Calculating a dogs real age

Meg, my West Highland Terrier died a couple of months ago. She’d reached the grand old age of 19 years and four months.

A few days later, I was reminded of the oft-quoted statistic that every human year equates to seven dog years. This mental calculation looms more largely in an owner’s mind as a dog gets older, and thoughts turn to how long the pet has left.

But if that stat were really true then Meg would have been 135 years old when she died, which seems very unlikely.

No human is known to have lived beyond 122.

So if the seven dog years to one human year is wrong how do we work out an accurate calculation?

Dogs are the most diverse mammal species on the planet. They can vary in weight from 6 lb (3kg) to 200 lb (90kg) when fully grown and have widely differing body shapes and hair types.

This also means that there is a lot of variation among breeds in terms of life expectancy. And unexpectedly, small dogs like Meg live longer than big ones.

“If you think about statistical correlation between average life span and body size in mammals it generally tends to be positive – gorillas, elephants and whales are much longer lived than shrews, voles and mice,” says Daniel Promislow, professor of genetics at the University of Georgia.

That would lead you to believe that Great Danes would live longer than Chihuahuas but it’s the other way round.

Promislow has his own theory why this is.

“The disease that shows the strongest correlation with size is cancer,” he says.

“We know that cancer goes up even faster with age than mortality does. The rate of cancer increases very dramatically with age – the same as in humans.”

So it may be because the risk of cancer increases so much, and because large dogs are at such a higher risk of dying of cancer (roughly 50% chance), that large dogs generally have shorter lives than small dogs (roughly 10% chance of dying of cancer).

Ben with Meg in 2011

This is true despite the fact that small dogs reach adulthood faster than big dogs.

“Small dogs reach skeletal and reproductive maturity sooner than larger breeds. Once they’ve achieved those measures of adulthood they carry on to live longer,” says Dr Kate Creevy, assistant professor of internal medicine at the University of Georgia.

In other words, small breeds have a shortened juvenile period and an extended adulthood.