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I read this webcomic today:

web comic abominable 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?

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Actually some parrots can live for 50+ years. And chickens are lucky to survive to maturity... –  Chad Aug 11 '11 at 15:32
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I think the obvious answer is "No", since many species don't have hearts. –  fred Aug 18 '11 at 19:18
    
A long time ago, Isaac Asimov wrote a regular article for Fantasy & Science Fiction magazine, and most were collected into books. In one of those, he calculated lifetime in heartbeats for a lot of animals, and concluded that everybody except humans had about a billion-heartbeat maximum lifetime, but humans hit four billion. (The methodology was flawed; he calculated max human lifetime as 114 years, and all other species got less atypical maxima.) –  David Thornley Aug 19 '11 at 3:07
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Don't exercise - conserve heartbeats! –  Coomie Jan 25 '13 at 5:26

3 Answers 3

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:

lifespan vs heartbeat
Full size

How to read the chart

  • The red line marks where the all the data points should be if this hypothesis were true
  • The green and violet lines mark the ±50% lines. According to the study above all mammals fall within those two lines.
  • Anything outside the lines pretty much disproves the hypothesis

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.

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+1 for doing extensive research to create your own graph. - There is such large variety in the animal kingdom, e.g. from the small to the big, body mass encompasses at least 6 orders of magnitude. A canary and a whale differ in mass by factor of ~1 million. But their number of total heartbeats differs only by a factor of ~10. I find it intriguing that despite all their differences most animals fall in between the 10e8 - 10e10 range. –  Oliver_C Aug 14 '11 at 1:25
    
@oli if you pick a number at random between 1 and 100 for life span and another between 10 and 100 for heart rate, 90% of the results will be within 2 orders of magnitude (unless I'm mistaken). This means that the available possible ranges for the parameters are the real concerns here. –  Ebenezer Sklivvze Aug 14 '11 at 16:29
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Yes. This begs the question why are those numbers as they are? - And when you multiply two variables that each vary by 2 orders of magnitude, the product should vary by 4 orders of magnitude. Unless the variables are not independant of each other and there is some sort of inverse relationship. –  Oliver_C Aug 15 '11 at 12:33
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@oli The product of two uniform random variables is not a uniform random variable. The range of possible products will span 4 orders of magnitude, but the distribution of probabilities will be lumped up in the middle, thus reducing the effective range of orders of magnitude. Add to that that the initial variables will not be really uniform, but likely normal and thus the lumping effect will be even greater. I am looking for some links to explain this further. –  Ebenezer Sklivvze Aug 15 '11 at 15:34
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Says a lot about the state of "peer-review" in science and what that's worth. –  Russell Steen Aug 15 '11 at 22:40

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...

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You are of course right that not all animals die after "1 billion heartbeats". That's why I said "some truth " in my answer. The "1 billion heartbeats" is basically the flashy and simplified main stream version of the underlying "metabolic rate is not independent of body mass" phenomenon. –  Oliver_C Aug 12 '11 at 7:14
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Well, there's "some truth" in almost anything by that criteria. You can find a handful of data points that will support any theory and say there's "some truth" to it... but that's not science. –  Russell Steen Aug 12 '11 at 13:30
    
To be clear, I do think your answer has some interesting and worthwhile information in it, but that the op's question is more directly and easily disproven as clearly not every species has around a billion beats, and there's not even enough convergence IMO to justify an "almost every" assertion. –  Russell Steen Aug 12 '11 at 13:40
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Okay, I see now that we look at the OP's question differently. I don't take it as literally as you. You are correct that the "1 billion" is not striclty true. I will change my answer to make it more clear what I mean. –  Oliver_C Aug 12 '11 at 14:00

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:

Table



Heart rate and life expectancy in mammals and humans:

Life span


Life expectancy and total heart beats per lifetime in mammals and humans:

heart beats


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.


Scaling

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

Kleiber's Law

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:

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I am already impressed with the nonimproved version, but being an informavore, I'd be happy to see the improved version too. –  rumtscho Aug 11 '11 at 9:38
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I immediately notice that the first graph suggests that dogs and cats live 30 years. Definitely not true. Pet dogs live around 10-14, cats a few years more. The second graph seems to correct this, although they're reversed in this case (cats live longer). –  Tesserex Aug 11 '11 at 13:30
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Please note that the second graph uses log scales which are probably making something appear out of nothing... –  Ebenezer Sklivvze Aug 11 '11 at 16:10
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Wow, talk about creating a signal out of noise. Your table shows that it's 1 billion +/- 1/2 a billion or more. 1 billion +/- 50+% isn't very precise. Not only that it shows a ridiculously small number of animals. The last chart is completely different, having nothing at all to do with number of heartbeats (though interesting). The defense of being right to "an order of magnitude" is also very weak. "Within an order of magnitude" is pretty sloppy from a scientific standpoint. –  Russell Steen Aug 11 '11 at 22:58
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We are talking about physiology here, not particle physics. Individual differences of 50% are not so uncommon. Just to take a random example, normal plasma glucose levels range between 3 and 8 mmol/l. So that's an average of 5.5 +/- 2.5...And for many hormones the range is way larger. –  nico Aug 12 '11 at 15:16

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