Almost every dog owner has multiplied their pet's age by seven to work out its equivalent in "human years," but the American Kennel Club reports that the formula has no scientific basis.
The tradition of counting dog years this way dates back to the 1950s. Kelly Cassidy, curator of the Charles Conner Museum at Washington State University, said the rule has become so ingrained that it is hard to eradicate.
Veterinarian William Fortney told the Wall Street Journal that the multiplier was likely invented as a simple way to show owners how much faster dogs age compared with humans, particularly in terms of health, while also encouraging them to take their pets to the vet at least once a year.
How to calculate a dog's real age
The American Veterinary Medical Association offers a different formula for medium-sized dogs. A dog's first year of life is roughly equal to 15 human years, its second year adds about 9 more, and every year after that counts for around 5 human years.
Why big dogs age faster than small ones
Scientists still cannot fully explain why large dog breeds tend to age faster than small ones, since in nature bigger animals usually live longer than smaller ones, the opposite of what happens with dogs. Biologist Cornelia Kraus of the University of Göttingen told Inside Science that the life of large breeds seems to unfold in fast-forward.
According to Kraus, every 2 kilograms of a dog's weight cuts its expected lifespan by roughly a month. Possible explanations include a greater susceptibility among large breeds to age-related diseases and cancer, linked to their faster growth.
A DNA-based alternative
In 2019, researchers at the University of California, San Diego proposed a different way to measure a dog's age, using DNA. As people and dogs get older, specific chemical "marks" build up in their DNA, and the number of these marks can reveal how much the body has aged.
The team analyzed the DNA of 104 Labrador retrievers ranging from puppies to 16-year-old dogs and used the data to devise a formula: take the natural logarithm of the dog's age, multiply it by 16, and add 31 to get its approximate human age.
The study only examined one breed, however, so the result from this formula may differ significantly for other types of dogs.
