Because we modify the environment in a way that can cause cancer in wildlife, humans can be defined as an “oncogenic species”, say the authors of a new paper in the journal Nature Ecology and Evolution.

Cancer incidence in humans is currently on the rise, and much of this increase has been linked with changes in diet and lifestyle, as well as exposure to pollutants.

Human activity is causing many wildlife species to also experience changes in diet and habitat, but how this affects cancer rates has not been as well explored.

Now, an international team of researchers led by Frédéric Thomas at Centre de Researches Écologiques & Évolutives sur le Cancer (CREEC), in France, has analysed multiple studies that collectively show a clear association between human activity and cancer risk in wild animals.

The scientists provide a summary of the mechanisms by which humans are inducing cancer in other animals.

First and foremost is pollution. Environmental contaminants disrupt cell growth through a variety of mechanisms, including DNA damage, interference with immune function, and disruption of hormonal balance.

One study showed that 27% of beluga whales in the highly polluted Saint Lawrence Estuary in Canada had cancer. In another study, chlorine-based pesticides were associated with increased cancer rates in California sea lions. Meanwhile, radionucleotide contamination from the Chernobyl disaster in 1986 has been linked to increased tumours in local birds.

The authors also caution that the global accumulation of micro-plastics represents a potentially serious cancer threat to wildlife, as does exposure to agricultural pesticides.

Human-sourced food is also a problem. We provide food to animals intentionally via feeding, and unintentionally through waste. This material can contain hazards, such as mycotoxins from fungus that grows on discarded food, or toxins derived from certain antibiotics that become carcinogenic in sunlight. Moreover, the food itself can be low quality, leading to nutrient deficiency and decreased immune health, and can also alter gut microbiota, all of which are linked with increased cancer risk.

Beyond contaminants and diet, humans may be increasing cancer prevalence in wildlife through light pollution. The authors propose that Artificial Light at Night (ALAN), could be considered “an environmental endocrine disruptor for wildlife.”

ALAN is linked with elevated cancer risk in humans, most likely through disruption of key hormones vital to sleep regulation and cancer suppression. Artificial lighting could have a similar effect on wildlife, making it easier for cancers to form.

Human-caused habitat change can also reduce genetic diversity within animal populations, which can increase cancer susceptibility, say the authors. Reduced genetic diversity in snow leopards and western barred bandicoots has been shown to reduce their capacity to fight off cancer-causing pathogens. Moreover, loss of variation in a single gene in California sea lions has been linked with a rise in urogenital carcinoma, while decreased genetic diversity in certain fox and zebra species has been followed by a rise in cancer prevalence.

Thomas and colleagues believe that human-activity driven cancer in wild animals is currently underestimated and more research is needed to better understand how human behaviour, cancer and ecology are intertwined.