One dog year is not equivalent to seven human years, despite widespread use of the ratio for calculating the age of canine companions. Presumably, the ratio is based on the average lifespan of dogs being 10 years and humans being 70 years, it’s not quite so simple. The formula is not based on any real science and it was debunked by veterinarians years ago.
But geneticists digging into the mysteries of aging have developed a new calculation to understand how our canine companions' ages correspond to our own.
To understand how dogs age, the team looked at a phenomenon called DNA methylation. As mammals get older, their DNA picks up methyl groups that "stick" to their DNA. While these groups don’t change the DNA itself, they attach to the genetic molecule and can turn certain genes on or off, which is an important part of epigenetics, or the way environmental factors cause certain genes to express themselves.
Methlyation occurs at a relatively steady rate as humans age, which allows researchers to estimate a person's age, a process they’ve dubbed the “epigenetic clock.”
In the new paper on dog years, which has yet to be peer reviewed and is currently posted on the preprint server bioRxiv, a team led by Tina Wang of the University of California, San Diego, compared the epigenetic clocks in people to canines to better understand the genes associated with aging. They picked dogs because most live in the same environments as humans and also receive some degree of medical care, like humans do.
The team looked at methylation rates in 104 Labrador retrievers between the ages of four weeks and 16 years old, reports Michelle Starr at Science Alert. They then compared them to published methylation profiles of 320 humans from age one to 103. (They also compared both to 133 mice methylation profiles.)
It turns out some parts of a dog’s life follows the same pattern as humans, though other longevity milestones don’t link up quite as nicely. For instance, the methylation rate showed a seven-week-old pup corresponds to a 9-month-old human baby, and both species begin to get their first teeth at this time.
But the comparison breaks down after early puppyhood. The dog clock ticks much faster with pups speeding through puberty and reaching sexual maturity within their first year. Then, the dog's epigenetic clock slows down as the dog ages, and begins to match up with humans again in its later years.
Overall, the average 12-year lifespan of a Labrador lined up with the average worldwide lifespan of humans, which is about 70 years.
While the study complicates the concept of “dog years,” it does show that the animals experience similar methylation processes as humans.
“We already knew that dogs get the same diseases and functional declines of aging that humans do, and this work provides evidence that similar molecular changes are also occurring during aging,” Matt Kaeberlein, a biogerontologist at the University of Washington who was not involved in the study, tells Virginia Morell at Science. “It’s a beautiful demonstration of the conserved features of the epigenetic age clocks shared by dogs and humans.”
The new formula for a dog’s ages based on the study requires a little more math than multiplying by seven. You multiply the natural logarithm of a dog's age by 16, then add 31 [human_age = 16ln(dog_age) + 31].
According to the formula, a 2-year-old dog is the equivalent of a 42-year-old human, but things slow down after that. A 5-year-old dog is the equivalent of a 56.75 year old human, and a 10-year-old dog is the equivalent of 67.8-year-old person.
Evolutionary biologist Steve Austad of the University of Alabama in Birmingham, who was not involved in the study, tells Morell that he’s not too surprised that the epigenetic clock applies to dogs, too. He says that by studying different dog breeds with different lifespans the researchers may find some interesting results.
This formula is not the last word on dog years, however, especially since it only looked at one breed. Erika Mansourian, writing for the American Kennel Club, reports that the American Veterinary Medical Association says the accurate way to calculate dog years for a medium-sized dog is to assume the first year is equivalent to 15 years and age two adds another nine years. After that, each year of a dog’s life is equivalent to five human years. It doesn't perfectly line up with the new formula, but both acknowledge that dogs age rapidly in their first years of life.
Whatever the case, dogs’ lives are all too short. That may be why people are excited about a project by the Dog Aging Project, which is currently recruiting 10,000 pets and their owners to participate in a new study that will look at the dogs' health, gut microbes, diet and exercise to understand aging. And 500 lucky dogs will test out a new drug that may help slow the aging process, which could help us someday, too.