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In human years, how old is a two-year-old dog? They’re almost certainly older than 14. Though most people grow up believing that one dog year equals seven human years, the math is actually much more complicated.
No one knows how or when the “Seven-Year Rule” became ubiquitous, but it’s probably based on average life expectancies for people and dogs around the mid-twentieth century. It’s also possible that the “rule” was just a savvy bit of marketing. That’s what William Fortney, a veterinarian at Kansas State University, guesses. This conventional wisdom, he suggests, was perhaps “a way to educate the public on how fast a dog ages compared to a human… to encourage owners to bring in their pets at least once a year.”
Challenging the “Seven-Year Rule”
Scientists have attempted to find a more accurate understanding of “dog years” since as early as the 1950s. French researcher A. Lebeau is credited with defining the stages of a dog’s life. His studies into canine puberty and adulthood found that dogs age about twenty times as fast as humans during their first year. After that, their aging rate gradually slows. Dogs age about five times faster than humans by the time they reach middle age.
Applying the Seven-Year Rule also ignores the differences between different sizes and breeds of dog. In general, WebMD writes that smaller dogs mature more quickly than larger ones, but they also tend to live longer. “Medium-sized pooches,” for their part, “are somewhere in the middle on both counts.” In 1997, another team developed their own formula for dogs in specific weight groups.
The discrepancy between simple adages like the Seven-Year Rule and the realities of canine aging are closely related to the difference between biological and chronological age. Whereas the latter refers to the amount of time since a person or animal’s birth, the former takes numerous factors into account. In humans and animals alike, these can include genetic traits as well as lifestyle choices.
While dogs age more quickly than we do, new research suggests that their cells age in much the same way. Some scientists have argued that the process of DNA methylation (the addition of methyl particles to certain DNA sequences) is evidence of biological aging in human cells. This new study’s authors believe the same is true for canine cells. By examining methylation in Labrador Retrievers, they have settled on a new formula for determining a dog’s biological age: multiply the natural logarithm of the dog’s chronological age by 16 and add 31.
Not a math expert? Don’t worry, Science Mag has a tool to help you calculate your dog’s age in just a few clicks.
Estimating Your Dog’s Age
Pet parents who’ve chosen to adopt may not know their dog’s chronological age. It’s important to work with a veterinarian to make as accurate a guess as possible and attain the right level of care. Thankfully, DNA methylation tends to coincide with a number of additional (more easily-observed) changes:
- Teeth: Examining a dog’s teeth is probably the easiest way to estimate its age at a glance. Most dogs have a full set of adult teeth by the time they’re three-and-a-half-months old. They typically begin to show signs of tartar build-up and discoloration between ages three and five.
- Eyes: An older dog’s eyes can begin to cloud as a result of lens hardening.
- Coat: As dogs age, gray hair usually begins to spread from their muzzle to other parts of their body.
Your dog’s age will go a long way in determining its healthcare needs and your pet care costs. Work with your veterinarian to address everyday concerns and plan for the unexpected.