Cancer Risk Varies Across Mammal Species
Cancer is a common cause of death among animals. Over the course of an animal’s life, its cells mutate. Most mutations are harmless, but some mutations cause a cell to grow out of control, resulting in cancer. Since every cell division comes with a risk of cancer, scientists predict that larger animals with more cells and longer-lived animals are more likely to get cancer. Indeed, within a given species, longer-lived and larger animals are usually more likely to have cancer. But this generalization might not be true across species. Regardless of size and longevity, many species are approximately equally likely to get cancer, an observation known as Peto’s paradox.
To investigate Peto’s paradox, researchers used the Zoological Information Management System, which collects information from zoos and aquariums, to get records on 110,148 non-domesticated mammals from 191 species. The system only records whether an animal had cancer if the animal is dead and the veterinarian thought that their cancer contributed to their death. Because the information comes from zoos and aquariums, the findings may not generalize to wild animals.
The researchers estimated the adult animals’ average life expectancy. They also calculated the cancer mortality risk, which is the percentage of dead animals in the sample who died of cancer. Cancer mortality risk excludes animals who are still alive, potentially causing bias. Therefore, the researchers calculated a different cancer mortality statistic, cumulative incidence of cancer mortality, which they expected not to have that bias. Cancer mortality risk and cumulative incidence of cancer mortality were highly correlated, suggesting that the bias didn’t affect the results.
Cancer mortality rate is very variable across species. About a quarter of species had no cancer deaths, while in about a fifth of species more than ten percent of animals died of cancer. However, at least one case of cancer was detected in every species where researchers had the records of at least 82 individuals. In other words, while some species are less likely to die of cancer than others, all species die of cancer sometimes.
Animals in the Carnivora taxon are more likely to die of cancer than those in other taxa. Some researchers think that hormonal contraception use and prevention of pregnancy might be linked to cancer in these animals. But female members of Carnivora aren’t more likely to have cancer than males. Other researchers argue that eating raw meat might transmit viruses, which sometimes cause cancer. Mammal species that regularly eat other mammals are, in fact, more likely to get cancer than species that rarely or never eat other mammals. Since pathogens are more likely to be transmitted between species that are more similar to each other, this finding suggests that pathogen exposure is a reason Carnivora is more likely to die of cancer. However, the sample was small enough that this finding requires more research.
Species that are larger (especially ruminant animals) have a lower rate of cancer deaths, while species that have a longer life expectancy are more likely to die of cancer. However, neither result is statistically significant, and the effect size is very small. These results suggest that Peto’s paradox is accurate. The most likely explanation is that species who evolve to be larger and to live a longer time also evolve better mechanisms for preventing cancer.
In many species of wild animals, individuals commonly die of cancer. Preventing cancer in wild animals (e.g. by reducing emissions of carcinogens) may thus improve wild animal welfare. However, animals in zoos and aquariums may be more likely to die of cancer than wild animals, because humans protect animals in zoos and aquariums from other common causes of death. Cancer affects some species more than others for reasons researchers don’t fully understand yet, so it is difficult to know which species would be most affected by cancer-prevention efforts. Because larger and longer-lived species aren’t more likely to get cancer, we can guess that preventing cancer wouldn’t disproportionately improve their welfare.