Scientists Probe the Mystery of Animal Lifespans

Why do some animals live so long while others sputter out so quickly?

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ALISON STEWART, host:

When it's your time, it's your time. But why do some animals have nice long lifespans while others sputter out so quickly?

NPR's Robert Krulwich has been mulling all this, and found a scientist who believes he's discovered why there was such a difference between how long different animals live.

ROBERT KRULWICH: Let's begin with mice - you know, ordinary field mice living in a barn. I asked Professor Geoffrey West, how long do mice live? Just an approximate number.

Professor GEOFFREY WEST (President, Santa Fe Institute): That number would be just two or three years.

KRULWICH: Okay, so that's two or three years for a mouse. Now, I want to conjure up an elephant. Elephants normally live for 60 years. So I need my elephant, please.

(Soundbite of an elephant sound)

KRULWICH: They're up to 60 years old, right?

Prof. WEST: That's one way of saying it - yes. Yeah.

KRULWICH: And now a lion…

(Soundbite of a lion roaring)

KRULWICH: …also a mammal, by the way. A lion…

(Soundbite of a lion roaring)

KRULWICH: …they last about 15 years.

Prof. WEST: Yes.

KRULWICH: And a squirrel, they last about five years. So here is my question: Why these very different lifespans? Why do different animals live for different lengths?

Prof. WEST: So let me just tell you. All my work in biology stemmed from asking that kind of question. Where do those numbers come from?

KRULWICH: So you start with the same ingredients, that living things come from cells…

Prof. WEST: Right.

KRULWICH: …and that the cells seem to have different timetables, depending on (unintelligible).

Prof. WEST: That's right. Why is it that if those cells are put together to form a rat, that the timescales are different than if it's put together to be a human being or an elephant?

KRULWICH: Yeah. Why? Why do elephants live longer than rats? Well, Geoffrey West thinks he has found the answer. With his colleagues at the celebrated Santa Fe Institute in New Mexico - it's a science think tank that he runs, right?

Prof. WEST: I am the president of the Santa Fe Institute, indeed.

KRULWICH: Okay. Professor West says that he and his team have discovered why different creatures die at different times - and even more amazing, he can predict when each species should die, on average.

Prof. WEST: Yes.

KRULWICH: Well, wait a second. Wait a second. Has anyone else found a mathematical logic like this that runs to all living things? That's kind of…

Prof. WEST: Well, this is it.

KRULWICH: And here is how it works. All living things take in energy.

(Soundbite of rustling sound)

KRULWICH: We eat food, and food has energy in the form of calories, so the energy in the food then has to get passed around to all the cells in my body. And to do this, in humans, we get help from our hearts. Hearts are our pumps. And those pumps push blood and fuel and oxygen around our bodies.

Prof. WEST: That is the fantastic observation.

KRULWICH: If you look at all creatures with hearts, starting with the big ones…

Prof. WEST: If I take the heartbeat of a whale…

KRULWICH: …which is very slow. And then you wait and wait - and there you go.

Prof. WEST: It beats once every maybe…

KRULWICH: …maybe every three seconds or so. Yeah and the next one…

(Soundbite of heartbeat)

Prof. WEST: …whereas if I listen to a shrew, which is the smallest mammal…

KRULWICH: A shrew's heart beats so much faster. Instead of 10 to 20 beats per minute…

Prof. WEST: It's over 1,000.

KRULWICH: Sometimes way over 1,000. Now, it is no big surprise that little creatures have fast beating hearts. We know that. But when Geoffrey West's team looked closely at individual species…

Prof. WEST: One of the things that comes out of the theory is this astonishing fact that even though the shrew only lives a year or two and an elephant lives 80 years, they all use the same number of heartbeats.

KRULWICH: Estimating from sample heartbeats, scientists calculated the oldest elephants on record last for about a billion and a half heartbeats. That's their limit. Surprisingly, the oldest shrews on record, they also last about a billion and a half heartbeats. That's their limit. And the oldest lion, a billion and a half heartbeats. And the oldest rat, a billion and a half heartbeats. And the more they look, the more they found this same number, meaning that a rat and an elephant, while they have very different lifespans -they have, again, roughly, the same number of beats. It's just that the elephant's beats are slower.

Prof. WEST: The elephant's beats are slower.

KRULWICH: So the elephant lasts longer. But the limit, the magic number is…

Prof. WEST: A billion and a half, roughly. Roughly.

