Elephants rarely get cancer, and their giant, hot testicles may be a clue as to why.
The idea boils down to a protein called p53, which helps prevent DNA damage in cells, including damage that could turn a normal cell into a cancer cell.
Elephants, unlike humans, have multiple copies of the gene that codes for p53 — meaning, the gene that provides the “recipe” for the body to make the protein. Fritz Vollrathan evolutionary biologist at the University of Oxford said this could help protect their sperm from high temperatures.
This hypothesis starts with “Peto’s paradox,” Vollrath told Live Science.
In the 1970s, an epidemiologist named Richard Peto described a puzzling phenomenon: Large animals, despite having many more cells that could potentially turn into cancer cells, do not seem to have a higher risk of developing cancer than smaller animals. This is especially amazing with elephants – they are statistically less likely get cancer than humans, despite being many times bigger than us.
Researchers discovered that a few years ago elephants have 20 copies of the gene that encodes the p53 protein. In comparison, humans only have one. The protein essentially works as a copy editor, reviewing genetic material as cells multiply and potentially killing cells with any damage that could lead to cancer. Because elephants have multiple copies of the gene that codes for p53, they could undergo multiple rounds of “copy editing,” which could vastly reduce the risk of a damaged cell surviving.
But why did elephants evolve 20 copies of this gene? Vollrath thinks it has to do with their testicles. Many male animals, including humans, have their testicles partially outside their bodies to cool them down, which is believed to be important for creating a healthy batch of sperm. The reasons for this are unclear, although it may have something to do with increased DNA damage at higher temperatures.
However, by a quirk of evolutionary history, elephants’ testicles are inside their bodies. As multi-ton, dark gray animals walking around in the sun, their testicles can get very hot – which is why the elephants may struggle to make viable sperm. But if they had more copy-editing proteins, the theory goes, that hot sperm could be protected from harm.
Vollrath published this hypothesis as nut in the journal Trends in Ecology and Evolution on June 27.
It is difficult to judge exactly why a particular trait evolved in a species, Vincent Lynchan evolutionary biologist at the University at Buffalo, who was not involved in developing this new hypothesis, told Live Science.
It is possible that multiple copies of the p53 gene evolved to protect elephant sperm from high temperatures. But it’s also possible that those multiple specimens evolved because elephants are large animals and so may be more susceptible to cancer, Lynch said. It can also be both at the same time.
Other large animals do not have multiple copies of the p53 gene. Whales, for example, are large animals with internal testicles, but they seem to have only one copy. But whales also have an internal system for cooling their testicles, Vollrath noted — plus, it doesn’t get that hot in the water.
Similarly, animals closely related to elephants, such as hyraxes, also have internal testes. But these animals are much smaller than elephants, and small animals are much more efficient at dissipating heat than large animals, Lynch said.
However it may have evolved, elephants seem to have a way of naturally evading cancer — and studying how it works can help us understand more about the disease, Vollrath said.
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