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Small dogs have an average life span of about 10 to 14 years, while larger dogs may live only five to eight. (iStock)

Why is it that tiny terriers and Chihuahuas tend to outlive big, robust Great Danes and mastiffs? An analysis of thousands of veterinary hospital records may have uncovered the answer.

"What we found was that large dogs are actually aging at a faster rate, so large dogs appear to actually be … falling apart faster than small dogs," said Daniel Promislow, a Canadian professor of genetics at the University of Georgia, in an interview with Quirks & Quarks that airs Saturday.

Promislow was discussing the results of a study he co-authored, published in this month's issue of the journal The American Naturalist, which relied on records of 80,000 dogs that died at 30 veterinary hospitals in the U.S. and Canada in recent decades.

Previous studies have shown that while small dogs typically live to the age of 10 to 14, and can reach the ripe old age of 20, larger breeds had an average lifespan as short as five to eight years.

The new study, which was led by Cornelia Kraus at the Laboratory of Survival and Longevity at the Max Planck Institute for Demographic Research in Rostock, Germany, found that a major reason for the differences was that cancer is much more common in large dogs.

Promislow suggested that may be because humans may have inadvertently selected for characteristics that predispose large dogs to cancer, such as rapid growth, while breeding them for larger size.

The fact that larger dogs don't live as long as smaller dogs is surprising in a way, because species of larger animals, such as whales and elephants, tend to live longer than smaller species such as mice.

On the other hand, Promislow points out, whales and elephants have many more cells than smaller animals, which would theoretically create more opportunities for cancer to arise.

"Presumably," he added, "they must have evolved some special defence mechanisms against cancer."

Those mechanisms likely evolved via natural selection over the very long period of time that these larger species evolved.

Most dog breeds arose through selection by humans over a much shorter period of time.

"It's really an evolutionary blink of an eye," Promislow said, "and the protective mechanisms haven't had a chance to catch up."