The old rule of thumb for comparing the relative age of a dog to a human says that every year of a dog's life is the equivalent of seven years of a human's life. But a new study has busted that rule of thumb, finding a new relationship that seems to do better for matching the relative maturity of canines and people through different points of their lifespans.
The authors of the study created the following chart to visualize the relationship they found for comparing the relative maturity of dogs with humans at different points in their lives, using the most dog-like human they could identify, actor Tom Hanks, and the most Tom Hanks-like dog breed they could identify, a labrador retriever.
While you can use the chart to convert a dog's age into human years, we've taken the mathematical relationship the study's authors found and built the following tool to do that math, since the chart doesn't necessarily provide a clear result. If you're accessing this article on a site that republishes our RSS news feed, please click through to our site to access a working version.
While we're having a bit of fun in setting up premise for the tool, there is real science behind it. Here's an excerpt from the authors' paper after presenting the mathematical relationship they determined:
We found that this function showed strong agreement between the approximate times at which dogs and humans experience common physiological milestones during both development and lifetime aging, i.e., infant, juvenile, adolescent, mature, and senior (Lebeau, 1953; Bogin and Smith, 1996; Bartges et al., 2012) (Figure 3D). The observed agreement between epigenetics and physiology was particularly close for infant/juvenile and senior stages. For instance, the epigenome translated approximately 8 weeks in dogs (0.15 years) to approximately 9 months in humans (0.78 years), corresponding to the infant stage when deciduous teeth develop in both puppies and babies (Bogin and Smith, 1996; Bartges et al., 2012). In seniors, the expected lifespan of Labrador retrievers, 12 years, correctly translated to the worldwide lifetime expectancy of humans, 70 years (Fleming et al., 2011; CIA, 2013). For adolescent and mature stages, the correspondence was more approximate, with the epigenome showing faster changes for dogs, relative to humans, than expected by physiological tables (Inoue et al., 2015; Arias et al., 2017) (Figure 2D). Thus, the canine epigenome progresses through a series of conserved biological states that align with major physiological changes in humans, occurring in the same sequence but at different chronological timepoints during each species’ lifespan.
Because of the nature of the math, we limited the tool to only consider results for a dog that is at least three months old (0.25 years) or older. A little below that arbitrary level, we found the study's dog-age-to-human-age conversion will indicate a negative age in human years, which doesn't make much sense. We think the relationship also breaks down somewhat at the higher end of the age spectrum, where an old dog would seem to hardly age at all compared to a human, but we haven't limited the tool's results at this extreme.
In that sense, the new relationship for converting dog years into human ages is like any other model. To paraphrase George Box' immortal words, "all models are wrong, some are useful", where part of the trick to making a model work for you is determining over what range it provides realistic results!
References
Wang, Tina et al. Quantitative Translation of Dog-to-Human Aging by Conserved Remodeling of the DNA Methylome. Cell Systems. July 02, 2020. DOI: 10.1016/j.cels.2020.06.006.