No, One Human Year Is Not Equivalent To Seven Dog Years

Instead of merely multiplying their dog's age in "human years" by seven, dog parents could now come up with a much more reliable measure of its age by considering its size and maturity into account.

Finding an exact way to do so is not easy; however, a team of researchers has developed a formula. The calculation is not based not on any arbitrary metric, but on changes to DNA over time.

The researchers' method based their results on an epigenetic mechanism called methylation. As both humans and domestic dogs (Canis lupus familiaris) age, methyl groups adds to our DNA molecules, which can change the activity of a DNA segment without altering the DNA itself.

The methyl groups have their own function, but an analogy is that DNA methylation can be used to measure age in humans. The feature is called the epigenetic clock.

Geneticists Tina Wang and Trey Ideker of the University of California San Diego set out to compare the epigenetic clock of humans to the epigenetic clock of dogs.

The lifespans of dogs could vary. Some large breeds, such as mastiffs, could survive in 6 to 7 years, while some miniature dog breeds, such as chihuahuas, could exist in 17 to 18 years. All dogs - despite this variation - show a similar developmental, physiological, and pathological trajectory.

The researchers used Labrador retriever dogs for this study. The team explained that a particular dog breed exhibits a substantial genome homogeneity, which improves the chance of distinguishing genetic factors linked with complex traits, including aging.

The scientists then analyzed their dog data to the declared methylation profiles from the blood of 320 humans - aged between one and 103 years - and those of 133 mice.

The researchers characterized the methylomes of 104 Labrador retrievers spanning a 16 year age range, achieving >150X coverage within mammalian syntenic blocks using targeted sequencing.

"[Correlating] with human methylomes [shows] a nonlinear relationship which [converts] dog to human years, [adjusts] the timing of major physiological milestones between the two species, and extends to mice," the researchers said.

The similarities were perceived when examining young dogs to young humans, and elderly dogs to elderly humans.

So what's the formula?

This matching of the epigenetic clocks enabled the team to determine a formula for measuring the 'human' age of dogs: human_age = 16ln(dog_age) + 31. Multiply the common logarithm of your dog's age in years by 16, then add 31. That will give you the dog's age in 'human years.'  Here's a calculator you can use to make your life simple.

Certain milestones matched up well using the formula. Seven dog weeks correspond to nine human months - the period when baby teeth are erupting in puppies and infants. 

The two species' average lifespans also paired up - 12 years for the Labradors and 70 for humans. Other milestones that said didn't match up quite well.

Dogs pass through puberty and reach sexual maturity quicker than humans, according to researchers. Therefore, the period between adolescence and middle-age of humans and dogs doesn't match up. A five-year-old Labrador is calculated approximately 56 in human years.

Now, there's a pesky problem with different breeds ageing differently. Hence, a formula based on the epigenetic clock is considerably more useful in calculating dog's age than simply multiplying by seven.

The researchers described it in a paper published on the pre-print resource bioRxiv ahead of peer review.