Cancer, the uncontrolled growth of cells, affects animals in their natural habitats. While it might seem rare, wild animals do develop various forms of cancer. This phenomenon has existed throughout evolutionary history. Understanding cancer in wild populations offers insights into disease ecology and ecosystem health.
Cancer’s Presence in Wild Populations
Cancer has been documented across a wide array of wild animal species, including mammals, birds, fish, reptiles, and amphibians. A comprehensive study of over 16,000 necropsy records from 292 vertebrate species found evidence of neoplastic disease across all taxonomic groups.
Specific examples include transmissible facial tumors in Tasmanian devils, which have severely impacted their populations. Marine bivalves, such as clams and cockles, can develop a leukemia-like cancer that spreads through seawater. Fish, including coral trout, have shown instances of melanoma. Cancer has also been observed in beluga whales, with high rates of intestinal cancer in some populations.
Why Cancer Appears Less Common in Wild Animals
The perception that cancer is rare in wild animals stems from several factors. Wild animals generally have shorter lifespans compared to humans or animals in managed care. Many cancers take time to develop, so wild animals may succumb to other environmental pressures before cancer becomes apparent.
Natural selection and predation also play a significant role. Animals weakened by disease, including cancer, are more vulnerable to predators. This natural culling means individuals with advanced cancers are less likely to survive long enough for their condition to be observed or documented.
Furthermore, wild animals are not routinely subjected to diagnostic tools common in human and veterinary medicine. They do not receive regular screenings, blood tests, or autopsies, making it difficult to detect internal or early-stage cancers. Many cancers likely go undiagnosed because they do not present obvious external signs, or the animal dies and decays before examination is possible.
Causes of Cancer in Wild Animals
Environmental contaminants are a significant concern. Exposure to pollutants like polycyclic aromatic hydrocarbons, heavy metals, and pesticides in their habitats can act as carcinogens, inducing genetic mutations that lead to cancer. For example, fish in polluted waterways exhibit high levels of liver and skin cancer.
Infectious agents, such as viruses, are also implicated in wild animal cancers. Papillomaviruses and retroviruses are known to induce tumors in certain species. Green sea turtles, for instance, are affected by fibropapillomatosis, a disease causing tumors on their skin and internal organs, suspected to be virus-related. Some transmissible cancers, like the facial tumor disease in Tasmanian devils, are spread directly through the transfer of cancer cells.
Age is another factor, as in humans, with older animals generally having a higher probability of developing cancer due to accumulated cellular damage. Genetic predispositions can also influence cancer susceptibility within specific populations.
Studying Cancer in Wild Populations
Scientists employ various methods to investigate and monitor cancer in wild animal populations. Necropsies, or post-mortem examinations, are crucial for identifying tumors and other neoplastic diseases in deceased animals. This allows researchers to analyze tissue samples and determine the type and extent of the cancer.
Field observations help document visible tumors or changes in animal behavior that might indicate illness. Researchers also utilize biomarker analysis, studying blood or tissue samples for molecular indicators of disease. Genetic sequencing is increasingly used to identify specific mutations linked to cancer development. This research is important for understanding disease ecology, informing conservation efforts, and providing insights that could benefit human health.