An international group of scientists have carried out a highly detailed study across more than a hundred thousand zoo animals for cancer research, and their findings were surprising.
Many of us think of cancer as occurring in humans. But animals, and not just pets, can also develop cancer as well.
Yet we don’t know to what extent wild animals develop cancer, because in the wild, their chances of survival are reduced, due to starvation or predation, a news release informs. Moreover, cancer affects older animals, and in the wild, it is difficult to estimate the age of a dying or dead animal.
Because of this, researchers focused on zoo animals, where their lives are closely monitored from their infancy to adulthood and old age, to find out how often they face cancer.
The scientists used data on 191 species and 110,148 adult mammals that died in zoos to study cancer. They found that cancer is a widespread disease that threatens mammals and that it can “emerge anywhere along the mammalian phylogeny.”
An important finding was that cancer risk is not uniformly distributed and that, for example, Carnivorans had a higher rate of incidence of cancer – “over 25 percent of Clouded leopards, Bat-eared foxes and Red wolves die of cancer”, while ungulates seem to be highly resistant to cancer.
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 Technica.
The team limited their search to data points taken after 2010 because, prior to that, the record-keeping was not as good, she said. And the reason the team studied animals in zoos was because it’s hard to collect information with this much detail 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.
The researchers found that eating animals, especially mammalian prey, increased cancer risk across mammals. They suggest that the reasons may be related to “their low microbiome diversity, limited physical exercise under human care, oncogenic viral infections to other physiological aspects of carnivorous mammals.”
The study also explored whether larger mammals were more likely to develop cancer, due to their increased cell numbers and cell divisions. Tumours are usually a result of mutations during cell division.
In humans, greater body size (height) is associated with a higher risk of cancer, the news release notes. So it would make sense that animals that have larger bodies and longer lifespans would be more prone to develop cancer.
Yet these correlations don’t affect animals as they do humans. This effect is known as Peto’s paradox: “The lack of correlation between body size and cancer risk.” The researchers write that the “logical challenge, first formulated by Sir Richard Peto, … noted that although mice have approximately 1,000 times fewer cells and >30 times shorter lifespans than humans, their risk of carcinogenesis is not markedly different.”
The research provides proof for Peto’s paradox, noting that “cancer risk is largely independent of body mass and life expectancy across mammals.” It means that during evolution, larger animals or those with longer lifespans evolved to have more efficient tumour suppressor mechanisms.
“Overall our work highlights that cancer might represent a serious and significant threat to animal welfare, that needs considerable scientific attention, especially in the context of recent environmental changes caused by humans,” says co-author Fernando Colchero, University of Southern Denmark.
The authors write that “cancer was detected in at least one individual in almost all species with more than 82 individual pathological records available.” Yet there were two species where no cancer was detected, the blackbuck (Antilope cervicapra) and the Patagonian mara (Dolichotis patagonum), despite postmortem pathological records being available for 196 and 213 individuals, respectively.
The news release notes that studying these animals can provide major breakthroughs in cancer medicine, with the development of biomimetic natural cancer treatments which, unlike most other cancer treatments, would be non-toxic to the patient.