For mammals, eating other animals can increase cancer risk

Cancer is a sad fact of life, as nearly 40 percent of people are diagnosed with it at some point in their lives. But humans aren’t alone in this. Many different species can also develop the disease—some more often than others. By studying these species and their habits and natural defenses (or lack thereof), we can learn new ways to combat the disease.

New research that involves a comprehensive survey of cancer shows that many mammals can indeed get cancer. To gain insight into this, the team looked at records for 110,148 animals from 191 species that died in zoos. The data came from Species360, an international non-profit that collects and unifies this kind of data from zoos across the world, according to Orsolya Vincze, a research fellow at the Centre for Ecological Research in Hungary and one of the paper’s authors.

Using the data gathered by the organization, the research team could “collect information on what the animals died of,” she told Ars.

The team limited their search to data points taken after 2010 because, prior to that, the record-keeping was not as good, she said. Further, the team only studied animals in zoos because it’s difficult to collect this kind of information from species in the wild. Animals in their native habitats that get cancer are also more likely to be preyed upon or starve to death—they tend to die earlier, Vincze said.

“You have to go to zoos where every individual is followed and you know when they die and you know what they died of,” she said.

Lessons learned

Most of the species the team studied had some cancer risk. The only two exceptions—as far as the data goes—were the blackbuck (a kind of antelope) and the Patagonian mara (a kind of rodent). The data included info on 196 and 213 individuals from those species, respectively.

Carnivores, however, were particularly prone to cancer. Within the dataset, more than a quarter of clouded leopards, bat-eared foxes, and red wolves died of cancer, for instance. According to Vincze, there are some hypotheses surrounding why this might be the case.

For one, carnivores have different microbiomes compared to other types of animals, which could be an issue as a rich community of microorganisms can help limit cancer. Carnivores, particularly those in captivity, also have limited ranges. A lack of physical activity could also contribute. Raw meat—like the kind most carnivorous mammals eat—can also contain bacteria or other microbes that can increase the risk of getting cancer. For example, raw cow meat can carry bovine leukemia virus, which some studies have suggested can increase the chance of a human getting breast cancer. Overall, though, Vincze said that more research needs to be done in this area.

The bigger they are

Somewhat surprisingly, animal size is not correlated to cancer risk. Cancer mutations usually occur when cells divide. In theory, a large, longer-lived animal should have more cell divisions than smaller animals and, thus, they should be more prone to cancer. This is seen in dogs and humans—larger members of both species tend to have higher cancer risk, Vincze said.

However, larger species aren’t particularly at higher risk for getting the disease—a phenomenon called Peto’s Paradox. According to Vincze, this is likely because these species evolved ways to combat cancer in their genetic pasts. By studying the mechanisms through which these large species suppress cancer, we could potentially develop ways to fight the disease. And, by studying why some species have higher instances of the disease, we can learn more about it in general, Vincze said.

"We could really look into the molecular mechanisms and identify them, and try to design new treatment methods for cancer in humans and animals alike,” she said.

Nature, 2021. DOI: 10.1038/s41586-021-04224-5 (About DOIs)