Dec. 17, 2010

Science Dad on What is a Dog Year

by Vince Harriman

Click to enlarge images

Young Stella

Thanks to all the great comments and guesses people posted last week -- no one really got close to guessing how big the solar system is, which was the whole point of the project. As for a scale model of the Universe, well, the universe is so big that even a scale model makes no sense. Even if we made the universe the size of the solar system, the Milky Way galaxy is vanishingly small. If you've never seen the Ray and Charles Eames film Powers of Ten, go watch it now. Carey and Michael Huang have also made a great interactive scale of the universe that helps to visually explain its size. Our scale model really put things in perspective, even if measuring the universe still ended up being a number followed by an incomprehensible number of zeroes.

This week we decided to look at another kind of scale to see if it made any sense. Beckett's birthday is right around the corner, so we've been talking about birthdays a lot lately. Beckett has often asked about our dog Stella's birthday. Now I am one of those dog owners who has no idea when the dog was born and for the longest time I had no idea even how old she was. Over 5 and under 10 was my usual answer. But little boys need details and real answers, and with his birthday around the corner, the need to know became even stronger.

A typical weight-based graph. Dog years represented on the x-axis, human years on the y-axis. The colors represent different weight classes.

Most people have heard that a dog year is equal to seven human years. But why? And what does that mean? It turns out there is no scientific basis for this scale. Canine lifespans vary greatly from breed to breed, and are often a function of size. You can see from the graph that life spans diverge based on weight. But even this graph doesn't tell you that giant breeds, for example, live only 7 or 8 years, while Chihuahuas and other 'toy' breeds can live as long as twenty years.

Stella is an active and healthy dog and has weighed almost exactly 60 pounds for years. While the math says she is older than I am, she still acts like a young, happy puppy -- full of life, very patient, and best friend to our two boys. While we've managed to get Rowan to stop trying to ride her like a horse, he still wants to pull her tail and ears.

Stella today

I made a half-hearted attempt to clarify the role of anthropomorphism and anthropocentrism in science, or in life in general, but Beckett had no interest. What for me was the most fascinating aspect of the whole human year/dog year calculation had absolutely no interest for Beckett. Which was fine. Since we had just last week made a scale model of the solar system, the idea of a scale model that describes dog years and lifespan relative to human years and lifespan made sense to him.

It is helpful to think of pet lifespans in a way we can relate to. But still, it's hard to believe our Stella is over 50! She still acts like a teenager, I can tell you that.

We'd love to hear your dog stories -- does your dog's human age correspond to the way your dog acts? Did your dog act like a puppy long after those years were past? And if you think the seven year rule is perfect, we'd love to hear that too. All in the name of science!

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-5 11/12 and Rowan-2 1/2 ask him 'why' approximately 6,523 times a day.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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