Determining a dog's age and establishing equivalences with human age is something we've tried to do for centuries. A classic conclusion for this exercise is that every canine year equals seven human years, but that conclusion is wrong. On more than one occasion, scientists have explained that a dog's age varies by breed and size, so... is it possible to make a precise calculation?
The Mathematical Problem
First, the problem is mathematical: if each dog year were equivalent to seven human years, our reproductive cycle would begin at age 7 and we'd live to 150, which doesn't happen. No one knows for sure the origin of the comparison between dog and human years, but one of the oldest records dates back to the 13th century, at Westminster Abbey. An ancient inscription there suggests a ratio of 9 to 1, with an average of 9 years for dogs and 81 for humans. Five centuries later, French botanist and naturalist Georges Buffon reached a similar conclusion, stating that dogs can live up to 10 or 12 years, and humans up to 90 or 100.
The Origin of the '7 to 1' Rule
Where did the '7 to 1' come from? A Wall Street Journal article published in August 2008 quotes veterinarian William Fortney of Kansas State University, who said it probably was a publicity stunt, and a way to educate people about how quickly dogs age, in an attempt to ensure more frequent visits. However, once subjected to scientific rigor, the '7 to 1' falls apart. 65 years ago, a French researcher named A. Lebeau determined that a dog can age between 15 and 20 times faster than a human in its first year of life, slowly decreasing until reaching a stability of five times.
Breed and Size Matter
Even so, there's an extra layer of complexity. David J. Waters, director of the gerontology center at Purdue University, was blunt in stating that "eight years in one breed do not equal eight years in another". The second table gives us a much more solid idea thanks to its separation by size. It has long been known that larger dogs live less, a fact that continues to challenge experts, but which has been seen reproduced in other species (mice, for example). With this, you're invited to do your own calculations. Whether it's a little hyperactive one that barks and runs, or a noble giant that knocks everything over, our final message is the same: Hug your dog.
