Comparative oncology offers unique insights into disease resistance. Many people assume large wild mammals, such as the American bison, are nearly immune to cancer due to their natural lifestyle. Investigating this requires examining the pathology of these megafauna to understand the true prevalence and mechanisms of cancer in a natural setting.
Documented Cases and Prevalence of Cancer in Bison
Yes, bison can develop cancer, a finding confirmed through opportunistic post-mortem examinations and veterinary case reports. The documentation of tumors in both the American and European species of bison, while present, is exceedingly scarce in scientific literature. Reported cases are typically isolated incidents of malignant growths, such as squamous cell carcinoma, which affects epithelial tissues, and malignant schwannoma, a tumor of the nerve sheath.
The scarcity of reported cases reflects the practical challenges of studying wild populations, not necessarily a low true incidence rate. Systematic health screening is virtually impossible for free-ranging bison; pathology is usually only discovered post-mortem or if the animal is brought into human care. Consequently, the available data is highly skewed, favoring the observation of health over the detection of internal cancers. The overall prevalence of cancer in large wild mammals like bison is consistently described as extremely low compared to humans or domesticated animals.
Comparative Pathology: Bison Versus Domesticated Cattle
Bison and domesticated cattle belong to the Bovidae family, sharing a close phylogenetic relationship, yet they exhibit dramatically different cancer profiles. Cancer incidence is significantly higher in intensively managed cattle populations, highlighting the influence of human-controlled environments and selective breeding. This disparity is particularly evident in the context of viral-induced cancers.
Bovine Leukosis Virus (BLV) is one of the most widespread infectious agents in domestic cattle, with US dairy herds showing an infection rate in nearly 90% of herds and affecting close to half of all cows. This retrovirus is transmitted through the transfer of blood and can lead to lymphosarcoma in a small percentage of infected animals, typically those between four and eight years old. In stark contrast, documented cases of BLV infection in free-ranging bison are virtually non-existent, with only one antibody detection reported in a European bison.
Ocular squamous cell carcinoma, often called “cancer eye,” is a common malignancy in older dairy cattle and certain breeds like the white-faced Hereford. Domesticated animals often live longer than their wild counterparts due to human protection, extending their lifespan into the age bracket where age-related cancers are more likely. High-density housing and shared veterinary equipment also facilitate the transmission of infectious agents like BLV among cattle, unlike the dispersed lifestyle of wild bison.
Ecological and Physiological Factors Affecting Cancer Rates in Wild Ungulates
The low observed cancer rate in bison and other large ungulates is partially explained by the powerful dynamics of natural selection in the wild. This mechanism, sometimes called the culling effect, ensures that animals developing debilitating diseases are quickly removed from the population by predation or resource competition. An animal weakened by a developing tumor is less able to evade predators or compete for mates, meaning it is unlikely to survive long enough for the cancer to progress and be documented.
Wild ungulates also possess a much shorter effective lifespan compared to their maximum biological potential, which limits the time available for age-related cancers to manifest. While a bison might live for over 20 years in captivity, their average lifespan in the wild is significantly shorter, reducing the number of cell divisions and opportunities for oncogenic mutations to accumulate. This contrasts with domesticated cattle, which are often protected and allowed to live to an age that increases their cancer risk.
Bison’s physiology and large body size contribute to their cancer resistance, aligning with Peto’s Paradox. This observation notes that larger species, despite having trillions more cells, do not exhibit a proportionally higher cancer incidence than smaller animals. This suggests that large-bodied species have evolved enhanced tumor suppression mechanisms, such as more copies of tumor-suppressor genes or specialized cellular checkpoints. The bison’s natural diet and pristine environment also reduce their exposure to anthropogenic carcinogens common in human-dominated areas.