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# How Many Heartbeats in a Lifetime? (Part 2)

By
Jason Marshall, PhD,
September 13, 2011

In the last post, we used a bit of simple unit conversion math to figure out roughly how many heartbeats there are in a typical human lifetime. (If you haven't read it, you should check it out before moving on.) Toward the end of the post, I posed the question:

How does the number of beats in a human lifetime compare to the number of beats in the lifetime of something like a honeybee? Or a cat? Or a whale?

As luck would have it, a few weeks ago I stumbled upon a blog post by Caltech professor Sean Carroll about Ten Things Everyone Should Know About Time. And down there at number ten on his list under the heading "A lifespan is a billion heartbeats," I found something pretty amazing. Sean writes:

...there exist simple scaling laws relating animal metabolism to body mass. Larger animals live longer; but they also metabolize slower, as manifested in slower heart rates. These effects cancel out, so that animals from shrews to blue whales have lifespans with just about equal number of heartbeats — about one and a half billion, if you simply must be precise. In that very real sense, all animal species experience "the same amount of time."

In other words, no matter if they're big or small, fast or slow, swimmers or sunbathers, it seems that the average length of a lifetime is around 1.5 billion heartbeats. So although it's certainly sad that many of our favorite pets live only 5, 10, or maybe 20 years, it's perhaps somewhat comforting to know that they've lived a "full" lifetime...they've just been travelling through "time" (in the sense that time is kept by the rate of their beating hearts) faster than you.

But wait a minute! In the first part of this post, we found that a good long human lifetime is made up of roughly 3 billion heartbeats—twice the 1.5 billion that we just said is the average universal lifetime for various species. What gives?

Well, I dug around a little and found this great post by Robert Krulwich (it's short, and well worth a read) which addresses exactly this question. In the post, Krulwich writes:

Human beings used to fit into this pattern, but now that we have learned to drink safe water, wash and bathe and create medicines, we last longer than our size would predict.

Just another one of the perks of being human, I suppose.

Image courtesy of Shutterstock