Why do animals have different life spans?

Is there some biological reason that my dogs, for example, may only hope to last about 15 years - with good fortune, exercise, genetics, and nutrition - whereas I, under similar conditions, might last about 100 years?

Thanks for the straight dope.

I recall an essay, I think by Isaac Asimov, that observed that with one exception mammals (or perhaps placental mammals) have an average lifespan of one billion heartbeats. That is, shrews with frenetically fast heartbeat rates have about a billion heartbeats a year and tend to die the year after their birth. Elephants, with massive hearts that pump large amounts of blood more slowly, average about 13.3 million heartbeats a year, and live until 75. Note that this formula does not AFAIK apply to birds or reptiles. And the single exception? Homo sapiens, with a life expectancy of three billion heartbeats.

Asimov stole that from me. I myself first made that observation and I wrote a paper on it in the early 1960’s. But thank you anyway for your mentioning of my observation. My paper further went on to track variations, or lack thereof, among varying lifespans of different humans who had various levels of lifetime activity, various modal heart rates, and normalized to an average total number of human heartbeats factoring out accidents and some diseases.


Aging is programmed into animal species at certain rates. We are meant to be born and die at a certain time based on the evolutionary strategy of the species. Some species do very well having lots of short-lived offspring that reproduce quickly whereas humans do better with a long lifespan that takes time to develop but offers long opportuntities for reproduction especially for males.

There is a whole lot of interesting work being done on this. The current breakthroughs look at telomere shortening which determine how many times cells can be copied before they lead to enough DNA errors to kill the individual. Think of it like a photocopy you would make of an original document. You copy it and then make copies of the copies until it isn’t similar enough to the original to be useful anymore. That is what telomere shortening does and what current research is trying to work around. Aging is probably treatable or reversible. It is likely there will be some more breakthroughs on that in the next couple of decades but they have already started. Scientists are currently experimenting with an enzyme called telomerase which can slow down aging caused by telomere shortening a great. This doesn’t solve all problems however. Individuals can still get physical damage like osteoarthritis and tooth decay that are just products of time itself.

There are many good articles on this if you interested.

Cite? What journal was it published in?

No offense, but I second the “cite” request. This sounds a lot like my mom, and she’s schizophrenic. Always claiming she wrote whatever song happens to be on the radio at the time (from the 80’s). She’s got quite a convincing backstory too. It’s just not believable given … um … every single other thing in her life.

I’d love to read a paper as interesting as the one mentioned here. Where’s it at?

Since I am adament about not giving away my identity by revealing cites of my own publications, I think it is only fair to allow Susanann the same privilege.

She shouldn’t claim credit for it and also accuse a scientist of stealing from her without being willing to back it up. Sorry.

She can, but it’s fair to disbelieve it as well, especially since in my experience on the board Susanann has never shown any indication of any expertise in biology. In any case, she’s given enough information now that I could find an article in the peer-reviewed literature through Google Scholar if I chose to make the effort, if she was indeed the first to make that observation. There are a number of recent articles that have documented the figure of 1.1 billion heartbeats for most mammals; checking their references (which I don’t have access to at home) should turn up early citations for the idea. However, I find Susanann’s claim sufficiently tenuous that I don’t think it’s worth investigating without more information.

If Susanann cares to, she can send me the citation by e-mail or PM, and if it checks out, I’ll vouch for it.

It appears that Asimov published the observation, which I recall reading, in Of Time and Space and Other Things (1965), a collection of 17 of his essays from Fantasy and Science Fiction.

Thanks for this. I’ve noted that my dog has a higher normal body temperature than humans; is body temp correlated?

I’ve also read about people adhering to low calorie diets on the theory that it slows down cell reproduction.

Fascinating stuff, much like the delusions in Susann’s head. :smiley:

Of course, even if Susanann did come to that conclusion first, it’s still possible that Asimov discovered it independently. Literature searches weren’t as easy in those pre-Internet days, and it’s quite possible that even an expert at it like Asimov might have missed a paper.

It’s not quite true, anyway, though. We’re not the only outlier; the other great apes are also significantly ahead of the pack (though not by as much as us).

[Moderator Note]

I don’t think remarks of this kind are appropriate for GQ. If you want to remark on Susanann’s possible mental health, please take it to the Pit.

General Questions Moderator

Aside from heartbeat, my understanding is that the time period necessary to procreate a new generation that is self sufficient also plays a role in aging.

So (generically) we live healthy lives long enough for our kids to become mature, responsible adults. After that our health starts falling apart since there is no reason for us to stick around and no evolutionary pressure to keep us alive (a grandparent who dies will not affect the survival of self sufficient young the way a young parent of young children will put their kids at a disadvantage). So natural selection doesn’t select for people who can live longer than it takes for their kids to become self sufficient.

