

A fascinating little beastie that loves prime numbers - jgrahamc
http://www.jgc.org/blog/2010/03/fascinating-little-beastie.html

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scott_s
_A prime numbered cycle means that the cicada rarely meets a predator (since
any predator would have to be synchronized to 17 years, for example) and mass
emergence would overwhelm any predator around._

I think that's half accurate. They meet many, many predators - hence the great
numbers overwhelming predators, as you point out. Their numbers are so great
that all existing predators can gorge themselves, and there's still plenty
left to mate.

Rather, their prime-number periodicity reduces the likelihood that a
_specialized_ predator will evolve. Many predators eat them, but none of those
predators have evolved to specifically eat those cicadas.

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btilly
Sorry, but you are wrong.

Generalized predators tend to have boom/bust cycles that are a small integer
number of years. When the boom aligns with the cicadas, it sucks for the
cicadas. When the bust aligns with the cicadas, it is great for them. Having a
non-prime period would synchronize that boom/bust cycle with the cicada's
cycle, and would make every generation a bad one for the cicada's strategy.
Eventually this would make the cicadas go extinct.

There is no possibility of a specialized predator evolving because said
predator would spend most of the time starving.

~~~
scott_s
_There is no possibility of a specialized predator evolving because said
predator would spend most of the time starving._

A specialized predator could evolve to have the same cycle as the cicadas.
Which is the point of the theory for why the cicadas have a prime-number
cycle: it decreases the chance that a predator species will happen-upon a
cycle that only sometimes aligns with the cicadas.

The cicadas themselves are so numerous that no general predator species will
have anything close to a chance to hunt all of them.

~~~
camccann
Also significant is that cicadas, while numerous, aren't very large. Predator
body size tends to be roughly proportional to their prey's body size, and
larger animals tend to live longer.

The net result is that the cicada cycle is a bit longer than the expected
lifespan of a hypothetical predator, making it even harder for such to evolve.

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machrider
I was half-expecting the prime-number-shitting bear:
<http://alpha61.com/primenumbershittingbear/>

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jrockway
Not what I was expecting. But upon reflection, that page is exactly as
advertised.

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defen
I was living in Princeton when these little bastards made their appearance in
2004. Just one bug can produce an impressively loud sound - so you can imagine
what millions sound like. And there's nothing like walking to work through
heaps of bug carcasses.

~~~
jrockway
Yeah, I hate experiencing nature for one week every 17 years. We should agent-
orange the trees they are feeding on, and stop the cycle forever.

Let's also pump a bunch of toxic gas into the atmosphere to block out the sun
and stop global warming. What could go wrong!

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rthomas6
Yes, I'm sure that's exactly what he meant.

Chill out man. People are allowed to think things are fascinating and kind of
annoying at the same time.

~~~
boredguy8
Human behavior, for instance.

~~~
jrockway
Yes, the human behavior where humans pretend they aren't one to elevate
themselves above others _is_ amusing.

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btilly
It is interesting, but I'm curious how a paper got written on this in 2001 and
accepted. It is the same explanation that Stephen J. Gould wrote a popular
essay on many years earlier.

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ANH
There's nothing like cycling every day through a cloud of cicadas for a couple
of weeks, as I did in Virginia in 2004. Not only are they noisy before mating,
but they make a very loud _thwack_ when hitting a bicycle helmet.

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Mz
And this is comment number 17!

(It would be funny if the discussion stopped here. :-D)

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itistoday
_A prime numbered cycle means that the cicada rarely meets a predator (since
any predator would have to be synchronized to 17 years, for example) and mass
emergence would overwhelm any predator around._

I don't understand. That sentence doesn't explain why it can't be a non-prime
number like every 16 years, or every 18 years. It looks like part of the
argument is missing.

~~~
jgrahamc
Well suppose it's not a prime number then if you have some predator whose
cycle is a factor of the cicada cycle then it's going to meet up with the
cicada every time.

Suppose you have an 18 year cicada, it has factor of 2, 3, 6, 9 so any
predator that peaks every 2, 3, 6 or 9 years can synchronize with the cicada
and be around to eat it.

But if the cicada has a prime numbered cycle then it greatly reduces the
chance of synchronizing with a predator.

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Daniel_Newby
The prime numbers have nothing to do with predators. Predators require regular
feeding from live prey. A prey species that hides for even one year causes
significant starvation in predators that could otherwise come to depend on it.
This effect is maximized after only a handful of years of hiding. No lizard or
bird could survive on stored fat for more than one or two years.

The prime numbers come from competition between strains of cicadas. First
consider that genetic timers are not perfect: most cicadas may emerge after N
years, but a fraction will emerge in N-1, N+1, or even farther years due to
randomness in genetics and the environment. By blind chance, occasionally an
early group will become plentiful, causing a predator population boom that
smashes the population of the "correct" year. It's like a badly-made gear that
occasionally jumps one tooth forward.

Second consider what such jumping does to strains that are not relatively
prime, such as 6 and 12 year cicadas. Eventually the random chance effect will
synchronize them so that they emerge in the same year. Because these year
skipping cicadas all evolved from the same parent species, they will be
closely related. This creates a cross-mating problem if they emerge at the
same time, causing matings to either produce no offspring, or to produce
hybrids that are synchronized with neither period. There may also be problems
with excess damage to the host plants. Therefore evolution favors strains that
synchronize as infrequently as possible. That means prime numbers.

IIRC a few years back some scientists created a simulation with plausible
numbers for fertility and hybrids, and the result was spontaneous emergence of
the prime number periods for the observed strains of cicadas.

Edited for clarity.

