# What mechanism accounts for the cicadas' 17-year-life cycle? How are the years counted? **URL:** \ **Category:** Factual Questions\ **Created:** [June 3, 2013, 9:14am UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017 "2013-06-03T09:14:22Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [June 5, 2013, 12:12am UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/21 "2013-06-05T00:12:15Z")
There are quite a few broods; twenty-three by major count. Some are extinct, which is the real oddity to me. [Here’s a very good page](http://www.magicicada.org/about/brood_pages/broods.php) on the broods and cycles. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [June 5, 2013, 12:34pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/22 "2013-06-05T12:34:49Z")
> [@dracoi](#): > > One additional factor is predation. If 10% of your population comes out every spring, the local predators will get used to this and they’ll adjust their own migrations or breeding times to maximize the number they can eat. But if they have to wait 13 or 17 years, whole generations of your predators will live and die without having you available as food. I totally understand this, and even why this makes prime numbers advantageous. I was asking about the OTHER posited explanation, that the prime numbered cycles are an adaptation to avoid occasional cold summers during the glaciation period (and NOT an adaptation to counter predators). I’m not sure exactly how this is supposed to work, or why the predation hypothesis isn’t sufficient. I can only read the abstract linked above, and it’s not very clear to me. Is there anybody who can explain it? ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [June 5, 2013, 1:28pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/23 "2013-06-05T13:28:01Z")
Read my link. Prime numbers are good because they mean that any two broods with different prime-number breeding times will rarely appear in the same year (if they are in the same area). A 13-year and a 17-year brood will only appear in the same year once in 221 years. On the other hand, a 16-year and a 24-year brood will appear together once in 48 years. ---
**Author:** ![Darth\_Ayebaw](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darth_ayebaw/32/9870_2.png) [@Darth\_Ayebaw](https://boards.straightdope.com/u/Darth_Ayebaw)\ **Post date:** [June 5, 2013, 2:07pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/24 "2013-06-05T14:07:18Z")
> [@Sailboat](#): > > The glacial explanation for their periodicity makes sense to me (very quick and dirty version: the long prime number period between emergences dramatically decreases the odds of emerging during a cold summer and having a massive die-off that could lead to extinction…or, knowing how evolution works, massive die-offs DID lead to extinction for most of the the cicada broods that were not on this cycle). > > However, it’s been pointed out that the 17 and 13 year cycles have been pretty much perfectly maintained over 10,000 years since the glaciers retreated – predator population probably plays a role in maintaining the cycles (the prime number prevents the cicadas from falling into “resonance” with booms in predator population). > > Fairly detailed article here that might interest you: [Bugs that Count,\* American Scientist\*](http://www.americanscientist.org/issues/pub/2004/9/bugs-that-count/1&css=print) > > edit: **septimus** , the article above has something to say about the tree cycles: > > Also, the article contains some pretty interesting speculation about _humans_: Wait, why? It doesn’t affect the weather in any way, and I figure the odds of emerging during a cold summer are roughly the same for any given year, right? I mean they’d see less cold summers only as an effect of seeing less summers overall. But the percentage should be the same given that the odds of a bad summer are roughly the same for any given year, right? ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [June 5, 2013, 2:25pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/25 "2013-06-05T14:25:50Z")
> [@billfish678](#): > > A thought. > > Could it not be possible once in a great while a part of a brood doesn’t stick to their calendar? So over long periods of time new broods keep getting created while once in awhile the original brood comes out during a bad summer and gets killed off. But we only see the broods we see now because we are basically looking at a snapshot in time compared to evolution and history. Yeah – I’ve read that scientists think exactly such “splitting” has occasionally caused a new brood to form out-of-synch with the original brood. > [@CC](#): > > So why is the east coast now having the 17 year emergence, and here in the midwest, we had it a few years ago? Probably the above-mentioned splitting. Historical accident gets the new brood on a different 17-year cycle. The factors that make 17 years an effective cycle should work for _any_ given 17 years, and keep each of the now-separate broods on its own 17-year cycle. > [@KarlGauss](#): > > But how does this “dramatically decrease the odds of emerging during a cold summer and having a massive die-off” which was the context? > [@Quercus](#): > > I was asking about the OTHER posited explanation, that the prime numbered cycles are an adaptation to avoid occasional cold summers during the glaciation period (and NOT an adaptation to counter predators). > [@Darth\_Ayebaw](#): > > Wait, why? It doesn’t affect the weather in any way, and I figure the odds of emerging during a cold summer are roughly the same for any given year, right? I mean they’d see less cold summers only as an effect of seeing less summers overall. But the percentage should be the same given that the odds of a bad summer are roughly the same for any given year, right? I apologize. I once read a brilliant explanation of how the prime-number cycles reduce the odds of cold summers killing them off, but can not find it now. The math was dramatic – according to my memory, over some long period (either 1,000 years or the whole 10,000 year period since the glaciers) the odds of a brood being wiped out by cold summers was something like 90% for annual emergence, 50% for a ten-year cycle, and only 1-3% for a 17-year cycle…or something like that (I am pulling these numbers out of thin air, but the numbers in the article made a big impression on me and were not very far from these figures). The math, assuming the author was accurate, immediately made clear that the 17-year strategy (and to a lesser extant, the 13-year one) would have indeed increased the odds of a brood surviving, even if the details were inobvious to the mathematically-challenged. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 5, 2013, 3:07pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/26 "2013-06-05T15:07:02Z")
Even if your expected number of generations of survival is constant, a longer dormant period will result in survival for a longer amount of time. ---
**Author:** ![Darth\_Ayebaw](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darth_ayebaw/32/9870_2.png) [@Darth\_Ayebaw](https://boards.straightdope.com/u/Darth_Ayebaw)\ **Post date:** [June 5, 2013, 5:45pm UTC](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017/27 "2013-06-05T17:45:06Z")
I’m just wondering what the methodology there is. Maybe you can’t find a cite but do you remember any of the math, even vaguely? It’s just that, if I bury myself now, why would I have more luck emerging in any given summer than any other? It should be like flipping a coin, every new toss has roughly the same odds as the one before. [Previous page](https://boards.straightdope.com/t/what-mechanism-accounts-for-the-cicadas-17-year-life-cycle-how-are-the-years-counted/660017.md?page=1)