It turns out that all animal species have about the same number of heartbeats during their lifetime. This nearly universal rule was addressed in one of the best books I’ve read in the past five years: Scale: The Universal Laws of Growth, Innovation, Sustainability, and the Pace of Life in Organisms, Cities, Economies, and Companies by Geoffrey West. Dr. West is a particle physicist and former president of The Santa Fe Institute. Previous IFODs have hit on West’s research noting that people walk faster in bigger cities (see Zipf’s Law, City Size and Walking Speed) and the fascinating notion that while human life expectancy has increased, the maximum human lifespan has not increased much (see The Upper Limit).

In Scale, West addresses how various things respond to changes in size and discusses a universal law known as allometry which holds that as many things grow in size their characteristics scale non-linearly. Here’s an excerpt from Scale explaining some of the interesting questions that result from looking at scaling laws:

Scaling simply refers, in its most elemental form, to how a system responds when its size changes. What happens to a city or a company if its size is doubled? Or to a building, an airplane, an economy, or an animal if its size is halved? If the population of a city is doubled, does the resulting city have approximately twice as many roads, twice as much crime, and produce twice as many patents? Do the profits of a company double if its sales double, and does an animal require half as much food if its weight is halved?

A particularly interesting aspect of scaling is the relationship between an animal’s size and its metabolism. As an animal’s size increases, it’s metabolic rate only scales by 3/4 power (this is known as sublinear scaling). That means that while “a whale weighs about 100 million times more than a shrew [and thus] you might expect its metabolic rate to be 100 million times greater, too. But it’s only a million times bigger because metabolic rate scales as mass to the three-quarters [100,000,000^¾ is 1,000,000]. The pattern holds with very few exceptions across all organisms.” Source. Another way to think about this is that larger animals are more efficient machines than smaller animals:

A profound consequence of [3/4 sublinear scaling] is that on a per gram basis, a larger animal (a woman for example) is actually more efficient than a smaller one (e.g. her dog) because less energy is required to support each gram of her tissue (by about 25 percent). Her horse, by the way, would be even more efficient. This systematic savings with increasing size is known as an economy of scale. Put succinctly, this states that the bigger you are, the less you need per capita (or, in the case of animals, per cell or per gram of tissue) to stay alive.

An astounding result of the sublinear scaling of metabolism and body size is that all animals tend to enjoy the same number of heartbeats over their natural lifetimes, about 1.5 billion beats, but have hugely different lifespans. This 1.5 billion heartbeats rule is true whether they live 1 year or 100 years on average. A shrew has a very fast metabolism — it averages about 1,300 – 1,500 beats per minute — and thus burns through its 1.5 billion heartbeats in a few years while an elephant lives about 75 years thanks to its low 30 beats per minute heart rate. So, elephant hearts don’t beat longer; instead, they beat slower.

We humans are outliers: we get 2.5 – 3 billion beats over our natural lifetimes. “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.” Source. So, that leads to the natural question – do humans with lower resting heart rates live longer? Based on a study out of Sweden, the answer appears to be yes. The reason may not be that a lower heart rate uses up less of our allotted beats, but rather that a higher resting heart rate may be indicative of health problems that negatively affect our longevity.