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?
I think this piece (and the calculator) arose out of a Radio 4/World Service programme called More or Less -- there was a section of the show on it a couple of weeks ago. The Beeb describes the show as “Tim Harford explains -- and sometimes debunks -- the numbers and statistics used in political debate, the news and everyday life.” Of course in an ideal world the media would check the validity and applicability of the numbers and statistics they report and the programme would be redundant.
Not sure if the World Service version has a more international focus than the one that goes out on R4, but it’s probably available as a podcast from the usual places if you want to check it out Mano.
What? The origins of the formula are inontrovertible: Lorne Greene.
Very interesting question. The answer, if yes, though, could be as simple as a correlation with the rate and number of cell divisions. Cell size is pretty constant, so larger animals must have more cells, derived from more cell divisions. Each DNA-replication event is a chance for a potentially carcinogenic mutation.
Lorne Greene? Old Ben Cartwright? More details, please. Inquiring minds want to know!
Dang, blockquote fail again. A bit of Googling reveals that Alpo commercials from the 1970’s, featuring Mr. Greene, popularized the 7:1 ratio but it was common from long before that.
http://www.slate.com/articles/news_and_politics/explainer/2009/05/a_dogs_life.html
I remember those commercials! But did not remember him using the 7:1 ratio.
The formula which I have long used (nope, can’t recall where I heard it) goes like this:
A dog is (just barely) able to reproduce by age 1, so that matches about age 15 in a human.
After 2 years, the dog has typically reached full growth and maximum physical potential, so that equals approximately 25 in a human.
For each dog year after that, add 5 human years.
After the first few years, the numbers come out quite close to the Alpo algorithm.
Oddly enough, size of a species does not lead to a higher incidence of cancer.
http://en.wikipedia.org/wiki/Peto's_paradox
if it did, apparently every single whale would die of cancer.
That is the interesting thing. If true, it seems to be something within a species and not across species.
This would be an interesting “model fitting” exercise: if a dog’s age was on the x axis and the equivalent human age was on the y axis, the graph would be what math professors call “concave down” or “concave” function (engineering usage).
Many of the largest dogs (Borzoi, for example) don’t reach full size until they are nearly four years old. Show judges are supposed to take that into account. Something like a lack of chest depth isn’t a big fault in a two year old, but is in a four year old.
Human meddling is undoubtedly a large part of this. Domestic dogs are all descended from wolves via selective breeding.
Compare it to ligers and tiglons. Ligers grow abnormally large, larger than the lion father or tigress mother. In captivity, all have the same lifespan, but a liger might not survive as long because of its bulk. Tiglons (tiger father, lion mother) are born sterile so they couldn’t propagate naturally, and those born to captive animals have much shorter lifespans than their parents.
http://en.wikipedia.org/wiki/Liger
http://en.wikipedia.org/wiki/Tiglon
The comparative lifespans of the largest and smallest dogs resembles that of humans. People with gigantism tend to suffer more and chronic health problems than those with dwarfism. Most of those listed as the tallest ever most live less than 50 years, while the shortest tended to live as long as most people.
http://en.wikipedia.org/wiki/Dwarfism
http://en.wikipedia.org/wiki/Gigantism
http://en.wikipedia.org/wiki/Tall_people
http://en.wikipedia.org/wiki/List_of_shortest_people
yeah. I was quipping.
Size differences within and between species may have a completely different basis, at least among mammals: larger species have larger cells of some kinds, while cells of other kinds have similar sizes across species but lower metabolic rate. Within species, I’m sure I’ve seen somewhere that cell size and metabolic rate don’t vary much, while cell number (and so number of cell divisions) does, but I can’t find a reference.