Bigger, Heavier Animals May Get More Cancer Than Others 

Animals with longer gestation times may have fewer cancers, but the way traits relate to cancer risk is complex.

Cancer occurs in animals as well as in humans, at widely varying rates among different species. How large and heavy the animals are, and how long their offspring develop before birth, may be linked to the cancer rates, according to study results published in the AACR journal Cancer Discovery.  

“Discovering why some species have particularly high or low levels of cancer may lead to a better understanding of cancer syndromes and novel strategies for the management and prevention of cancer,” wrote the team led by Zachary Compton, PhD, a postdoctoral fellow in the  University of Arizona Cancer Center’s NCI T32 fellowship program and first author of the study. 

The study of cancer across different species—ranging from lizards and frogs to cats, dogs, and elephants—is called comparative oncology.  

“We want to learn more about what makes humans unique in their cancer susceptibility,” Compton said. “There’s some innate cancer risk that’s just part of our species identity, and comparative oncology can help us learn why.” 

Compton and his colleagues based the study a total of 16,049 necropsy records from 292 species at 99 animal care institutions in the United States and London. All species included in the analysis had at least 20 available necropsies. The researchers assessed whether neoplasia (abnormal and excessive growth of cells and tissue) was present, whether it was benign or malignant, and in what part of animal’s body it occurred.  

Across all species, the median prevalence of neoplasia was 4.89%, and the median prevalence of malignancy was 3.2%. Mammals had the highest rates of both neoplasia (12%) and malignancy (7%), and amphibians had the lowest rates (1.2% and 0%, respectively).  

By comparison, approximately 39.3% of people in the United States will be diagnosed with cancer during their lifetime, according to federal estimates; but Compton warned that we may not know the exact prevalence of malignancy in humans, as most natural deaths do not warrant autopsies, and there is currently no robust way to estimate benign neoplasia at the population level. 

Larger body mass was significantly associated with a higher prevalence of neoplasia, according to the study. For every tenfold increase in body mass, the risk of neoplasia increased by 2.1. Other factors associated with malignancy and/or neoplasia included increased maximum longevity, larger litter size in mammals, and, in a subset analysis of 15 species, somatic mutation rate, or the rate at which new mutations occur in the body.  

Longer gestation time was significantly associated with a lower prevalence of both neoplasia (5.3% decrease in neoplasia risk per tenfold increase in months of gestation) and malignancy (5.65% decrease in malignancy risk per tenfold increase in months of gestation). 

The analysis identified animals with exceptionally high and exceptionally low rates of neoplasia. Ferrets and opossums had the highest rates, with 63% and 56% having neoplasia at necropsy, respectively. Species with low rates of neoplasia included the Nubian ibex, the tammar wallaby, and several species of bats.  

Many species had no malignancies observed, which may be strong candidates to study cancer suppression mechanisms, Compton said. He added that species with high rates of neoplasia may serve as models of spontaneous cancer development, and of some human cancer predisposition syndromes. 

“This research gives us a wide range of hypotheses to pursue, including what molecular pathways and exposures are promoting or preventing cancer development across these species,” Compton said.