An international research team led by a member of the ELKH Centre for Ecological Research (CER) examined the cancer risk of around 200 species of zoo mammals by comparing the susceptibility of each species to cancer. In their studies, the researchers found that while cancer is rare in ungulate herbivores, it is very common in predatory mammals. The importance of the research is shown by the fact that the results can be used in human and veterinary medicine in the future. The results of this study, the first in the world, were published in the prestigious journal Nature.
When it comes to cancer, we think almost exclusively of human cancer. This is understandable, of course, but it is also important to know that animals are just as vulnerable to this disease. In addition, due human activity, particularly pollution, animals are being exposed to increasingly powerful carcinogens. Yet, very little is known about the incidence of cancer in animals and which groups are more susceptible. Now, researchers are one step closer to answering this question, with the first truly comprehensive study on the inter-species variation in the risk of animal tumors.
“Until now, almost nothing was known about the risks and incidence of cancer in animals. Animals with tumors in the wild are likely to die quickly of hunger or fall prey to predators, so random sampling of live animals does not give a true picture of the prevalence of tumors in animals, says Orsolya Vincze, a researcher at the Institute of Aquatic Ecology of the CER, and the leader of the research project.
“That is why we used the database of Species360, an international organization to look at data from zoo animals. This database also contains the anatomical findings of individual animals, providing reliable information on cancer,” he adds.
In this study, data were collected from more than 110,000 individuals of 191 mammal species, and used to estimate the risk of developing tumors in the animals studied. The results showed that the exposure of mammal species varies widely in this area. Some species develop cancer very rarely, while, in others, half of the individuals die of cancer. Of the species studied, for example, an even-toed ungulate and large-bodied rodent native to South America, the large mara, appears to be highly resistant to tumors. In contrast, cancer is very common among predatory mammals, especially mammal-eating predators such as lions, pumas and leopards. It remains to be seen whether the different susceptibility to developing tumors is a consequence of genetic background or whether it is linked to the different lifestyles of the species.
“Ungulates, which have a low cancer risk, tend to graze, while predators feed on raw meat. Although it is possible that the higher cancer risk in carnivores may be explained by meat consumption (e.g. through the accumulation of carcinogenic contaminants, a diet low in fibre, or the presence of cancer-causing viruses), no firm conclusions can be drawn. It is also possible that the high susceptibility to cancer in zoo predatory mammals is due to lack of exercise or other conditions associated with captivity,” the researcher stresses.
The study also addresses a fascinating evolutionary question. Tumors are diseases of mutational origin, and mutations usually arise during cell division. Consequently, animals with a larger body size and longer lifespan that have undergone more cell division are expected to have a higher risk of cancer. The theory is supported by several studies in dogs and humans, for example, where greater body size (height) is associated with a higher risk of cancer. However, these relationships do not seem to hold across species, as an elephant and a mouse are similarly likely to develop cancer, even though their lifespans and body sizes are orders of magnitude different.
This contradiction is also known as Peto’s paradox, which researchers have now been able to prove for the first time. This is important because it points to the fact that the evolution of long life and large body size is associated with molecular mechanisms that naturally protect against tumors. The study of large, long-lived animals could also shed light on natural anti-tumor mechanisms that could later be used in human or veterinary medicine. In the next phase of research, the authors of the study will also try to answer the question of whether there are differences in tumor types across different species.