I read this webcomic today:
From: abominable.cc
I have no idea where the author got his factoid about the billion heartbeats. But it sounds interesting. The examples I can think of (rabbits, humans, elephants, said hummingbird) seem to support the theory that bigger creatures both have a slower heartbeat and live longer. Is there really such a trend, or am I missing the counterexamples? And if yes, how close does it come to the "one billion" number?
YES, there is some truth behind the "1 billion heartbeats " claim.
And when I say "some" I mean the creator of the comic didn't just make it up out of thin air.
While it's not literally true that all animals get 1 billion heartbeats before they die, a relation between metabolic rate (which is related to heart rate) and life span has been observed.
San José State University - Animal Longevity and Scale:
Heart rate and life expectancy in mammals and humans:
Life expectancy and total heart beats per lifetime in mammals and humans:
As a corollary, the basal energy consumption per heart beat and heart mass may be the same for all animals.This suggests that the life span is predetermined by the basic energetics of the living cells, and that the apparent inverse relation between life span and heart rate reveals the heart rate to serve as a marker of the metabolic rate.
This may be exemplified by considering the vast range of physiological cardiac parameters between one of the smallest, the shrew weighing 2 g, and the largest extant mammalian, the blue whale of 100 000 kg.
Despite a difference of many millions in body weight, heart weight, stroke volume, and total blood pumped per lifetime, the total oxygen consumption and ATP usage per unit mass and lifetime are almost identical together with the total number of the heart beats per lifetime.
[Source]
Mice and Elephants: A Matter of Scale
As animals get bigger, from tiny shrew to huge blue whale, pulse rates slow down and life spans stretch out longer, conspiring so that the number of heartbeats during an average stay on Earth tends to be roughly the same, around a billion.
Mysteriously, these and a large variety of other phenomena change with body size according to a precise mathematical principle called "quarter-power scaling".
[...]
It might seem that because a cat is a hundred times more massive than a mouse, its metabolic rate, the intensity with which it burns energy, would be a hundred times greater. After all, the cat has a hundred times more cells to feed.
But if this were so, the animal would quickly be consumed by a fit of spontaneous feline combustion, or at least a very bad fever. The reason: the surface area a creature uses to dissipate the heat of the metabolic fires does not grow as fast as its body mass.
To see this, consider a mouse as an approximation of a small sphere. As the sphere grows larger, to cat size, the surface area increases along two dimensions but the volume increases along three dimensions. The size of the biological radiator cannot possibly keep up with the size of the metabolic engine.
Things behave differently at different scales, but there are orderly ways -- scaling laws -- that connect one realm to another.
Metabolic Rate and Kleiber's Law:
The first accurate measurements of body mass versus metabolic rate in 1932 shows that the metabolic rate R for all organisms follows the 3/4 power-law of the body mass,
R ~ M3/4
This is known as the Kleiber's Law.
The reasons behind the power law are not yet fully understood, although there are of course theories. But since the OP's question doesn't actually ask for an explanation I feel it's okay to leave it to the interested reader to click through the links above and below to learn more about the proposed theories (plus, I believe it would make my answer just unbearably long if I include them in my post).
More:
No, this is not true. The Daphniidae only live for about 18.2 million heartbeats, which isn't even close to the (very loose) criteria of "within an order of magnitude". There are other species as well. All animals just do not have "around a billion" heartbeats.
A good mammalian counter example is the North American black bear. Even when not hibernating they only have about 55 beats per minute and only live around 25 years. Even if it never hibernated, that's only 700 million beats per lifetime. When they hibernate their heartrate slows to 14 beats per minute for up to seven months. Even on a short hibernation season of only 5 months (every year for their entire lifespan) they'd get only 500 million beats.
The rumor you found hit the "main stream" with an USA today story and is just one of those trendy things to say at a cocktail party to sound smart.
It doesn't help that this whole thing gets mixed up with metabolic rate, which is not the same thing as heart rate. Most of the actual research is about things like metabolic rate, production of free radicals, etc. If you read this abstract used by that story you'll see that it's talking about metabolic rate (usually lumped together as 'rate of living') and even that is strongly challenged because actual observations tend to conflict with the conclusions of the theory. The abstract also makes it very clear that this isn't a matter where there is yet a clear answer.
A more accurate statement may be "All the animals a news reporter could think to look up were somewhere in the same vague vicinity of a billion heartbeats". Or for a better explanation of how this became news...
It's true within 50% error for mammals and the study that started this idea is the following:
Rest heart rate and life expectancy
Among mammals, there is an inverse semilogarithmic relation between heart rate and life expectancy. The product of these variables, namely, the number of heart beats/lifetime, should provide a mathematical expression that defines for each species a predetermined number of heart beats in a lifetime. Plots of the calculated number of heart beats/lifetime among mammals against life expectancy and body weight (allometric scale of 0.5 x 10(6)) are, within an order of magnitude, remarkably constant and average 7.3 +/- 5.6 x 10(8) heart beats/lifetime.
However, I did not find this article straight away, so I've created my own plot and data set. Guess what? I've found that the conclusions of the above article are actually quite wrong when extended to non-mammalian species. I got most of my data from peer-reviewed literature.
The results can be read on the following diagram:
How to read the chart
Discussion
Clearly reptilians, birds, insect and mammals do not share the same heart physiology. So expecting the same kind of mileage is unjustified.
Compare for example the canary and the elk: both have the same life-expectancy of 22 years, but the former has a heartbeat of ~1000bpm, whereas the latter ~50bpm. Therefore it follows that a canary's heart will beat 20 times more than the heart of an elk. There are many examples like this in the data set.
Dataset
The dataset is available here (along with 32 references).
There was no way of making it fit in here.
By now, I have read multiple books in which the authors take for granted that a simple mathematical relationship exists between size and metabolism (sometimes measured in heartbeats) except for humans, who are surprisingly long-lived for their size. The modern research on that topic has been spearheaded by Geoffrey West, whose book "Scale" I have put on my reading list. I don't have the exact formula at hand. But I would count the claim as "true" even if the number "a billion heartbeats" might be a wrong-but-placative retelling.
Curiously, today I was reading an older popular science book and it contained a chapter dedicated to that relationship.
This states that the ratio of breath time to heartbeat time is 4.0 in mammals of any body size. In other words, all mammals, whatever their size, breathe once for each four heartbeats. Small mammals breathe and beat their hearts faster than large mammals, but both breath and heart slow up at the same relative rate as mammals get larger.
Lifetime also scales at the same rate as body weight (.28 times as fast as we move from small to large mammals). This means that the ratio of both breath time and heartbeat time to lifetime is also constant over the entire range of mammalian size. When we perform a calculation similar to the one above, we find that all mammals, regardless of their size, tend to breathe about 200 million times during their lives (their hearts, therefore, beat about 800 million times). Small mammals breathe fast, but live for a short time. Measured by the internal clocks of their own hearts or the rhythm of their own breathing, all mammals live the same time.
It states that one of the first people to note the existence of such relationships was Galileo, but that the methods for empirical calculation were developed by "W. R. Stahl, B. Günther, and E. Guerra in the late 1950s and early 1960s".
The numbers the chapter discusses are about mammals, but it also makes a qualitative claim that the relationship holds for many other animals (except humans, which is explained by our evolutionary strategy of neoteny).
The source I am citing is Stephen Jay Gould's book "The panda's thumb", chapter 29, "Our allotted lifetimes".
