All of us know the rule of thumb that says that one calendar year for a dog corresponds to seven years for a human, though the origins of the formula are unknown. But this is a very rough approximation because small dogs age more quickly early on (for the first two years it is 12.5 years per human year for small dogs, 10.5 for medium-sized dogs, and 9 for large dogs) and then age more slowly later. In other words, smaller dogs have a truncated childhood and extended adulthood when compared with bigger dogs. There is a calculator that enables you to calculate more accurately the equivalent age of your dog for a certain number of breeds.
There are two things about dogs that immediately strike an observer and those are the huge diversities in size and lifetimes within this single species. Dogs range from the tiny Chihuahua (2-6 lbs) to the massive Spanish Mastiff (121-154 lbs). Such diversity gives us a unique window to study how size affects aging.
“It doesn’t happen in any other animal,” says Kate Creevy. “There isn’t any other species which has within a single species the same degree of size diversity that dogs have. It’s possible that by creating all of these diversely sized dogs that we unmasked this ageing phenomenon.”
We know that large dogs tend to have shorter lives than smaller dogs. This negative correlation of longevity with size is a puzzle since in the animal kingdom, when we compare different species, large animals tend to live longer, i.e., the correlation is positive.
“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).
Teasing out causation and generalizing to other species is always rather tricky. Is it true that larger size leads to greater chance of cancer within all species?