An Unspoken Carcinogen For Wildlife: Human Activity

Just like humans, many wild animals are experiencing anthropogenic changes in their environments. Several studies have already confirmed that many nonhuman animal species can develop cancer, and that tumor incidences in wild populations link to the populations’ exposure to pollution—just like with humans. But, even though we might expect increased cancer rates in wild animals due to anthropogenic modifications to their environments, wildlife cancer research is still a new field of study.

In this literature review, a group of scientists propose that human activities might increase cancer rates in wild populations through several processes. And as experts predict human impacts on wildlife to increase in the future, acknowledging such a possible link is crucial, especially if researchers want to develop and conduct more research in this direction. You’ll find four categories of anthropogenic effects below that the authors have identified as potentially carcinogenic to wildlife.

Pollution And Cancer

Scientists have most extensively studied the category that concerns cancer risk from pollution. And the authors highlight some of the key results so far: cancer resides in beluga whales who live in highly polluted habitats, cancer prevalence in California sea lions is linked to contaminants, and carcinomas in sea lions are shown to contain high levels of polychlorinated biphenyls (PCBs)—contaminants that bear both carcinogenic and immunosuppressive effects.

The authors suggest that plastic pollution might also be an understudied candidate contributing to the general increase in tumors in marine environments. Bisphenol A (BPA), for one, possesses endocrine disruption properties, and scientists think it contributes to cancer development both in human and nonhuman animals. Similarly, we expose wild animals to high levels of pesticides and herbicides, which could have a negative impact on the rate of hormone-dependent tumorous growths.

Although little literature exists on the topic, another type of environmental contamination is radiation. The review found one study that had already shown a link between background radiation and tumors in birds in Chernobyl. More importantly, scientists seemingly never considered how interactions between pollutants might influence cancer growth in wildlife. As yet, there is a lack of answers to questions such as ‘Would exposure to two pollutants lead to a twofold risk of cancer?’

Light Pollution In Urban Environments

Scientists established the link between exposure to artificial light at night and cancer development in humans back in 2009: in female employees working night shifts. Melatonin, a hormone with anticancer properties that is present in all vertebrates, is rhythmically secreted—mostly at night—and light suppresses it. So, the authors suspect similar oncogenic effects in nonhuman animals, not to mention the possibility that light pollution could have negative effects on sleep duration and sleep quality in wild animals. Scientists have described before about how increases in the amount of sleep can decrease cancer burden.

Anthropogenic Food And Cancer

It is increasingly the case that both humans and wildlife live in a nutritional environment that differs from that which they’ve evolved for. In wildlife, this typically manifests in behaviors such as supplementary feeding to help animals survive, humans using some individuals for recreational monitoring or hunting, and, of course, unintentional food provisioning—eating our garbage.

Some further issues related to anthropogenic food sources that the authors highlight are as follows:

• Fungal growth and the resulting toxic metabolites—mycotoxins—contaminate wildlife feed very frequently. Scientists have already associated these contaminants with carcinogenicity in both human and nonhuman animal populations.
• Farmed animal carcasses that farmers often discard at avian scavenger feeding stations contain residues of antibiotics. In fact, 92% of tested Eurasian griffon vultures showed variable concentrations of residues of an antibiotic that becomes carcinogenic upon UV exposure.
• Another aspect of this is that these anthropogenic foods can often be of low quality, which may also increase the risk of developing cancer for wildlife. A recent review demonstrated that nearly half of the analyzed studies found protein or micronutrient deficiencies in these anthropogenic foods.
• Finally, feeding stations can attract crowds of animals, potentially facilitating the transmission of pathogens and leading to stress, which can have immunosuppressive effects.

Loss Of Genetic Diversity And Inbreeding

The researchers report that more and more evidence supports a link between reduced genetic diversity, inbreeding, and cancer development in nonhuman animals. Dogs and cats illustrate this well—they have a significantly lower genetic diversity compared with their wild ancestors, and scientists have linked this to the relatively high cancer prevalence in our favorite animal companions.

It is clear that, due to their ubiquitous distribution, wild animals are on the front lines of environmental change and exposure to toxic hazards. Very little is known about cancer because of the difficulty in detecting and measuring oncogenic processes in wild species. So, the authors argue that cancer is undoubtedly a health consequence of modern anthropogenic changes, and that we underestimate the extent to which this is the case. But they are hopeful as “most ecological impacts are sooner or later followed by evolutionary responses that lead to new equilibria.” They predict that natural selection, in a world increasingly exposed to the risk of cancer, should mostly favor adaptations that prevent malignant growths and alleviate their detrimental effects. Whether there will be anyone left to reap these evolutionary resistance benefits, though, remains unclear.

Animal advocates will appreciate the ongoing research in the field of anthropogenic effects on wildlife. The link that the researchers proposed—between us changing natural environments and an increased cancer risk in wild animals—is an especially welcome metric as it enables us to quantify our impact in a scientifically sound manner. Plus, the addition of less discussed factors such as light pollution and wildlife feeding enables new advocacy frontiers.