How old is your faithful four-legged companion really? For a long time now, dog owners have relied on the old ‘multiply by 7’ method to estimate the age of their pet in human years. But a new approach suggested by American researchers shows that the calculation is in fact far from simple.In a study published on July 9th in the Cells Systems journal, researchers from the American Institutes of Health (NIH), among others, explain that the ratio used to transpose a dog's age into human years actually changes over time. Thus, if multiplying by 7 is reliable enough to calculate the age of your dog towards the end of its life, it is not the same for the other periods of its life.Unequal genetic ageingTo reach this conclusion, researchers studied the evolution of canine and human genomes as individuals age, according to developmental stages that are the same for both species: birth, childhood, youth, puberty, adulthood, and death. However, this genetic ageing through DNA alteration, called methylation, does not occur at the same rate for dogs and humans, as illustrated in the graph below.After precisely studying this process in 104 labrador retrievers of all ages, Trey Ideker from the University of San Diego and his colleagues were able to generate a new, complex formula to accurately calculate the human equivalent of the age of the animals, realising that we often underestimate the genetic age of our furry friends, especially in their early years.A one-year-old dog is already in his thirties…Thus, an eight-week-old puppy is about the same age as a 46-week-old human baby, or nine months old. A one-year-old dog would be comparable to a 30-year-old human, and a four-year-old dog would be as old as a… 52-year-old human. But fortunately, this ageing process begins to slow down considerably after the age of seven. ‘It makes sense when you think about it. After all, a nine-month-old dog can have puppies, so we already knew that the 1:7 ratio was not an accurate measure of age,’ says Trey Ideker.And while the usefulness of this new ‘epigenetic clock’ may not be immediately obvious, one should still be aware that knowing your pet's age, and its potential fitness, will help you to better monitor and care for it on a daily basis. ‘It's also important that we better understand their ageing process because veterinarians frequently use the old 1:7 ratio to determine a dog's age and use this information to guide diagnosis and treatment decisions,’ says Trey Ideker.‘I have a six-year-old dog, she still runs with me, but I realise now that she's not as “young” as I thought she was,’ he illustrates. However, Ideker acknowledges a limitation to his method: only one breed of dog has been studied so far, and canine life expectancy can vary between breeds. To complete their epigenetic clock, researchers are already planning to test their method on other breeds.