If your dog has been alive and kicking its paws about for a decade, the widely held belief is that it has aged as much as a human would have done by the grand old age of 70. This conversion factor – each year of a dog’s life accounting for seven human years – comes from dividing human life expectancy of around 77 by the canine life expectancy of around 11.
The underlying assumption is that each calendar year a dog lives though is equivalent to seven human years at any stage of a dog’s life. But new research suggests that things aren’t so simple. And if we look at some basic developmental milestones, it’s clear why.
For example, most dog breeds reach sexual maturity between the ages of six and 12 months – the upper end of that range corresponding, by the traditional conversion, to a human age of seven. And at the other end of the spectrum, although unusual, some dogs have been known to live for over 20 years. Under the “factor-of-seven” conversion rule, this would equate to an unfathomable 140 human-equivalent years.
To make matters more complicated, dogs’ life expectancy depends significantly on the breed. Smaller dogs tend to live significantly longer, suggesting that they age more slowly than bigger dogs.
All of this raises the question of what exactly we mean by age. The most obvious way to describe it is simply the length of time that has passed since birth. This is known as the chronological definition of age.
However, there are other descriptions. “Biological age”, for example, is a more subjective definition, which relies on assessing physiological indicators to identify an indivdual’s development. These include measures like the “frailty index” – surveys which take into account an individual’s disease status, cognitive impairments and levels of activity.
For example, if you’ve spent a lot of time eating junk food and smoking cigarettes instead of taking exercise and eating healthily, the chances are your biological age will exceed your chronological age. Or, you might be a 60-year-old with the body of a 40-year-old if you’ve looked after yourself well.
A new look at a dog’s life
When it comes to comparing animal ages across species, the biological definitions of age are far more useful than their chronological counterparts. Knowing a hamster is six weeks old doesn’t give you a good picture of that animal’s life stage even if you know the life expectancy of a hamster is only three years. Learning that a hamster has reached an age at which it can reproduce gives a much better picture of its level of maturity.
The authors of the new ageing study suggest that a sensible way to measure biological age is though so-called “epigenetic clocks” – changes to the packaging of our DNA that accumulate over time in all mammals.