The science community's past and present views on most animals having the same number of heartbeats

[lazybratsche]

@chorpler

That was the point of the original story (every time I heard this, anyway) – that humans live much longer, in terms of heartbeats, than other mammals.

This is definitely true. Our cardiovascular system and metabolism are fairly ordinary, when compared to related animals. Our life span really sets us apart. In general, it’s rare for animals to live very long past the age of reproduction. The standard evolutionary biology explanation for this says that when there is a high mortality rate from predation, disease, or competition, there is very little reproductive benefit from a longer life span. Thus, there is positive selection for individuals who reproduce early and often, negative selection for individuals who spend their resources on longevity instead of reproduction, and negligible selection for longevity alone. This can be demonstrated mathematically with population biology models, though obviously as we’re not all mayflies the real world is more complicated.

Humans, then, are unusual since it appears that evolution has selected for individuals that survive long enough to be grandparents.

[lazybratsche]

@jtur88

Largely because they have medical intervention to expand their lifespans. Longevity beyond the 30s might be rare among humans living in the wild, without access to medical care, and other factors like technologies that minimize starvation, exposure, accidents and predators. Domesticated animals, provided with similar human care, probably live somewhat longer than their wild ancestors did.

That’s not really true. When you see “life expectancy” in the 30s or 40s for premodern societies, it’s usually due to infant and early childhood mortality. Individuals who survived childhood mortality could expect to live 50 or 60 years, even in societies without anything approaching modern medicine.

Like I just said, there is strong evidence that there has been selection for unusually long life span in humans. Menopause, for example, is not merely the gradual and passive loss of fertility with age. It is an acquired trait that is practically unheard of in other animals. Evolution has selected for individuals who survive as post-reproductive grandparents, presumably so they can provide some advantage to their children and grandchildren.

[Chronos]

The figure of a billion heartbeats for other animals is also based on the extreme outliers of various species who are lucky enough to reach the maximum span for their species, not the average. Think of a 25-year-old housecat, for instance: They’re quite rare, and you won’t get any much older than that, but they do occasionally come up.

[Learjeff]

This issue is discussed in Nick Lane’s “Power, Sex, and Suicide,” which is about mitochondria. I don’t remember the details, though later I’ll see if I can find it. I believe he discusses it as interesting conjecture that doesn’t end up being terribly significant. He gives more attention to the relationship between metabolic rates and lifespan, which show a similar relationship.

BTW, the average heartbeat rate for hunter-gatherers might be closer to 60 – I’d be surprised if it wasn’t considerably lower than the average today, just due to getting more exercise. Not that it matters much.

[NPham2005]

@njtt

Rejected. - But to say it that way is a bit misleading because it suggests that the idea was, once, at least seriously entertained by at least a few biologists or cardiologists, and I am pretty sure it wasn’t.

Wrong. It has been widely accepted since at least the 1960s.

@lazybratsche

That’s not really true. When you see “life expectancy” in the 30s or 40s for premodern societies, it’s usually due to infant and early childhood mortality. Individuals who survived childhood mortality could expect to live 50 or 60 years, even in societies without anything approaching modern medicine.

Like I just said, there is strong evidence that there has been selection for unusually long life span in humans. Menopause, for example, is not merely the gradual and passive loss of fertility with age. It is an acquired trait that is practically unheard of in other animals. Evolution has selected for individuals who survive as post-reproductive grandparents, presumably so they can provide some advantage to their children and grandchildren.

There is stronger evidence arguing AGAINST selection for longer lifespans than there is stronger evidence arguing FOR selection for longer lifespans. And if, by premodern societies, you mean societies prior to 30,000 BC, people could indeed expect to live to 50 or 60…rarely. Longer lifespans weren’t something that we originally had.

[NPham2005]

@Chronos

The figure of a billion heartbeats for other animals is also based on the extreme outliers of various species who are lucky enough to reach the maximum span for their species, not the average.

Surely, you can’t be serious? The spans I’ve seen listed for almost all the other animals are very much their regular lifespans.

@ac05

Actually, I believe the average heart rate of humans is 72bpm, not 60bpm.

72 bpm is not the normal heart rate. The majority of Americans are overweight, and the 72 bpm info comes from this. Being overweight is not normal. Mr. Bill and LearJeff are correct; the normal heart rate, believe it or not, is actually 60 bpm. Most mammals hearts beat about a billion times (I came up with a figure of 997,199,304 times but I will have to get more confirmation on this specific figure), and with a 60 bpm, humans would reach a billion heartbeats at the approximate age of 31 years, 252 days, 1 hour, and 34 minutes. The natural age meant for human reproduction, and thus adulthood, is age 15 (or about 15.7 years). A billion heartbeats would have been just the right amount of time for a person to raise a child into adulthood. For example, If Blah had a child at age 15 (the age nature intended), then Blah would have 15 years (ages 16-30) to raise that child until the child turns the adult age of 15 (and likely produces a child of his/her own in the very same year). Blah could then die happy a year later at age 31. Nevertheless, we now live longer than our original lifespan of 31 years because we now have safe drinking water, we wash our hands, and we have modern medicine. So if you’re 31 or older, you’re an old person.

