Bigger Bodies, Bigger Cancer Risk? New Study Challenges Decades-Old Assumption
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The study used veterinary autopsy records from 31 amphibians, 79 birds, 90 mammals and 63 reptiles. Despite different growth patterns, some species growing indefinitely, others stopping at maturity, the trend remained: bigger animals had more cancer.
The study found that larger species consistently show higher tumour prevalence.
For decades, scientists believed large animals were no more prone to cancer than smaller ones. But fresh research has flipped that idea on its head. Elephants, giraffes and pythons? They actually get more cancer than tiny creatures like bats, mice and frogs. A team from the University of Reading, University College London and Johns Hopkins University School of Medicine analysed data from 263 species across mammals, birds, reptiles and amphibians. Their findings challenge Peto’s paradox, a 1977 concept suggesting an animal’s size had no bearing on cancer risk.
Published on 24 February in Proceedings of the National Academy of Sciences (PNAS), the study found larger species consistently show higher tumour prevalence. However, species that evolved rapidly to large sizes, like elephants, developed stronger natural cancer defences.
Professor Chris Venditti, senior researcher at the University of Reading, noted that “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.”
The study analysed the largest dataset of its kind, using veterinary autopsy records from 31 amphibians, 79 birds, 90 mammals and 63 reptiles. Despite different growth patterns, some species growing indefinitely, others stopping at maturity, the trend remained: bigger animals had more cancer. Yet, species like elephants evolved ways to suppress tumours, keeping their risk comparable to much smaller animals like tigers.
Dr Joanna Baker, co-author from the University of Reading, explained, “When species needed to grow larger, they also evolved remarkable defences against cancer. Elephants shouldn’t fear their size – they developed sophisticated biological tools to keep cancer in check.”
For humans, the picture is complex. Medical advancements make it difficult to determine where we fit into this size-cancer equation. The research confirms a general trend, bigger species tend to get more cancer, but also highlights how some evolved to counteract it. Understanding these natural defences could inspire new cancer treatments.
The study also revealed some unexpected exceptions. The common budgie (Melopsittacus undulatus), a tiny bird weighing less than 30 grams, had cancer rates 40 times higher than expected. Meanwhile, the naked mole rat (Heterocephalus glaber) appears almost entirely immune to cancer.
Dr George Butler, lead author from 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.”
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