According to the well-known ‘rule of paw’, one dog year is the equivalent of seven years. Now, in a study published recently in Cell Systems, US scientists say it’s wrong. Dogs are much older than we think, and the researchers devised a more accurate formula to calculate a dog’s age based on the chemical changes in the DNA as organisms grow old.
Dogs share the same environment as their owners and receive almost the same standard of health care as humans, providing a unique opportunity for scientists to understand ageing across species. Like humans, dogs follow similar developmental trajectories that lead them to grey and become more susceptible to age-related diseases over time. However, how they age on a molecular level is more complicated—ageing rapidly at first and slowing down later in life.
“In terms of how physiologically mature a one-year-old dog is, a nine-month-old dog can have puppies. Right away, you know that if you do the math, you don’t just times seven,” said senior author Trey Ideker of the University of California, San Diego.
“What’s surprising is exactly how old that one-year-old dog is—it’s like a 30-year old human.”
Human and dog DNA doesn’t change much throughout the course of life, but chemical marks on the DNA, called methylation marks, do.
“I tend to think of it very much like when you look at someone’s face and guess their age based on their wrinkles, gray hair, and other features,” Ideker said. “These are just similar kinds of features on the molecular level.”
The researchers studied 104 Labrador retrievers spanning from few-week-old puppies to 16-year-old dogs and compared the changes in the methylation pattern to humans.
The comparison revealed a new formula that better matches the canine-human life stages. Based on this, an eight-week-old dog is approximately the age of a nine-month-old baby, both being in the infant stage where puppies and babies develop teeth.
The average 12-year lifespan of labrador retrievers also corresponds to the worldwide life expectancy—70 years—of humans.