Bigger animals have increased cancer risk, study confirms
Elephants' rapid evolution has meant they have managed to reduce their cancer risk.
A study has challenged long-standing theories by revealing that larger animals do have an increased risk of developing cancer.
The findings challenge the idea of ‘Peto’s paradox’: a theory from 1977 that suggested that there was no correlation between animal size and cancer risk.
The new study saw researchers from three universities collaborate to examine cancer data from 263 species. This was spread across the four major animal groups: amphibians, birds, mammals and reptiles.
The data had been gathered from veterinary autopsies of 31 amphibians, 79 birds, 90 mammals and 63 reptiles. Using advanced statistical methods, researchers analysed the relationship between cancer rates and body sizes, while taking into account the different species involved.
Researchers discovered that, overall, bigger animals displayed a higher prevalence of benign and malignant tumours.
This finding proved to be consistent in all species, regardless of whether they naturally stop growing (birds and mammals) or grow throughout their lives (amphibians and reptiles).
However, the researchers also identified that rapid evolution was a contributing factor to cancer defence. Animals which have rapidly evolved to a large size over short evolutionary timescales also evolved improved mechanisms to control cell growth and prevent tumours.
This means that, despite being ten times bigger, elephants have approximately the same cancer risk as a tiger.
There were some outliers to the researchers’ theory though, where a subset of species had much more or much less cancer than expected for their body size.
The common budgie, which weighs less than 30g, was found to have cancer rates which were more than 40 times higher than expected for its body size. The naked mole rat, however, had almost no recorded rate of cancer.
Advances in modern medicine means that researchers were unable to apply this theory to humans.
Overall, the research revealed a trend of larger species recording higher cancer rates. However, it also highlighted the role of evolution in developing mechanisms for cellular defence.
George Butler, lead author of the research at University College London, 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."
The full study can be found in the journal Proceedings of the National Academy of Sciences.
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The BSAVA has opened submissions for the BSAVA Clinical Research Abstracts 2026.
