On Jul. 9, 2020, a study, published in the journal Cell Systems, laid out a new framework for comparing dog-to-human aging. The researchers found the first eight weeks of a dog;s life is comparable to the first nine months of human infancy, but the ratio changes over time.

All mammals experience the same overarching developmental timeline: birth, infancy, youth, puberty, adulthood and death. But researchers have long sought specific biological events that govern when such life stages take place. One means to study such a progression involves epigenetics — gene expression changes caused by factors other than the DNA sequence itself. Recent findings have shown that epigenetic changes are linked to specific stages of aging and that these are shared among species. 

Researchers focused on one type of epigenetic change called methylation, a process in which molecules called methyl groups are attached to particular DNA sequences, usually parts of a gene. Attaching to these DNA regions effectively turns the gene into the “off” position. So far, researchers have identified that in humans, methylation patterns change predictably over time. These patterns have allowed the creation of mathematical models that can accurately gauge the age of an individual — called “epigenetic clocks.” 

But these epigenetic clocks have only been successful in predicting human age. They do not seem to be valid across species, such as in mice, dogs, and wolves. To see why the epigenetic clocks in these other species differed from the human version, researchers first studied the epigenetic changes over the lifetime of a domestic dog and compared the results obtained with humans. Dogs are a useful model for such comparisons because much of their environment, diet, chemical exposure, and physiological and developmental patterns are similar to humans.  The researchers also attempted to correlate the human epigenetic clock with dogs, using this as a proxy for converting dog years to human years. 

Based on the data, researchers identified similar age-related methylation patterns, specifically when pairing young dogs with young humans or older dogs with older humans. They did not observe this relationship when comparing young dogs to older humans and vice versa. The study also found that groups of specific genes involved in development can explain much of the similarity, which had similar methylation patterns during aging in dogs and humans. 

The new formula is more complicated than the “multiply by seven” method. When dogs and humans experience similar physiological milestones, such as infancy, adolescence and aging, the new formula provided reasonable estimates of equivalent ages. For example, by using the new formula, eight weeks in dogs roughly translates to nine months in humans, which corresponds to the infant stage in both puppies and babies. The expected lifespan of senior Labrador retrievers, 12 years, correctly translates to 70 years in humans, the worldwide average life expectancy.

The group acknowledges that the dog-to-human years formula is largely based on data from Labrador retrievers alone. Hence, future studies with other dog breeds will be required to test the formula’s generalizability. Because dog breeds have different life spans, the formula may be different among breeds.