Of course, it doesn’t take much to pierce the myth of the 7:1 ratio. Just look at all the humans out there who are 119 to 140 years old. Oh. Wait. There aren’t any. But 17- to 20-year-old dogs? Plenty of them.

Busted.

So, what can take the place of that miscalculated ratio? Finally, some people are trying to figure it out. So far, they’ve figured out that it’s really complicated.

The problem is that smaller dogs tend to live a lot longer than larger dogs. Say you have a Chihuahua and a Great Dane growing up together. Over the course of their lives, the Chihuahua is aging slower than the Great Dane.

But smaller dogs also mature much faster than larger dogs. In fact, it might take two years for a large dog to reach full skeletal size.

Consequently, in the first two years, that Chihuahua is aging faster than the Great Dane — and then he starts aging slower. That means that after two years, the Chihuahua dog is “older” than the Great Dane, but after five years he becomes “younger.”

Or so says Dr. Kate Creevy, assistant professor of internal medicine at the University of Georgia, who’s been sorting all this out, according to the BBC. She’s found that this aging weirdness is unique to dogs.

“It doesn’t happen in any other animal,” she said. “There isn’t any other species which has … the same degree of size diversity that dogs have. It’s possible that by creating all of these diversely sized dogs that we unmasked this ageing phenomenon.”

So how can we wrap all that up into a handy equation to tell how old our dogs are? It’s tough. According to the story, if you want a broad average across all breeds, six dog years to one human year is a better guideline to use.

But that changes wildly on the far ends of the spectrum. For example, a Bulldog will age an average of 13 years per human year, whereas for a Miniature Dachshund, it’s just 4 years per human year.

As for where the 7:1 ratio came from, experts point to textbooks in the 1960s, which featured the ratio in math problems. No one, however, has come forward to take credit for it.

Via the BBC