You may think you know how old your dog is — how their age translates into human years, roughly. But do you, really? Some researchers think that we have been calculating dogs' ages wrong this whole time. In a new study paper, they explain why.
Dogs are some of humans' favorite four-legged friends, but while they are loyal and loving companions, their lifespans are, unfortunately, much shorter than our own.
Yet, understanding what a dog's age means, in terms of their development and life stage, can be difficult.
Traditionally, people have converted dog years into human years by estimating that 1 dog year equals 7 human years. But this may not be accurate, at all.
The American Veterinary Medical Association argue that aging is not clear cut. They recommend that, instead of converting age, people who live with dogs look for specific signs of physical development and aging in order to better understand their canine companion's stage of life.
Now, a team of researchers — led by Prof. Trey Ideker, a geneticist at the University of California, San Diego — has developed a more accurate method of calculating a dog's age and translating it to human years.
This method, which the investigators explain in a study paper published online ahead of print at bioRxiv, involves looking at how subtle chemical shifts in the body affect gene expression over time. This process is known as DNA methylation.
These subtle chemical modifications play a key role in human aging, and they are also present in animals.
Dogs are excellent candidates, when it comes to comparing biological aging, as they share our environments and often receive similar medical treatments for similar health issues.
8 weeks for dogs like 9 months for humans
In the current study, the researchers focused on a single breed: Labrador retrievers. This allowed them to make more accurate comparisons between dog and human years, as it eliminated the possible inherent differences in aging among different dog breeds.
The team searched for changes in gene expression patterns in 104 Labrador retrievers aged between 4 weeks and 16 years, which veterinarians typically consider to be a venerable old age for dogs.
Then, the investigators compared these patterns with those in humans. They found that, in both humans and Labradors, certain gene expression mutations occur in similar ways among genes that play roles in developmental processes.
Based on this observation, the team believes that some aspects of aging are likely part of the developmental process.
"While the biology of aging has historically been considered as separate from that of development, their strong association, demonstrated here, supports a model in which at least some aspects of aging are a continuation of development rather than a distinct process," the researchers write in their study paper.
Moreover, when they compared the rate of DNA methylation in Labradors with that in humans, the researchers were able to match dog ages to human ages. The calculation, however, turned out to be quite complex.
The formula that the investigators have developed involves determining the natural logarithm of the dog's age, multiplying this by 16, and adding 31. A calculator can help work out the natural logarithm.
By this reckoning, the development of an 8-week-old puppy is roughly equivalent to that of a 9-month-old human baby. As the researchers explain in their paper:
"Approximately 8 weeks in dogs (0.15 years) [translate] to approximately 9 months in humans (0.78 years), corresponding to the infant stage when deciduous teeth erupt in both puppies and babies. In seniors, the expected lifespan of Labrador retrievers, 12 years, correctly translated to the worldwide lifetime expectancy of humans, 70 years."
In the future, the researchers hope to take their study further, looking at why aging patterns can differ among dogs and breeds, leading to some dogs developing certain health problems much earlier in life than others.