However I’m sure there are tons of holes in that theory. For one, many animals don’t seem to care one whit about their young so I have no idea how they age (why frogs last about 10 years when all they do is lay tadpoles they don’t follow up on, etc). I think only mammals protect and nurture their young after they are born. I have no idea how aging works for other species in that scenario. For another the general life span for humans now is about 80 (that is with modern medicine, protection from microbes and good nutrition). Good nutrition and protection from microbes alone will push life expectancy to 70 or higher. And historically most people have started having children around age 13 so a person could be a grandparent by age 26. I don’t know why they’d need to live to be 70 if they are a grandparent at 26.

So great tortoises hearts beat super slowly? I thought elephants’ hearts beat slower than humans and they do not live way longer than humans.

Forgive my ignorance, but it seems like little mammals have fast beating hearts, while larger ones have slower beating hearts. How fast does a blue whale’s heart beat and how long do they live until a natural death?

As has been mentioned, humans live longer than expected based on their size and heart rate.

Here’s a graph of the relationship between lifespan and total heartbeats in mammals. And here’s an article documenting the relationship.

Large whales may have a heart rate of 20 beats/minute and live 80 years. See here for some charts.

Tortoises are reptiles, and Polycarp singles out:

Though having said that, it wouldn’t surprise me if giant tortoise hearts did beat slowly.

The idea that aging is linked to reproductive age makes no real sense at all. The vast majority of mammals will begin breeding by their second year, at the latest. Wolves, lions, buffalo, mice, three toed sloths, duck billed platypus. They are all naturally grandparents by the age of 4 and great grandparents by the age of of 6. While two or three mammal species do indeed only live to produce one set of offspring, it is very much the exception. The vast majority of mammals will reproduce multiple times, with the earlier offspring invariably being older than their own uncles and aunts.

There is no correlation between reproductive age an lifespan anyway. While there is a very approximate correlation between size and lifespan, and an approximate correlation between size and reproductive age, the correlation between lifespan and reproductive age is much weaker than either. Leopards, rabbits, pigs and housecats have the same reproductive ages, yet the lifespans differ wildly.

Even if there were some sort of correlation between reproductive age an lifespan, it doesn’t make any sense to suggest that the “time period necessary to procreate a new generation that is self sufficient” plays any role. Since mice can produce 10 generations in a typical “wild” lifespan, cats can produce 10 and horses 15, why would the time period necessary to procreate a new generation that is self sufficient play any role in lifespan? Why not just keep reproducing for a hundred years?

Even for mammals, it makes little sense. A buffalo only cares for its calf for 6- 9 months, yet lives for 10 years and breeds every year and the young themselves breed in their second year.

As far as we can tell, the average age of first childbirth for women has been around 16 throughout history. I am unaware of any time or place where 13 would have been considered other than shocking. Remember, until recently menarche was 14-15, so women would have been physically unable to have children at 13. Even today I imagine that >50% of women would be unable to conceive at 13. Age of first parenthood for males is harder to know because it leaves no osteological evidence, but written records and common sense both suggest that it was several years older than that for women, so probably ~18.

Another point to note is that life expectancy is not the same as life span. Life span is related to aging, life expectancy is not. To highlight that, the life expectancy of a turtle is about 2 months. The life span is about 150 years. For antelope the life expectancy is about 2 years, the lifespan is about 10. This is equally true of humans.

While the life expectancy without modern medicine was about 40, the life span was always around 80, or as someone said 3000 years ago: “The days of our years are threescore years and ten; and if by reason of strength they be fourscore years”. As far as anyone can tell, modern medicine has not increased the human life span by even a single day. The point being that life expectancy isn’t at all related to lifespan or aging. Animals can have high lifespans yet much lower life expectancies than animals with low lifespans.

I’m not saying I believe her. I think it’s unfortunate she made a specific claim that it impossible to judge without her input. I’m careful to separate specifics from the generalities about the writing business. She did make a bad decision.

However, Asimov was no longer a working scientist when he was writing that column. It originally appeared in the Feb. 1964 issue of The Magazine of Fantasy and Science Fiction, BTW, so that sets a date. Most of his columns were based on his reading of what was happening in science around that time. He rarely gave any sources for anything, though. It is certainly plausible that he read a paper with the heartbeat data in it - or even a reference to such a paper - and then expanded it for use as a column. He obviously took his data on animals’ lifespans and average beats per minute from some outside source. The total number of human heartbeats in a lifetime was something that had long been known - references predate Asimov by decades in Google Books (I haven’t tried to use Scholar); the only issue was who put it together and when. Many people could have done so by that time. In fact, he cheats a bit to make it more dramatic by using a 4 billion maximum when most earlier references gave the total for the average human as 2.5 billion, a number he acknowledges.

Asimov could have thought of it first. It’s exactly the type of number play that he loved in his columns. Or he could have read it elsewhere and exploited it, which he also did frequently. That’s how you get all those essays. I just don’t know.

His number was higher because he was using absolute maximum ages for all of his animals, not typical ages. IIRC, his figure for humans was based on something like a 115 year lifespan.

Thanks for the links to all the charts. This is really interesting.

Humans live longer because we maintain our health, then? I guess that makes sense.