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Multiply by seven is the conventional wisdom. But this formula appears to be roughly based on the average dog lifespan of 10 years and an average human lifespan of 70 years (worldwide average), giving a ratio of 7 to 1. No one knows where the dog years rule came from, though virtually all dog owners know it. According to that popular myth, every year a dog spends on the planet is equivalent to seven years for a human. So if a dog lives to be 15 years old, she's actually 105 in human years. Researchers and others who have simply taken the time to think about the ratio have recognized that it's illogical for decades. It is obvious to dog owners that puppies age much quicker than human infants in the early years at least.
New research from the School of Medicine at the University of California San Diego looked at molecular changes over time of 105 Labrador Retrievers. They studied epigentic changes in gene expression over time. This is the same phenomena that causes humans to show aging even though their DNA has not been modified, genes are expressed differently as we age, for example with facial wrinkles. To understand how dogs age, the researchers looked at a phenomenon called DNA methylation. As mammals get older, their DNA picks up methyl groups that "stick" to their DNA. While these groups don’t change the DNA itself, they attach to the genetic molecule and can turn certain genes on or off. This process occurs in humans in a linear progression, but the researchers found a none linear relationships in the rate in the subject Labs. The formula they came up with is:
[human_age = 16*ln(dog_age) + 31]
the ln() function refers to the base of natural logarithms and occurs in many mathematical interpretations of natural phenomena. The formula produced the following curve:
This is not surprising to most dog owners as we have seen our puppies mature quickly during the early years then more slowly later on. The researchers acknowledge the limitations of the study being restricted to just one breed, but other studies have related age to size by apparent maturity. These results are shown in the following table;
It seems that more research is required here since the above chart disagrees substantially with the epigentic study. For example the apparent maturity chart shows 4 dog years to be equivalent to 32 human years for all body types whereas the epigentic study indicates 4 years is equivalent to 52 years, at least in Labrador Retrievers. Because the slower ageing rate after maturity indicated in the epigentic study, the two methods are equal by about 11 dog years.
Although interesting, all this is really academic since dogs generally have regular checkups and dog owners are very attuned to their pets well being.