Dogs live an average of 12 years. Life expectancy in humans, by contrast, is at least five times that.
That discrepancy is in part the basis for the common rule of thumb that one “dog year” is the equivalent of seven “human years.”
But according to a new study published in the journal Cell, that one-to-seven ratio is wrong. That’s because new genetic evidence shows puppies and younger dogs age faster than their older counterparts do.
A 6-year-old dog would be the human equivalent of about 60 years old, the researchers found.
Although an animal’s DNA does not change during its lifetime, these chemical markers – called methyl groups – do. As years pass, methyl groups accumulate in the DNA, causing certain genes to turn on and off. After tracking how these groups accumulate in different parts of labrador genomes over time, the researchers compared that accumulation rate to the same methyl group changes in humans.
Ideker’s team found that in dogs’ first year of life, the animals accumulate more methyl groups than humans do. So an 8-week-old puppy is equivalent to a 9-month-old baby.
Then as years pass, that accumulation rate slows down in dogs to better match humans’ accumulation rates – suggesting older dogs age more slowly once they get older.
Ultimately, one dog year isn’t equal to seven human years. In fact, in order to calculate your dog’s human age equivalent, you’ll need a calculator.
The researchers formula is: A dog’s human age = 16 ln * your dog’s age + 31. (The ln in this formula refers to the natural log of a number.)
Essentially, for each year older a dog gets, the corresponding increase in “human years” gets smaller and smaller.
So based on that formula, a 6-year-old lab is 60 human years old. But a 12-year-old lab is 70 human years old.
“If we think about aging in terms of how old our cells are, this new paper is really useful in matching up human and dog years,” Lucy Asher, an animal behaviour expert who was not affiliated with the study, told The Guardian.
Going forward, Ideker wants to replicate the study in other dog breeds that have different average lifespans.
The new research found that methyl groups accumulated on some of the same genes in dogs and people as the two species aged. This makes sense, according to the study authors, given that dogs share the same living environment as their owners and receive almost the same standard of healthcare.
But even so, age matching doesn’t apply to the two species’ hormones and behaviours.
So there’s a reason your 1-year-old canine won’t act his human age, and instead prefers to chase sticks and squirrels with child-like abandon.
“The development of dogs is not just a shortened version of the human development, which is why it’s difficult to find a clear match-up between a dog’s age and a human’s age,” Asher added.