The old way to think about your dog's "human age" — the age in actual years times seven — is wrong. And researchers now have a new formula they think will calculate your dog's age more accurately.
Simply put, compared with humans, dogs age very quickly at first, but then their aging slows down, a lot.
Trey Ideker of the University of California, San Diego was part of a team of researchers that looked at aging on the molecular level.
Humans and dogs have DNA that basically doesn't change over the course of life, Ideker says. However, "we have these additional chemical marks, called epigenetic marks. The particular epigenetic mark that has turned out to be pretty important for the study of age is called methylation," he says.
"It's basically a wrinkle on your genome. So you have DNA that does not change, but then you have these additional marks that do change as you age. And they change in a very predictable way, so that you can use the pattern of marks to read out your age."
The comparison of humans and dogs was made easier because they both often live in similar environments and get similar access to health care.
For the study, they drew the blood of 104 Labrador retrievers ages 4 weeks to 16 years.
"All I have to do is take dogs in a certain age group, like in a 1-year-old age group, and look at what are the most similar molecular profiles in the humans," Ideker says. "And what ages those humans are. And it turns out that if you do that for like a 1-year-old dog, you find that the matching humans at the molecular profile level are surprisingly old. They're about 30 years old."
Using a dog age calculator based on the research, a 2-year-old dog is about 42 in human years. But a dog twice as old, 4 years, is roughly equivalent to a 53-year-old human. Double it again and an 8-year-old dog is only 64 in human age.
The researchers broke down their comparison by developmental period:
"Juvenile refers to the period after infancy and before puberty, 2-6 mos. in dogs, 1-12 yrs. in humans;
"Adolescent refers to the period from puberty to completion of growth, 6 mos. to 2 yrs. in dogs, approximately 12-25 yrs. in humans;
"Mature refers to the period from 2-7 yrs. in dogs and 25-50 yrs. in humans;
"Senior refers to the subsequent period until life expectancy, 12 yrs. in dogs, 70 yrs. in humans."
For a more personal example, Sadie, NPR's Peter Breslow's family dog, is 7 in human years. So the Breslow family might have once thought of her as being 49, but now it looks like she's about 62.
But there's another way to think about it when we humans get to be about that age.
"If I'm a 60-year-old," Ideker says, "wow, I'm a 7-year-old dog!"
SCOTT SIMON, HOST:
Our dogs grow old more quickly than we want. The old rule of paw, if you please, is that for every human year, a dog ages seven. Scientists have come up with a new way to calculate that. And their formula may be more accurate, but it doesn't do much to help our hopes.
Our Peter Breslow and a member of his family explain.
PETER BRESLOW, BYLINE: This is Sadie.
(SOUNDBITE OF DOG GROWLING)
BRESLOW: She's a black Australian labradoodle and pretty much the best dog in the world. She loves playing tug of war, is obsessed with bunnies and sometimes accompanies me on the harmonica.
(SOUNDBITE OF DOG HOWLING)
BRESLOW: And this is...
TREY IDEKER: Trey Ideker. I am a professor of genetics at UC San Diego School of Medicine.
BRESLOW: Professor Ideker was part of a team led by Dr. Tina Wang that helped come up with the new formula. He says that over the past decade or so, the old adage that a dog's age is human years times seven has been improved upon by molecular studies. The explanation is complicated.
IDEKER: Well, I - hopefully it's not dreadfully complicated.
BRESLOW: Let's start out with DNA, says Ideker, which basically doesn't change over the life of humans or dogs, who live in much the same environment as we do and get pretty good health care, too.
IDEKER: However, on top of that basic genetic code, that DNA, we have these additional chemical marks called epigenetic marks. The particular epigenetic mark that has turned out to be pretty important for the study of age is called methylation. And we can there get as complicated as you want, but I probably will stop.
BRESLOW: So he dumbed it down a bit.
IDEKER: It's basically a wrinkle on your genome. So you have DNA that does not change. But then you have these additional marks that do change as you age. And they change in a very predictable way so that you can use the pattern of marks to read out your age.
BRESLOW: Scientists can use these molecular marks to read a human's age very accurately. So why not apply them to man's best friend? For the study, they drew the blood of 104 Labrador retrievers ages four weeks to 16 years.
IDEKER: Now all I have to do is take dogs in a certain age group, like in the 1-year-old age group, and look at what are the most similar molecular profiles in the humans and what ages those humans are. And it turns out that if you do that for, like, a 1-year-old dog, you find that the matching humans at the molecular profile level are surprisingly old. They're about 30 years old.
BRESLOW: That's right. The bouncing, fluffy youngster happily gnawing the leg of your coffee table is really old enough to have a mortgage on a condo. But don't worry, the aging curve slows way down as your dog grows up. My dog Sadie is 7. So I have in front of me, at the smithsonian.com website, this handy-dandy calculator. So Sadie is 7. So we're thinking she's 49. But I punch in seven on the calculator, and it says Sadie is 62.1 years old, which - that's kind of a bummer.
IDEKER: So she's older than you thought. But what you'll see on that curve is she's currently aging at a much slower rate than 1 to 7. In the first couple of years of life, her molecular makeup progressed really quickly. And now she's slowing down. So maybe that's the glass-half-full version of this.
BRESLOW: As a matter of fact, when I punch in 10 - Sadie's age in three years - which would be 70 in traditional dog years, the curve says she's only 67.8 years old. And maybe, according to geneticists Trey Ideker, there's another way to think about all this if, say, you're a baby boomer.
IDEKER: Maybe the question isn't, why is my 7-year-old dog 60? But, if I'm a 60-year-old, wow, I'm a 7-year-old dog (laughter).
BRESLOW: Which is a pretty good thing. Right, Sadie?
(SOUNDBITE OF DOG GROWLING)
BRESLOW: Peter Breslow, NPR News.
(SOUNDBITE OF DOG HOWLING) Transcript provided by NPR, Copyright NPR.