Have you ever wondered why you seem to know a lot more elderly Chihuahuas than aged Rottweilers? It's just a fact of life that small dogs, if they escape disease or misadventure, live longer lives than larger ones. This trend has puzzled veterinary researchers for as long as we've been fussing over our faithful four-legged friends, and it's a mystery that hasn't been definitively solved.
Conventional wisdom in the animal world dictates that between species, larger animals live longer than smaller ones — an elephant can live into its 70s while a mouse might have only a year or two to do its earthly business. However, the opposite seems to be true within single-species subsets, and our dogs, although they come in lots of different shapes and sizes, are all the same species. The average life span for an enormous dog is around seven or eight years, while a tiny dog can reliably live, variables aside, to the ripe old age of 14 or 15.
The popular adage that one human year is equal to seven canine years isn't supported by modern veterinary research, although it's easy math, and therefore sticks around. The truth is more complex — the rule of thumb seems to be that the first year of a dog's life gets it to the physical maturity level of a 15-year-old human, the second year adds another nine years and, after that, the rate of aging varies widely based largely on breed and size. A small dog is considered "senior" around age eight, while a large dog hits old age around five or six.
"We're unsure of the reasons small dogs live longer than larger dogs on average, but this difference in life spans is likely due to a combination of factors rather than just a single factor," says veterinarian and epidemiologist Dr. Sandra Lefebvre, co-author of a 2019 paper about risk factors associated with the life span of pet dogs.
Life Span Can Depend on Breed
Part of the problem is that a dog's size isn't the only, or even the primary, factor that determines life span — genetics and breeding also play a role in longevity, with certain breeds being more vulnerable to certain life-threatening or life-shortening diseases and conditions than others. The aging process for dogs is very different than it is for humans, but it's also different between dogs.
One reason for shortened life span seems to be breeding. According to a study published April 28, 2022, in the journal Nature, based on a massive database of veterinary records from the United Kingdom, longevity seemed to have more to do with breed than it did with size — at least among purebred dogs. Small Jack Russell terriers and midsized border collies differ substantially in size, but according to the study they live pretty similar life spans — 12.7 and 12.1 years, respectively. However, flat-faced breeds like pugs and French bulldogs were found to have much lower life expectancies due to breathing problems, disease and difficulty giving birth. The French bulldog's life expectancy is only 4.5 years, and none of the other flat-faced breeds were found to have an average life span of more than 7.8 years.
The Speed of Aging Is Key to Canine Life Span
One reason small dogs live longer on average seems to do with how long it takes a dog to become full-grown, and the difference in size between its puppy and adult bodies. It seems that growing very large, very quickly takes a toll on a dog's overall life span.
"Large dogs are known to grow and age at rates faster than small dogs," says Lefebvre. "For example, although it takes a Great Dane about 18 to 24 months to grow to full size, this means growing up to 32 inches (81 centimeters) in height and 175 pounds (79 kilograms) in weight over that period. That's remarkably fast, and physiologically demanding!"
Large breeds die of cancer more often than small breeds — a trend researchers believe has to do with how quickly they grow in their first year of life. With all that rapid expansion, it's possible abnormal cell growth is more likely, or because their lives play out more quickly, that they succumb to the diseases of old age earlier than, say, a Chihuahua. It takes a Chihuahua about 10 to 12 months to grow to full size, but that size usually maxes out at only about 8 inches (15 centimeters) and 6 pounds (2.7 kilograms). It's just not as taxing on the body to become a full-grown Chihuahua as it is to become a complete Great Dane.
And another element that's related to size? Obesity. Obese dogs are known to have shorter life spans than other dogs, not unlike human beings. "This is likely due to the health problems like diabetes or cardiovascular disease from the lack of activity that accompanies obesity," says Lefebvre.
Finally, lifestyle and access to veterinary care can affect longevity, and these factors can depend on the role of the dog within the family. For example, toy dogs may be pampered more than larger breeds, given their lap-friendly size.
Now That's Interesting
Dog research is important to humans because dogs die of the same illnesses humans do, including cancer, heart disease and diabetes.
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