I find it curious that nature actually chooses to adhere to this
formula by enhancing elephant cell efficiency. There is no obvious
reason to even do this. This is a subtle question but possibly an
important question.
What is plain is that the three quarter power rules prevails for life
as we know it or at least for two standard deviations.
Since we are now managing to reverse aging or at least to slow or
even halt it, it will be interesting to observe the effect on this
rule. Real reversal may run into something more subtle.
Nature Has A
Formula That Tells Us When It's Time To Die
By ROBERT KRULWICH
January 22, 2013
We wax, we wane. It's
the dance of life.
Every living thing is
a pulse. We quicken, then we fade. There is a deep beauty in this,
but deeper down, inside every plant, every leaf, inside every living
thing (us included) sits a secret.
Below the pulse, which
you see here, elegantly captured by Shanghai photographer/designer
Yunfan Tan, is a life/death cycle, a pattern that shows up in the
teeniest of plants, (phytoplankton, algae, moss), also in the bigger
plants, (shrubs, bushes, little trees) — and even in the biggest,
the needle bearing giant sequoias.
Everything alive will
eventually die, we know that, but now we can read the pattern and see
death coming. We have recently learned its logic, which "You can
put into mathematics," says physicist Geoffrey West. It shows up
with "extraordinary regularity," not just in plants, but in
all animals, from slugs to giraffes. Death, it seems, is
intimately related to size.
Life is short for
small creatures, longer in big ones. So algae die sooner than oak
trees; elephants live longer than mayflies, but you know that. Here's
the surprise: There is a mathematical formula which says if
you tell me how big something is, I can tell you — with some
variation, but not a lot — how long it will live. This
doesn't apply to individuals, only to groups, to species. The formula
is a simple quarter-power exercise: You take the mass of a
plant or an animal, and its metabolic rate is equal to its mass taken
to the three-fourths power. I'll explain how this works
down below, but the point is, this rule seems to govern all life.
A 2007 paper checked
700 different kinds of plants, and almost every time they applied the
formula, it correctly predicted lifespan. "This is universal. It
cuts across the design of organisms," West says. "It
applies to me, all mammals, and the trees sitting out there, even
though we're completely different designs."
It's hard to believe
that creatures as different as jellyfish and cheetahs, daisies and
bats, are governed by the same mathematical logic, but size seems to
predict lifespan. The formula seems to be nature's way to preserve
larger creatures who need time to grow and prosper, and it not only
operates in all living things, but even in the cells of living
things. It tells animals for example, that there's a universal limit
to life, that though they come in different sizes, they have roughly
a billion and a half heart beats; elephant hearts beat slowly,
hummingbird hearts beat fast, but when your count is up, you are
over. Plants pulse as well, moving nourishment through their veins.
They obey the same commands of scale, and when the formula says
"you're done," amazingly, the buttercup and the redwood
tree obey. Why a specific mathematical formula should govern all of
us, I don't completely understand, but when the math says, "it's
time," off we go …
Of course these rules
do not tell any particular bee or dog or person when they are going
to die. Every individual is subject to accident, caprice, luck. No,
this is a general rule. It governs species. Modern humans have
managed, because of medicines and hygiene, to become an exception,
but 50,000 years ago, we were probably part of the pattern. If you're
interested in quarter power scaling, you can check out "Of Mice
and Elephants: A Matter of Scale," by George Johnson or go back
to an earlier blog post I wrote here. But to summarize,
nature goes easy on larger creatures so they don't wear out too
quickly. An elephant has trillions more cells than a shrew, and all
those cells have to connect and communicate to keep the animal going.
In any big creature, animal or plant, there are so many more
pathways, moving parts, so much more work to do, the big guys could
wear out very quickly. So Geoffrey West and his colleagues found that
nature gives larger creatures a gift: more efficient cells.
Literally.
The cells in an
elephant do more work in a minute than the cells of a mouse. That's
why an elephant cell can beat at a slower rate than the rattatat-tat
of a mouse cell. Both wear out after a billion and a half beats, but
the elephant does it more slowly. As for the peculiar quarter power
scaling differences, that rule emerges from the data when you plot
the different lifespans of animals or plants on a graph. Notice how
plants, big and small fall along the same quarter-power line? Here it
is, from a paper by Marba, Duarte and Agusti, cited in my blog post.
Yunfan Tan is a
young Shanghai artist/product designer who calls these short
animations "Dancing Leaves." He graduated college last June
(DongHua University, Shanghai), went to work for some American ad
agencies and is now on the web with something new seven times a week.
He calls this project "Make Something Cooool Every Day."
I've been a little
surprised by some of the comments posted down below. A bunch of you,
mostly scientists, almost always biologists, have suggested that
Geoffrey West's ideas, or my version of his ideas, are so simply
stated, or based on such flimsy or fudged evidence, that the
relationship between size and mortality is not to be trusted.
Many of you admit that
size may correlate with lifespan in theory, but in the real world,
animals get crushed, eaten, get diseases and size doesn't say
anything about that. Others of you think the data is rigged, and
animals or plants that don't fit the pattern have been spliced out,
or that the size/mortality correlation is too vague to be trusted.
"Congratulations," says reader Tom Benton, "You
plotted some data points and then ran a regression. I see nothing in
this article about its explanatory power."
Well, the "you"
being charged here isn't me. I didn't do the correlations, but I did
decide to write about them, and chose to treat them with certain
respect. And here's why: Yes, West's work is controversial. But it's
hardly new. He was inspired by an earlier biologist, Max Kleiber, who
worked at the University of California, Davis, in the 1930s, and
first noticed that the metabolic rate of a creature is equal to its
mass taken to the three-fourths power. So this idea has been around
for a long time. It was West and his colleagues at the Santa Fe
Institute who in the late 1990s (and others in this century) who
mapped it onto real creatures.
'That's Not Science,
That's Just Taking Notes'
Of course, there are
many exceptions to their correlation. Many of you point them out.
Little birds live longer big dogs; some parrots live longer than
lions. When Professor West published his paper in Science, biologists
all over the world peppered him with "But what about ... [this
animal]?" objections. He called them "quibblers," and
told The New York Times Magazine, "There are always going
to be people who say, 'What about the crayfish?' ... "Well, what
about it? Every fundamental law has exceptions. But you still need
the law or else all you have is observations that don't make sense.
And that's not science. That's just taking notes."
West is a physicist.
Big pictures interest him. Small stories don't. He's not the gentlest
of beings when criticized, but science has never been a gentle
occupation. It may be that many of you biologists writing in are
irritated by an overreaching physicist. But I notice you also aren't
crazy about an over-simplifying reporter. If what I have written
seems outright wrong, or too sloppy to be credited, I welcome your
corrections. I try to read everything that people write in, and when
I make an error, I try to correct it. This is, as many of you point
out, a blog, not a news column, and I'm here to give people a taste
of ideas that excite me, images that fascinate. I am here to share,
to tickle, to amaze; not to lecture.
But that said,
Professor West's paper on quarter power scaling has been cited by
other scientists more than 1,500 times since it was published. By any
measure, that's a lot. The New York Times called it one of
the most "influential papers in modern biology." So while
it may irritate, this idea has weight.
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