When it comes to calculating a dog's age in human years, you might think it's as easy as multiplying its age by seven. But scientists have now said we may be barking up the wrong tree.
Experts have created a new formula for working out the relative age of a dog compared to a human based on the rate molecules are added to DNA.
The formula, which is more complicated than the old rule of thumb – which the researchers say is just a "myth" – involves finding what is known as the "natural logarithm" of a dog's age, multiplying it by 16 and then adding 31 to the total.
This means an 18-year-old dog would be 77, and a two-year-old dog would be 42 in human years, according to the team from the University of California.
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The formula looks at changes to DNA over time, specifically to the methyl groups, which add themselves to DNA at a consistent rate. They can be used to measure the age in humans and dogs in a process known as the epigenetic clock.
Looking at this process, the research, published on the BioRxiv website, showed similarities between dogs and humans in the timing of major milestones in development.
But the team, led by geneticist Tina Wang, say it isn't easy to come up with a comparison as dogs reach maturity at different rates and different dog breeds have different life spans.
It is not known how the myth that a dog year is equivalent to seven human years originated, but some suggest it was popularised by adverts in the mid-20th century.
The experts also disputed that one cat year is equivalent to seven human years.