For nearly half a century, a dogma seemed to be commonly accepted in the biology community. Postulated by Richard Peto, an English epidemiologist and statistician, it stipulated that the incidence of cancer was not linked to the size of the organism. In other words, a whale did not necessarily have a higher rate of tumors than a mouse.
At first glance, this "Peto paradox" seems counterintuitive. Since cancer is the result of cells that begin to malfunction and proliferate excessively in complete anarchy, it seems logical that the more cells a living species has, the greater the probability that one of them will become cancerous. Consider that a single small centimeter of size difference between two individuals roughly translates to a billion cells more or less!
263 animal species examined
However, Richard Peto noted in 1977 that, given the same number of cells, humans, for example, were much less prone to cancer than mice. He then concluded that there must be evolutionary mechanisms in large animal species that protect organisms from the risk of carcinogenesis.
But a new study conducted by researchers at the University of Reading (UK) has now refuted their compatriot's conclusions. It is published in the journal PNAS (Proceedings of the National Academy of Sciences). The team examined existing cancer data from 263 animal species from four distinct groups: 31 amphibians, 79 birds, 63 reptiles, and 90 mammals.
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The elephant escapes the rule
Armed with this wealth of data, which Richard Peto did not have in the 1970s, the researchers discovered that the larger an animal, the higher the prevalence of developing a tumor, malignant or not. This is true whether the organism stops growing once it reaches a certain size—as is the case with birds and mammals—or, as is the case with amphibians and reptiles, whether the animal grows throughout its life.
However, the study revealed some peculiarities. For example, in the largest species, including the elephant. Despite its size and therefore its high tumor rate, the animal's evolution has favored the development of mechanisms to control cell growth and prevent the appearance of malignant tumors. As a result, the pachyderm has the same cancer risk as a mammal like the tiger, ten times smaller.
A budgie and a mole rat stand out
Taking the analysis a step further, the researchers compared the group of birds and mammals, which were, based on their sizes, the winners and losers in the great tumor race. They found that, of the 169 species studied, 28 birds and 42 mammals were unfortunate enough to have a higher prevalence of developing malignant cancers than their sizes would suggest. The big loser being Melopsittacus undulatus, the budgerigar. Although tiny at 30 grams, the bird is 40 times more likely to develop a tumor than any other bird or mammal of the same size.
And, sitting at the top of the podium of the lucky ones, the researchers were not surprised to find the famous naked mole rat (Heterocephalus glaber) for which there are no known examples of cancer, confirming that this underground rodent does indeed possess mysterious anti-tumor defense mechanisms.
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