RIVERSIDE, Calif. — Dogs can be particularly susceptible to various forms of cancer, but new research is revealing that the size of man’s best friend may have a lot to do with their chances of getting sick. Specifically, a new study finds that medium-sized dogs appear to face the worst odds of developing cancer. Researchers from the University of California-Riverside report both the largest and smallest breeds of dogs have a lower risk of developing cancerous tumors than their medium-sized counterparts.
Initially, researchers intended to test out a model of how cancer develops in the first place. That model, called the multistage model, works by predicting that size is a risk factor for cancer. Sure enough, size is indeed a cancer risk factor – but only while considering the variations in size within a single species.
It is quite common for cells to acquire errors or mutations while they divide and form copies of themselves. Bigger animals and animals that tend to live longer boast more cells and a longer lifespan. During that longer life, those cells divide. According to the multistage model, this means they have more opportunities to acquire mutations that may eventually become cancerous.
“The question that arises is why, then, don’t we get more cancer than a mouse? We don’t. There is no increase in cancer risk as animals increase in size from species to species,” says UC Riverside evolutionary biologist and study author Leonard Nunney in a media release.
Crucially, however, researchers say this does not hold true for animals of the same species.
“Studies on humans show that tall people get more cancer than short people. It’s about a 10% increase over the baseline risk for every 10 centimeters in height,” Nunney adds.
In pursuit of further insights regarding these risk factors, Nunney needed a species with an even larger difference between the smallest and biggest individuals. That led the research team to dogs.
“Testing this in dogs is even better because you can compare a tiny chihuahua to a Great Dane. That’s a 35-fold difference in size, and people can’t come close to that,” Nunney explains.
After surveying dogs’ death rates with three different data sets, Nunney uncovered that the smallest dogs, including Pomeranians, miniature pinschers, shih tzus, and chihuahuas, have a roughly 10 percent chance of dying from cancer. In comparison, many larger dogs, like Bernese mountain dogs, have more than an over 40 percent chance of dying from cancer.
It’s important to note this study included some outliers as well; flat-coated retrievers had the highest mortality rates from cancer, experiencing a variety of sarcoma with higher frequency than they should have, considering their size. Also, Scottish terriers appear to develop cancer more often than other small dog breeds.
“Terriers in general get more cancer than expected for their size,” Nunney notes.
In general, the study, published in the journal Royal Society Open Science, supports the idea that size is a major risk factor for cancer in dogs. Still, paradoxically, the very largest breeds, such as great Danes, develop cancer less often than medium-sized breeds. Researchers explain this can be attributed to a well-known but still unexplained phenomenon: the life expectancy of dogs grows shorter with size.
“For every pound increase in typical breed size you lose about two weeks of life. A very big dog, you’re lucky if they live past nine years, whereas small dogs can go about 14,” Nunney comments.
Cancer is predominantly a disease of old age. So, by having a reduced lifespan, the largest dogs enjoy a reduced cancer risk. According to the study, dog breeds are a clear fit with the multistage model of cancer development that shows larger sizes and longer lives offer more cell mutation opportunities.
“I was surprised how well dogs fit the model,” Nunney says. “But that doesn’t happen when you compare a mouse to an elephant or a human to a whale. So, does that undermine the model in some way?”
Nunney believes that any animal’s ability to avoid cancer increases with the size of the species.
“My argument is that preventing cancer is an evolving trait, so a whale will have more ways of preventing cancer than a mouse does,” the researcher states.
While datasets are limited when it comes to occurrences of cancer in whales, there is more information about cancer rates among elephants since they are kept in zoos.
“Elephants don’t get much cancer. Their ancestors, long before mastodons, were much smaller, so how, en route to today’s size, did they avoid cancer?” Nunney wonders. “The secret to preventing cancer could lie within the biology of larger animals.”
