AILSA CHANG, HOST:
You know that old formula to calculate your dog's age in human years, where you multiply by seven?
MARY LOUISE KELLY, HOST:
Well, a new study suggests it's a little more complicated than that. Tina Wang, the lead author, says you might want to dust off your scientific calculator.
TINA WANG: 16 times the natural log of dog years equals plus 31.
KELLY: And if your math is as rusty as mine...
TREY IDEKER: A 1-year-old dog amounts to approximately a 30-year-old human. A 4-year-old dog amounts to a early 50-year-old. And at that point, you're slowing in terms of the dog aging rate versus humans.
CHANG: That's Trey Ideker, the study's senior author and a professor at UC San Diego. His lab had discovered a way to track human age by looking at the chemical changes that accumulate on our DNA over time. Then Wang, who was his grad student at the time, approached him with a question.
IDEKER: She came into my office one day, and she says, you know, Trey, gosh, I appreciate this aging diagnostic you guys have, but aren't dogs so much more interesting than people?
KELLY: Absolutely. Well, Wang says her interest arose after she adopted a rescue dog.
WANG: People at the dog park would always ask me how old she was, and I was never satisfied that I couldn't tell them how old she was.
KELLY: So perhaps science could help.
CHANG: So they gathered blood samples from 100 Labrador retrievers, from pups to old dogs, and studied the chemical decorations on their DNA. In doing so, the scientists were able to build a clock to compare biological age across species.
IDEKER: So now we have a way of measuring age that works as well for dogs as for humans as for mice.
CHANG: The work appears in the journal Cell Systems.
KELLY: And as for that old formula, Ideker says it was just an average. Humans live about seven times longer than dogs. Simple observation, though, would tell you there was something off about that calculation.
IDEKER: A 9-month-old Labrador retriever can bear puppies.
CHANG: Now that Wang has scientifically deduced her dog's age in human years, which is around 60, she offers these words of advice.
WANG: Just, you know, enjoy your time with your dog.
KELLY: Yeah. After all, it's only cats who live nine lives.
(SOUNDBITE OF SNARKY PUPPY'S "BLING BLING") Transcript provided by NPR, Copyright NPR.