[Mijin]

@Carl_Pham

Indeed, to be a scientist at all, you have to trust your own judgment and experience much more than the opinion of others. If you are the kind of person who can be convinced that something is true merely because lots of other people – even *everybody * else on the planet – say its true, then you are not a scientist. A scientist trusts only his own senses, and what is consistent with that experience.

I think that’s going a bit far, or you’ve just phrased it badly.
Scientists don’t have time to evaluate a significant proportion let alone every claim that is out there.

To take an extreme example, consider quantum mechanics. Virtually all scientists outside of the field of QM, have to accept most of the claims without further investigation, because the proofs may involve more complex mathematics than they require for their own field. Or just would take eons to evaluate.
And I’m sure you wouldn’t claim than QM is consistent with anyone’s experience.

So at one level or another scientists (and the general public) have to trust the scientific community, but of course this trust is nothing like blind faith or dogma. Scientists follow rigorous methods designed to refute incorrect models, and only accept claims that make useful predictions. As long as they are doing that, we have reason to have confidence in their findings.

[naita]

@NPham2005

Wrong. It has been widely accepted since at least the 1960s.

Cites?

[SciFiSam]

@Wendell_Wagner

There are two real correlations: First, smaller animals (well, all the examples given are mammals) tend to have faster heart beats. Second, larger animals tend to live longer. These two things just happen to combine to mean that, vaguely, animals tend to live about the same number of heart beats (about 100,000,000 to 10,000,000,000). Read Isilder’s link. There’s no reason to think that the number of heart beats is actually any limit on lifespans.

Jack Russel Terriers, small dogs, tend to live longer than Labradors, large dogs. I chose those two because they’re well-known dogs without any major defects due to their breeds, like if I’d chosen a pug and a Great Dane, which would both have predictable problems caused by their breed. Nothing in those two breeds makes them more likely to have breathing problems, arthritis, weight problems, etc, any more than any other domestic dog.

But the smaller one routinely outlives the older one. Jack Russells are more active, generally, than Labs too - I’m sure there are exceptions but the general case is true.

I think this might be an easier way to understand that heartbeats and/or size are not a be all and end or all thing.

[Wendell_Wagner]

Which is why I used the word “tend.” The notion that large animals tend to live longer than small animals is just a rule of thumb, just like the rule that mammals tend to live one billion heartbeats, within one order of magnitude. I wouldn’t have used the word “tend” if I didn’t mean that statement to be just a rule of thumb, not an absolutely valid statement.

[Andy_L]

@ac05

I’m curious about the theory that most animals, as a general rule, have the same number of heartbeats. In the 1970s, did the scientific community generally accept this theory, or did it generally reject the theory? And today, does the scientific community generally accept this theory, or does it generally reject it?

Previous discussion:

http://boards.straightdope.com/sdmb/showthread.php?p=14032554&highlight=asimov+heartbeat#post14032554

[Chronos]

Quoth NPham2005:

…and with a 60 bpm, humans would reach a billion heartbeats at the approximate age of 31 years, 252 days, 1 hour, and 34 minutes.

If that’s the approximate age, then what’s the exact age?

Seriously, if you’re making an approximation, then you don’t care about the difference between 60 and 72. That’s what “approximation” means.

[DSeid]

FWIW a 2014 editorial on the subject in no less than Circulation.

An increasing body of scientific research and observational evidence indicates that resting heart rate (HR) is inversely related to the life span among homeothermic mammals and within individual species. … Mammals that have a slower average HR tend to live much longer than those that have a faster HR.1,2 Although some variability inevitably exists and is observed in humans, estimations yield a mean value of ≈1 × 109 (1 billion) heartbeats in a lifetime across almost all homeothermic mammals … Although exercise itself elevates HR significantly, resting HR is significantly reduced, and overall total heartbeats over 24 hours are reduced as well. …

The meat of the editorial is more about the prognostic utility of heart rate variability and recovery of heart rate from exercise as signs of autonomic function, but that bit included, again FWIW.

[LSLGuy]

@Wendell_Wagner

… just like the rule that mammals tend to live one billion heartbeats, within one order of magnitude. …

I agree with your overall point. I’m quoting here something you said which deserves standalone emphasis.

Back to the OP’s contention … A rule of thumb which is considered “valid” within plus or minus one order of magnitude is pretty useless. To say something costs $1 plus or minus one order of magnitude says it usually costs between 10 cents and 10 dollars. That’s not really very useful “information”. A billion heartbeats plus or minus one order of magnitude predicts a human lifespan between roughly 3 and 300 years. No kidding Sherlock.
Said another way, I can correlate damn near any two truly random unrelated variables to within plus/minus one (which really equals two) order(s) of magnitude.

An idea supplied with no causal mechanism supplied and such limited predictive power can’t be called anything but a spurious observation. It doesn’t even rise to the level of well-formed anecdote, much less a theory.

[DSeid]

Logarithmic scale correlation is still correlation. Not causation but that does not dismiss its reality.

Put it this way. No mammal with an average heart rate over 300 has a typical lifespan over three years old. No mammal with an average heart rate under 100 has an average lifespan under ten years.

