especially rats and mice usually develop cancer and tumors very fast, reducing their life expectancy.
From an evolutionary point of view, how come that these mammals are so prone to cancer and tumors even though they reproduce at a very fast pace?
Shouldn't they develop a resistance towards these illnesses through natural selection?

Source: https://www.thesprucepets.com/rat-tumors-1238498

  • 1
    $\begingroup$ Can you please add a source supporting your claim? Additionally both animals are not getting very old at all (a lot of mice for example only 2-3 years), so what do you mean by "in just a few years"? $\endgroup$
    – Chris
    Mar 8, 2020 at 13:11
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    $\begingroup$ Counterpoint: the naked mole rat is a small mammal that rarely gets cancer. $\endgroup$
    – acvill
    Mar 8, 2020 at 16:52
  • $\begingroup$ Here's some clues: researchgate.net/figure/… and google.com/… and try the web search for the same words. $\endgroup$ Mar 8, 2020 at 17:11
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    $\begingroup$ Do we really have data on anything other than lab rats and mice, which are often bred to develop cancer, or have cancer given to them for experiments? WRT the natural selection argument, almost everything in nature gets eaten by something else long before cancer or other age-related diseases develop. $\endgroup$
    – jamesqf
    Mar 8, 2020 at 18:52

1 Answer 1


The topic is called Peto's paradox, you can find references to studies on the wiki page, here's a summary:

A rat cell is more likely to get cancer than an elephant cell in the same time span. Mice have cancers in 2 years as often as humans have in 60 years.

enter image description here

Studies find that 4 ton elephants have only 4 times more blood cells than a 12 gram shrew. They have big mammal cells that divide at a slower rate. Rats can double their body weight every 10 days when they are teenagers, as if a puppy with a lot of appetite could become an adult dog in 30 days. enter image description here

It is thought that the genetic safeguards that have evolved to protect large mammals against cancer would slow the growth and reproduction rates of small mammals. For example, an elephant has 14 copies of the cancer suppression gene TP53, humans have three copies, hyraxes and manatees have one copy.

Evidence for the paradox began to emerge in the 1970's. Studies from zoos on many mammal species have confirmed that big animals are protected from higher cancer rates.


  • $\begingroup$ You could also look at it as a large slow growing animal had better be resistant to cancer or it is unlikely to live long enough to reproduce, simply due to the high turnover rate the impact of cancer on a small short lived line of animals is much less. $\endgroup$
    – John
    Mar 9, 2020 at 1:18

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