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Medium-sized dogs have a greater risk of developing cancer

Medium-sized dog breeds are more prone to cancer than both the smallest and the largest breeds, according to a recent study led by the University of California, Riverside.

These findings challenge the multistage model of cancer, which associates increased size within a single species to a heightened risk of cancer due to more frequent cellular mutations as cells divide and replicate.

Size and cancer risk

“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,” explained Leonard Nunney, a UC Riverside evolutionary biologist and the study’s author. 

The paradox lies within species where size appears to correlate with cancer risk, as seen in humans, where taller individuals have about a 10% increase in cancer risk for every 10 centimeters in height.

Dogs and cancer risk

To delve deeper into these relationships, Nunney looked at dogs – a species with significant size variation – to understand how size impacts cancer risk. 

“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,” he explained. 

His findings showed that smaller dogs like Pomeranians, miniature pinschers, and chihuahuas have approximately a 10% chance of dying from cancer, while some larger breeds like Bernese mountain dogs exhibit more than a 40% chance.

However, the research also identified outliers, such as flat-coated retrievers and Scottish terriers, which exhibit higher cancer rates than typical for their size. Despite these anomalies, the study generally supports the multistage model’s assertion that larger size within a species may increase cancer risk.

Lifespan and cancer risk 

Surprisingly, the study also found that the very largest breeds, like Great Danes, actually show lower cancer rates compared to medium-sized breeds. This is attributed to their shorter life spans. 

“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 said. Since cancer is predominantly an affliction of older age, these reduced lifespans in larger dogs correlate with decreased cancer risk.

These insights suggest that size and life expectancy within species are significant factors in cancer risk, aligning closely with the multistage model of cancer. “I was surprised how well dogs fit the model,” Nunney said. “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?”

Cancer prevention mechanisms 

Nunney proposes that larger species have evolved more sophisticated cancer prevention mechanisms. “My argument is that preventing cancer is an evolving trait, so a whale will have more ways of preventing cancer than a mouse does,” he explained.

This evolutionary perspective is supported by observations of elephants, which rarely suffer from cancer despite their size and longevity.

The broader implications of this research extend beyond veterinary science to potential insights into cancer prevention in humans. Understanding the natural mechanisms that larger species use to protect themselves from cancer could lead to breakthroughs in human cancer treatments and prevention strategies.

Cancer in dogs 

Cancer in dogs is a significant health concern and one of the leading causes of death in older dogs, though it can also affect younger ones. The disease occurs when cells grow uncontrollably, forming masses or invading tissues throughout the body. 

Cancer types

Different types of cancer in dogs include lymphoma, melanoma, bone cancer, and mammary gland cancer, among others. Symptoms vary widely depending on the type of cancer, but common signs include abnormal swelling, sores that don’t heal, weight loss, and changes in appetite.

Diagnosis of cancer in dogs

Diagnosis typically involves a combination of physical exams, blood tests, imaging like X-rays and ultrasound, and biopsies.

Treatment options are similarly varied, depending on the stage and type of cancer, and can include surgery, chemotherapy, radiation, and immunotherapy. These treatments aim to remove or kill cancer cells while minimizing damage to normal cells.


Early detection and treatment can significantly improve outcomes, which is why regular veterinary check-ups are important. Advances in veterinary medicine have improved the management of cancer in dogs, leading to better quality of life and longer survival rates for many dogs diagnosed with the disease.

The approach to treating canine cancer often involves a team of specialists, including veterinarians who specialize in oncology.

The study is published in the journal Royal Society Open Science.


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