Kleiber's Law

A mouse’s heart beats pretty fast, but an elephant’s heart beats at a slow, steady pace. Over their lifespans, the mouse and the elephant get about the same number of heartbeats.

It seems as though there is some kind of "biological timekeeping" where lifespan and metabolic rate are intertwined. The faster metabolism in smaller animals leads to a faster "pace of life," with quicker heart rates and shorter lifespans. Conversely, larger animals with their slower metabolism experience a slower pace of life, with lower heart rates but longer lifespans.

A Swiss biologist named Max Kleiber was thinking about this phenomenon a lot, and in the 1930s he formulated a principle that reveals a fascinating pattern in the biological world: an organism's metabolic rate—the speed at which it uses energy—doesn't scale directly with its body mass. Instead, it scales to the ¾ power of its mass.

As animals get bigger, their metabolisms don't speed up at the same rate as their size. This means a large animal like an elephant has a slower metabolic rate per unit of body mass compared to a small animal like a mouse.

This observation is a curious one, and it reminds me of a couple of other observations. One is from physics and one is from economics.

In physics, there is a direct relationship between space and time. To oversimplify a bit by way of a summary, the idea is that if you take away time, you add space, and vice versa. You can see real life examples of this in our GPS system, where corrections for relativity have to be made.

This is hard-and-fast rule in physics, with no cheating allowed. Kleiber's Law isn’t quite like that, but it does give a pretty useful guiding principle.

In economics, there’s a direct relationship between interest rates and business activity. If you want to slow the economy down, you raise interest rates (hello 2023!), and if you want to speed the economy up, you lower them. Once again, I’m oversimplifying some, but I just want you to get the idea of this sort of inverse relationship.

This is a much softer rule, subject to many other influences, but it’s nevertheless useful in a similar way to Kleiber's Law.

This law uncovers a profound truth about life: larger animals are more energy-efficient than smaller ones.

You’re probably familiar with the term “economies of scale”, where producing more of a thing makes that thing cheaper. In the same way that a factory can make a million pens cost less per pen than a dozen, it “costs less” for an elephant to eat per unit of weight.

In biology, as the size of an organism increases, the energy needed to sustain each unit of its body mass decreases. This slower metabolism contributes to longer lifespans… but the same number of heartbeats.

By understanding the metabolic needs of different species, we can better predict their responses to environmental changes, human-induced or natural. This knowledge is crucial in a world where habitats are rapidly changing, and species face unprecedented challenges.

Did you know about Kleiber’s Law before today? If you’re like me, you probably knew there was a relationship between size and metabolism, but had no idea there was a formula to describe this.

What else can we conclude from this observation?