People have long thought that a dog's age can be estimated by substituting one dog year with seven human years. Recently it's been suggested that a dog's age is dependent on its breed and size, with younger dogs tending to have longer lifespans than very large dogs. Recent research reported in Cell Systems has now suggested that the most accurate way to calculate the age of a dog is by using a mathematical formula that takes into account how a dog's DNA has changed over time.
Dogs get age-related illnesses like people do. However, at the molecular level, dogs tend to age quickly when they're younger and their aging rate slows as they get older.
"In terms of how physiologically mature a one-year-old dog is, a nine-month-old dog can have puppies. Right away, you know that if you do the math, you don't just times seven," said the senior study author Trey Ideker of the University of California, San Diego. "What's surprising is exactly how old that one-year-old dog is--it's like a 30-year old human."
The epigenetics of DNA, chemical tags that are added to the genome and can influence gene expression, can change over time. One common epigenetic marker is called methylation, in which a methyl is added to the genome.
In this work, the methylation patterns in 104 Labrador retrievers of various ages were assessed and compared to those seen in humans. This showed that canines and humans have life stages that can be compared in the following mathematical formula: human age = 16 ln(dog age) + 31. For example, a nine-month-old baby is has the equivalent age of an eight-week-old dog. This is when both organisms form teeth. Labrador retrievers have a life expectancy of about twelve years, the human equivalent of 70.
"I like to take my dogs on runs, and so I'm a little bit more sympathetic to the six-year-old now," says Ideker, who used the formula to determine that his dog is nearly 60.
The formula contains the natural logarithmic function ln, so as dogs reach their late teens, their human equivalent doesn't change that much. At 15, a dog is about the age of 74 in humans but when the dogs reach 16 it's only added one 'human' year to make them about equal to age 75.
The methylation changes seemed to occur mostly in genes that are related to development. Once adulthood comes, "you've largely shut off these genes, but they're still smoldering. If you look at the methylation marks on those developmental genes, they're still changing," said Ideker.
The research team created a developmental clock of sorts that can measure age and physiology in different species. Other processes used to estimate age based on methylation tend to only work with one species.