How old would my dog be if he were human? If you have had dogs, surely you have heard this question at some point, chatting with other owners, walking in the park. The most common is to multiply the dog’s age by 7 .
It is often said that each year of a dog is equivalent to 7 human years . But it is not true . Let’s imagine we have a 12-year-old dog. We multiply 12 by 7 and we get that our dog would be an elderly adult of 84 years . So far it makes a lot of sense for large breeds .
But things change with the smaller breeds , some of which live an average of 16 years. That would be 112 human years ! And although people can live more than a century, our life expectancy is not that high. So something is wrong.
A popular fix is to multiply by 5 when we talk about very small breed dogs. Thus, a small chihuahua of 15 years would be like a 75-year-old adult . Well then, fixed? I’m afraid not.
When calculating its aging we find two problems . The first is that the life expectancy of dogs changes a lot depending on their breed , as we have seen. Large dogs show greater wear and tear, living less than small ones.
The second problem is that dogs, like other animals, do not age at the same rate from birth to death .
Chronological age and biological age
The chronological age of our dog is easy to measure, just count the time that has elapsed since our adorable puppy was born. Biological age is something else, and refers mainly to the state of biochemical deterioration of their cells, tissues, systems and organs.
This is where a valuable aid comes into play: the epigenetic clock , a tool that allows the aging process of the organism to be measured. Knowing the changes produced in the genome of the dogs, and comparing it with the human genome , we can know how they age , and how we do.
A team of researchers from the University of California San Diego , led by geneticists Tina Wang and Trey Ideker , proposed in 2019 a new way to calculate how dogs age compared to humans .
To do this they studied an epigenetic mechanism called DNA methylation . As we age, methyl groups are added to DNA molecules. And there are genes that, by mutating, speed up the aging process .
All dogs share similar developmental stages , both physiological and pathological , which in turn are similar to those of humans. However, to increase the likelihood of finding genetic factors linked to aging, the team focused on just one breed : the Labrador retriever .
The researchers analyzed DNA methylation patterns in the genomes of 104 Labrador retrievers between four weeks and 16 years of age. His research revealed that dogs age very quickly up to the age of two .
This explains why a six-month -old dog has a similar biological age to a 20-year-old human . For us, he will still be a puppy that does not stop still, but his DNA tells us that he is a full adult , biologically speaking. Our popular (and incorrect) formula would have given 3.5 “human years” for the six month old pup.
However, despite going full speed at puberty, later on his aging slows down . So much so that if the dog’s 6 years were equivalent to 60 human years, when our pet is 12 years old, his DNA will tell us that he “would be” a 70-year-old human.
The dog will be twice as old, but if it were a human it would have been only 10 years older. We definitely need another method to calculate .
Multiplying by 7, or by 5, is not valid, it is clear. What alternative do we have?
Based on methylation data , certain life stages of dogs and humans appear to match very closely. For example, our 7-week-old puppy is as young as a 9-month-old human baby . Both of them are starting to get their milk teeth.
But, on other occasions, the aging process does not coincide, it is not linear. To more accurately compare age in dogs and humans, Tina Wang and her colleagues have devised a nonlinear formula that uses logarithms .
To know how many human years our dog’s years are equivalent to, we calculate the natural logarithm of the dog’s age (x), multiply it by 16 and add 31 . The formula looks like this:
16 ln(x) + 31
Why do we try to estimate how old our dogs would be if they were human? It really is something curious, but it doesn’t make much sense: they live less, we more. Of course we love them so much and they are so integrated into our daily lives that we can consider them members of our family .
So it is normal that we want to know how they age , being ourselves the most direct reference. Be that as it may, the truth is that they live much less than their owners . And that’s a shame.
References:
Clyde, D. (2020) ‘From dog days to human years. Nature Reviews Genetics’, 21(9), 508-509. DOI: 10.1038/s41576-020-0267-3
Horvath, S., et al. ‘DNA methylation-based biomarkers and the epigenetic clock theory of aging’, Nat. Rev. Genet. 19, 371–384 (2018) DOI: 10.1038/s41576-018-0004-3
Levine, ME et al. (2018), ‘An epigenetic biomarker of aging for lifespan and healthspan’, Aging (Albany NY), 10(4), 573. DOI: 10.18632/aging.101414
Wang, T. et al. (2020) ‘Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome’, Cell systems, 11(2), 176-185. DOI: 10.1016/j.cels.2020.06.006