KRULWICH: A billion and a half beats, and then you're over.

Prof. WEST: Then you're over.

KRULWICH: Huh.

Prof. WEST: It's almost like, you know, if you had a lot of the three score and 10 or something…

KRULWICH: Yeah.

Prof. WEST: …the biblical…

KRULWICH: Who knew there was this deep symmetry inside all animals?

Prof. WEST: I certainly didn't, but I came across it. I was so intrigued. I don't know what words to use - sort of primitively exciting about it. It somehow was - it must be what makes people religious when they, you know - I don't know what it is. I don't know what to say, exactly. I'm not a religious person. But God almighty, you know, that's what - if these laws are true, what they're telling you is there is an extraordinary unity to all of life.

KRULWICH: And yet, we die a different times, says Professor West, because we come in different sizes. Lifespan, it turns out, is very connected to size. The bigger a species in general, the longer it lives.

Prof. WEST: That's correct.

KRULWICH: Okay, set this up. I'm going to bring a small cat into the room, weighing maybe five pounds, a lovely little cat.

(Soundbite of cat screeching)

KRULWICH: I can be a little lovelier than that. Let me - a little…

(Soundbite of cat meowing)

KRULWICH: Yeah, better. Good. And then into the same room, I'm going to bring an ordinary-sized cow.

(Soundbite of a cow mooing)

Prof. WEST: Yes.

KRULWICH: Now, everybody knows that a cow is bigger than the cat. But here is something you don't know. If you take a cell from a cow and then you take a cell from a cat…

(Soundbite of cat meowing)

(Soundbite of laughter)

KRULWICH: So now you've got two cells. They are more or less identical.

Prof. WEST: They look the same. They act the same. They do same - they have the same functionality.

KRULWICH: But when you look closely, there's a big difference. It turns out that a cow cell is much more efficient. A cow cell will do its business. It'll make proteins, take things in, push things out. Take things in, push things out. Take things in, push things out. It does this so efficiently, you can do a lot of work in a relatively few beats. While the cat cell, to make the same number of proteins, it has to go…

(Soundbite of rustling sound)

Prof. WEST: You would find that the cell in a cat has to work much harder.

KRULWICH: So while the cat cell has to pump and pump and pump, a cow cell will take it easy - real easy.

(Soundbite of rustling sound)

Prof. WEST: So what that says - that the cow is more efficient than a cat.

KRULWICH: Apparently - and this is true across all animals and even plants -the bigger the creature, the more efficient its cells. It's nature's gift. You're big. You have more cells. You got to cut down on the wear and tear, and we do that by giving you cells to work a little less, a little slower. So that's the formula. The bigger you are, the slower you go. So bigger creatures wear out more slowly.

Prof. WEST: The pace of life is much slower.

KRULWICH: In the bigger animal.

Prof. WEST: In the bigger animal. In a very - in a calculable, predictive way.

KRULWICH: Big things last longer most of the time, but not all the time. There's one rather crucial exception.

Prof. WEST: That is human beings. Because if you look at all the data, everything I've talked about, about lifespan, we are actually - we, today, are in violation.

KRULWICH: After all, we, human beings are much smaller than elephants. So, according to the theory, we should live shorter lives than elephants. But now, more and more of us are living elephant-long lives. We didn't used to. But now we live 60, 70 years. There's a reason for this change, says Dr. West. We humans have invented medicines.

Prof. WEST: And probably, most importantly, we use hygiene.

KRULWICH: Because we wash and drink clean water and therefore don't get as many parasites or diseases, we don't wear out as quickly as we used to. And with medicines, we now know how to repair ourselves better. So our size no longer determines our average lifespan. We've broken free. But while we are an exception…

Prof. WEST: On the average, it pretty much fits.

KRULWICH: And it's a powerful formula. The size of a creature predicts a lot, says Professor West.

Prof. WEST: I can tell you how many children, how many offspring it's expected to have, the gestation period. I can tell you its heart rate. I can even tell you what its growth (unintelligible) looks like…

KRULWICH: All the way up to - and including the big one - when, on the average, it's likely to die.

Prof. WEST: Yes, exactly.

STEWART: Discovering that size does matter, NPR's Robert Krulwich.

(Soundbite of music)

STEWART: That's it for THE BRYANT PARK PROJECT. Thanks for listening. We're always available online at npr.org/bryantpark.

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