Scientists have figured out the reason that smaller dogs live so much longer than larger dogs, and unfortunately, it's kind of our fault.

Dogs appear to break a rule seen across the animal kingdom that a larger body results in a longer lifespan, with elephants living for up to 70 years and mice living a mere fraction of that. However, dogs are the opposite, with smaller dogs living much longer than larger ones.

According to a study published in the journal American Naturalist, larger dogs are more likely to die earlier because our selective breeding to make them larger has made them more susceptible to cancer.

The researchers suggest that these higher rates of cancer in larger dogs may be a result of there being an "evolutionary lag" in their natural defenses against cancer after we rapidly bred them to be larger.

Evolution of the Dog

Domestic dogs as we know them began as wolves sometime between 27,000 and 40,000 years ago, and have since been bred by humans to fulfill certain roles, from hunting and sheep herding to rat catching and simply looking cute.

These traits are gained by selectively breeding dogs with those traits with others with the same characteristics, then subsequently breeding the most favorable offspring of that litter with others with the same trait.

Over time, the dogs diverge into distinct breeds, ranging in size and stature from Chihuahuas and dachshunds to Great Danes and Newfoundlands.

Smaller dogs have an average lifespan of many years longer than larger dogs, with Chihuahuas living between 15 and 17 years, and Great Danes only lasting between 8 and 10 years, according to the American Kennel Club.

Dogs & Cancer

In the study, the authors describe how they examined the causes of death and life span across 164 dog breeds of a range of sizes, using published data about dog breeds from across the globe, and found that larger dogs were more likely to die of cancer at a younger age than smaller dogs.

"Larger dogs didn't necessarily age faster than the smaller breeds, but the research did show that as the breed's average body weight increased, so did the rates of cancer," Jack da Silva, a co-author of the paper and an evolutionary geneticist at the University of Adelaide in Australia, said in a statement.

"We believe the relationship between a dog's body size and their lifespan may be caused by an evolutionary lag in the body's cancer defenses, which are unable to keep up with the rapid and recent selective breeding of bigger dogs," he said. "Most of the 400 or so dog breeds we know today have only been established in the past 200 years. Larger dogs haven't had time to evolve better cancer defense mechanisms to match their size."

According to the paper, this is consistent with a theory of how animals optimize the use of energy across their lives known as the "disposable soma" theory, first proposed by British biologist Thomas Kirkwood in a 1977 review article published in the journal Nature. This theory states that animals can invest a lot of their energy into breeding earlier in their lives in order to produce a large number of offspring, at the cost of investing that energy into cancer defense and repairing damages to their cells and DNA. Those that have fewer offspring and have them later in life, on the other side of the trade-off, are more effective at cancer defense and live for longer, such as the naked mole rat.

"In all organisms, the focus is on reproducing early, even if it comes at the expense of maintaining and repairing the body and living longer," said da Silva.

It's not all bad for the big dogs of the world and their owners, however, as da Silva predicts that larger dogs will eventually evolve to regain their cancer defenses. However, as per the trade-off, this may also come hand in hand with these dog breeds having smaller litters, due to more energy being invested in keeping the dog alive for longer and staying cancer-free.

"This may occur naturally or through selective breeding, as people focus on breeding large dogs that have lower cancer rates and thus greater longevity," da Silva said.

The authors hope that this research will eventually help to study the process of aging in humans, and the onset of age-related cancers.

"Dogs represent a good model for studying ageing in humans. Dogs, like humans in the industrialised world, live in an environment that tends to protect them from accidental and infectious causes of death and are thus more likely to die from age-related diseases, such as cancer," da Silva said.

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about dog lifespan? Let us know via [email protected].