Q: They used to say dog years were like seven of ours. But dogs live anywhere from five to 15 years, depending on their breed. So how old really is my dog in human years?
A: Conventional wisdom once determined the correct ratio between dog years and human years to be one dog year for every seven human years. Somewhere around the 1950s, veterinarians latched on to the notion that a 1-year old-dog is to a 7-year-old child what a 10-year-old dog is to a 70-year-old adult human.
As you’ve astutely pointed out, however, dog years aren’t the same for all dogs. Just as Chihuahuas and Great Danes differ vastly in their sizes, their lifespans vary widely, too (giant breed dogs live significantly less long than their tiny counterparts). But there are other problems with this truism apart from just the breed and size thing.
After all, humans aren’t reproductively active by the age of 3 1/2 (dogs typically reach puberty around 6 months of age), nor do humans ever live past 140 (meanwhile, the rare dog does occasionally hit the 20-year mark).
This “dog years” convention was almost certainly conceived as a rule of thumb designed to conveniently help us simple-minded humans roughly estimate our dogs’ age relative to ours. By no means was it meant to be taken literally.
Still, this mythology stuck. This, despite a more accurate system developed by French researcher A. Lebeau that related specific life markers (puberty, adulthood and lifespan) to the same in humans. Via a method that discounted puberty, Lebeau established that, as adults, dogs age about five times faster than humans.
As a veterinarian, five to one definitely makes more sense. But that still doesn’t explain why small breeds of dogs live so much longer than bigger breeds. It’s not as if small mammals live longer in general. (That theory wouldn’t explain an elephant’s extreme longevity, for example.)
Nor does this five-to-one ratio explain why some similarly sized dog breeds differ dramatically in their longevity.
So how old is your dog in human years? My best guess is to look up the average lifespan for your dog’s breed type (shepherd, Lab, toy poodle, etc.) and then divide the average lifespan of a human in the United States by that number. It won't be a perfect estimate of reality, but it’ll get you way closer.
Dr. Patty Khuly has a veterinary practice at Sunset Animal Clinic in South Miami. Her website is drpattykhuly.com. Send questions to firstname.lastname@example.org.