For generations, dog lovers have used a simple rule to calculate their pets’ ages — one dog year equals seven human years. But scientists say the popular formula does not accurately reflect how dogs actually age.
A 2020 study published in Cell Systems found that dogs age rapidly during the early part of their lives before the process slows considerably.
Dogs age faster when they are young
Researchers studied blood samples from 104 Labrador retrievers ranging from puppies to 16-year-old dogs. They examined DNA methylation patterns and compared them with data from 320 humans aged between one and 103.
DNA methylation changes in predictable ways as organisms age. These patterns can be used to create an “epigenetic clock”, providing an estimate of biological age.
The researchers found that an eight-week-old puppy had an epigenetic age roughly comparable to a nine-month-old human. However, the rate of ageing changed substantially as the dogs grew older.
New formula changes the calculation
The researchers developed a formula to estimate a dog’s equivalent human age:
Human age = 16 × ln(dog age) + 31
Using the formula, a one-year-old dog is equivalent to about 31 human years, while a two-year-old dog is around 42. A four-year-old dog would be approximately 53 in human terms.
By 12, a dog would be roughly equivalent to a 71-year-old human.
Breed can make a difference
The study has an important limitation: all the dogs examined were Labradors. Researchers cautioned that different breeds could have different ageing patterns.
Dogs also vary considerably in lifespan depending on factors such as size and breed, meaning a Chihuahua and a Newfoundland may not age in exactly the same way.
Research could help humans too
The study was not simply about finding a better birthday calculation. Researchers identified hundreds of genes whose age-related methylation changes were similar in dogs, humans and mice.
The findings suggest that some biological processes involved in development may continue to change throughout life.
For dog lovers, however, the research offers a useful party fact: the next time someone says their dog is 47 in human years, the seven-year rule may not tell the whole story.