkins University School of Medicine, examined cancer data from 263 species across four major animal groups—amphibians, birds, mammals and reptiles.

The findings challenge “Peto’s paradox,” a longstanding idea based on observations from 1977 that suggested there was no link between an animal’s size and its cancer risk.

The study, published in the journal Proceedings of the National Academy of Sciences, revealed that bigger animals consistently show a higher prevalence of both benign and malignant tumors. The researchers also discovered that species which evolved to be larger more rapidly, like elephants, developed better natural defenses against cancer.

Professor Chris Venditti, senior author of the research at the University of Reading, said, “Everyone knows the myth that elephants are afraid of mice, but when it comes to cancer risk, mice are the ones who have less to fear. We’ve shown that larger species like elephants do face higher cancer rates—exactly what you’d expect given they have so many more cells that could go wrong.”

Keeping cancer in check

The study involved the analysis of the largest dataset of its kind to date. The researchers studied cancer records from veterinary autopsies of 31 amphibians, 79 birds, 90 mammals, and 63 reptiles. They used advanced statistical methods to analyze how cancer rates related to body size while accounting for how different species evolved.

Birds and mammals (which stop growing at a set size) and amphibians and reptiles (which can grow throughout their lives) were observed separately. Despite these differences in growth patterns, both groups showed the same overall trend—larger species had higher cancer rates.

However, the study shows species that rapidly evolved to large sizes over short evolutionary timescales, like elephants, evolved improved mechanisms to control cell growth and prevent tumors. An elephant, for instance, has about the same cancer risk as a tiger—an animal just one-tenth its size.

Dr. Joanna Baker, co-author from the University of Reading, added, “When species needed 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.”

Modern medicine makes it difficult to know exactly how humans fit this size-cancer pattern. The research shows a general trend where larger species have higher cancer rates, but also reveals how some species evolved better cancer defenses as they grew larger.

Overall, the findings support the idea that superior mechanisms of cellular defense—such as improved control of cell growth—have evolved at specific points in time and in larger species. This opens the door to new approaches to understanding the mechanisms responsible for cancer, potentially providing novel insight into human disease and its treatment.

Budgies vs. naked mole rats

On average, larger species get more cancer compared to smaller species. However, the researchers also identified a subset of species that get much more or much less cancer than you would expect given their body size.

For example, the common budgie (Melopsittacus undulatus) was found to have rates of cancer more than 40 times higher than would be expected for its body size—weighing less than 30g. Conversely, the naked mole rat (Heterocephalus glaber) has almost no recorded prevalence of cancers.

Dr. George Butler, lead author of the research at University College London and Johns Hopkins said, “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.”