Why big animals do not get more cancer: Revisiting Peto’s paradox

“The relevance for the biology of cancer and ageing of Peto’s paradox is off the scale and remains a highly valid paradigm of study.”

BUFFALO, NY — July 2, 2025 — A new research perspective was published in Aging (Aging-US) Volume 17, Issue 6, on May 29, 2025, titled “Peto’s paradox’s relevance is off the scale.”

In this perspective, Dr. Mirre J.P. Simons from the University of Sheffield argues that Peto’s paradox—a well-known concept in cancer biology—remains a vital framework for understanding cancer resistance in large animals. Dr. Simons challenges recent claims that dismiss the paradox and emphasizes that the unexpectedly low cancer rates in large species still require explanation. This insight is especially relevant for aging and cancer research.

Peto’s paradox highlights a puzzling observation: larger animals like elephants and whales, despite having far more cells than smaller animals, do not have proportionally higher cancer rates. If each cell had an equal chance of turning cancerous, bigger animals should develop cancer much more frequently. But in reality, they do not. This suggests that evolution has equipped these animals with powerful biological defenses against cancer.

“The field of comparative biology into ageing and cancer was given a strong impetus when Peto identified that humans have substantially more cells than mice, but do not have substantially larger incidence of cancer.”

Dr. Simons explains that recent studies showing small increases in cancer with body size do not disprove the paradox. The expected increase, based on basic mathematical models, would be massive—many times greater than what is observed. Instead of rejecting Peto’s paradox, the field should focus on understanding how large animals suppress cancer so effectively.

The author points out that the key to resolving this paradox may lie in traits that evolved alongside body size, such as tissue environments or specialized cell-control mechanisms. These features might reduce the probability of cancer developing, even in animals with millions or billions more cells than humans.

Importantly, this perspective underscores the clinical potential of studying species that resist cancer naturally. Studying these natural defenses may help researchers uncover new ways to understand, prevent, or manage cancer. Because cancer risk increases with age in most species, understanding how some animals limit both aging and cancer may also help explain how these two processes are connected.

Dr. Simons cautions against oversimplifying cancer biology by focusing only on genetic mutations. Instead, understanding how cells interact with their environment, known as the tissue microenvironment, may offer deeper insight into how cancer develops or is prevented.

By reaffirming the importance of Peto’s paradox, this research perspective encourages the scientific community to explore the evolutionary tools nature uses to fight cancer. These insights could improve our understanding of cancer and inspire new strategies to support healthier aging.

Read the full paper: DOI: https://doi.org/10.18632/aging.206258

Corresponding author: Mirre J.P. Simons – m.simons@sheffield.ac.uk

Keywords: aging, cancer, evolution

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