Within humans resting heart rate (RHR) correlates with lifespan as well, at least from cardiovascular disease, and there are mechanisms for causation that make sense. It seems to actually be at least partly a direct effect of the heart rate.

Faster resting heart rates (RHRs) are associated with shorter life expectancies.[1] Epidemiologic studies[2–19] have demonstrated that elevated RHRs increase the risk of mortality, from cardiovascular disease (CVD) in particular. The relationship has been demonstrated in diverse subgroups, including the general population,[2–11,19] hypertensives,[3,18,20] and those with established coronary artery disease (CAD).[12–17] Resting heart rate has previously been shown to fulfill several of the criteria for causality.[21, 22] In men, the relationship has been shown to be strong and graded.[4,8,9] Animal studies have demonstrated reduced rates of atherogenesis post experimental heart rate reduction.[23, 24] In addition, several biologically plausible mechanisms for the effect of elevated RHR have been proposed including the antiischemic and antiarrhythmic benefits of a low heart rate and the atherogenic hemodynamic effects of an elevated heart rate; a faster heart rate will necessarily impose more shear stresses than a slow one. …

… In men, on univariable analysis CVD, CHD, and total mortality rates increased with each successive increase in RHR quintile. Rate ratio comparing extreme quintiles was 3.87 (2.24–6.68) for CVD mortality …

… In women, on univariable analysis, the associations with mortality end points were also strong with a hazard ratio of 3.29 (2.32–4.65) comparing extreme quintiles for CVD mortality. However, in general, the relationships were less strongly graded than in men …

… In multivariable analyses (Table III), RHR as a continuous variable remained a significant predictor of CVD mortality after full adjustment. … To investigate whether elevated RHR was an independent risk factor or merely a marker of subclinical disease, we reanalyzed the effect excluding all fatal events that occurred within the first 2 years of follow-up. This resulted in virtually no difference in the hazard ratios … This analysis has clearly demonstrated the association between resting heart rate and CVD, CHD, and total mortality. We have confirmed the strength of the relationship. For example, in men, RHR >90 beats/min compared to RHR <60 beats/min was associated with an almost 2-fold increased risk of CVD mortality in men and 3-fold increased risk in women. This effect was independent and similar in magnitude to the risk associated with current smoking (data not shown). These estimates are similar to those of previous studies, which ranged from 1.5- to 3.3-fold increased risk in the higher RHR category.[3,4,6,8,10,11,25]

We have confirmed the graded nature of the relationship, particularly in men. The risk seems to increase more steeply in the highest quintile of RHR, in line with the findings of previous studies.[28] …

… Previously, there was doubt concerning the independence of the effect, particularly regarding possible confounders such as blood pressure, physical activity, and comorbidities.[19] This analysis has demonstrated that RHR is strongly related to other risk factors including smoking, SBP, BMI, total cholesterol, HDL cholesterol (inversely), and physical activity levels. Because of these associations, the effect of RHR on CVD end points attenuated on addition of the other CV risk factors to the model. However, despite this, the effect of RHR on CVD end points remained statistically significant even after inclusion of all of the risk factors, including physical activity as a categorical variable and persisted within strata of physical activity. In addition, the relationship remained after exclusion of those with comorbidities, which could potentially have been confounding the relationship. The effect of RHR was not substantially altered by the addition of waist circumference and triglycerides …

That is more than a “spurious observation.” The current evidence suggests that it may even be more than a correlation but of a causative nature.

[Chronos]

Besides which, the actual correlation is a lot stronger than just “within an order of magnitude”. It’s more like “within 20%” (aside from humans). When something ranges in price from 80 cents to $1.20, yeah, it’s fine to call it “about a buck”.

[Wendell_Wagner]

LSLGuy writes:

> A rule of thumb which is considered “valid” within plus or minus one order of
> magnitude is pretty useless.

Not true. Suppose there are two variables which both vary between 1 and 1 billion. Then it could easily be true that there might be some relationship between the two variables which is mostly valid but varies randomly by plus or minus one order of magnitude. What really matters is what the correlation coefficient is between the two variables. If you don’t know what the correlation coefficient is, learn about it here or just take a statistics course:

Pearson correlation coefficient

It’s necessary to do a mathematical calculation like this to really be able to say whether two things are correlated. (Note: Correlation is not causation. This calculation will only prove that the two things tend to vary together. Why they vary together is an entirely different matter.) I wanted to make this point because SciFiSam had mentioned the case of two breeds of dog where one is smaller but lives longer. That’s true, but it’s just an anecdote, not a mathematical calculation of the correlation between length of life and size. You have to look at the entire set of objects and do the calculation to determine how good the correlation is.

I only said that the correlation was only approximate to the nearest order of magnitude because I didn’t know the entire set of values for length of life and size, so I guessed about the amount to which they might vary. Chronos says that it’s more like 20% rather than an order of magnitude. To be able to really say how well this correlation works, it would be necessary to be given the entire body of data.

[LSLGuy]

That’s what I get for over-extrapolating from somebody else’s guesswork. :o

Thanks all for setting the record (and me) straight.