The title basically says it all:
What mechanism accounts for the cicadas’ 17-year-life cycle (or 13-year)? In particular, how are the years counted?
Thanks!
How they’re counted? One, two, three, four, and so on. (Emergence in 2013, means next emergence in 2030.)
As for the evolution, it’s to do with complicated things that confuse me:
What physical mechanism counts (and keeps track) discretely in integers?
I don’t think that’s been determined yet.
You need seventeen steps each triggered by a seasonal change like spring. Didn’t find the exact mechanism in the one cited article from the wikipedia page I could bother reading, but it had these two hypotheses:
http://hydrodictyon.eeb.uconn.edu/projects/cicada/resources/reprints/Williams%26Simon_1995.pdf
Recently I read, though I won’t hunt up a cite, that they somehow count annual tree cycles. In an experiment the cicadas emerged a year early when the researcher mimicked an extra life cycle of a tree.
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*
edit: septimus, the article above has something to say about the tree cycles:
Also, the article contains some pretty interesting speculation about humans:
Can you explain this to me? I read the abstract in the wikipedia cite, and it wasn’t helpful.
From what I gather, the first part says that if there are randomly-occuring cold summers, waiting underground a decade or so means that you can ignore 9 out of 10 summers, so you’ve got a better chance of having your lineage survive over a century. Which makes perfect sense.
But I don’t get how having the entire population emerge on the same year helps in this situation. A stable population would have about 1/10 of the population emerges in any given year. That way, a cold summer is guaranteed to kill 10% of the population, but won’t wipe it out completely.
What am I missing?
either emerge either when it’s clear the current summer is warm, or don’t emerge as a group, but rather spread out over different years so that someone will get a warm summer.
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.
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.
But it’s also worth pointing out that there are multiple broods. If you look at the Wikipedia page, there are broods that come out virtually every year, just in different places. And the 13 vs 17-year timing is a case of dominant and recessive genes. So… even if all the 17-year cicadas in an area are wiped out in one particular year, that area could still be populated by 13-year broods on a different cycle, or cicadas from other regions might wander in and start a new brood in the now-empty area. This means that individual broods are at high risk of dying out, but the species as a whole is reasonably safe.
Overwhelm any predator with sheer numbers.
Some predator insects will themselves be on cycles, e.g. 3 or 4 years. With a 13-year cicada cycle, such a predator will need to wait 13 of its cycles for its next taste of the cicada … and will “forget” how good they taste. 
With a 12-year cicada cycle, the predator would need to wait only 4 or 3 of its cycles. Thus the advantage of a prime-number cycle.
But how does this “dramatically decrease the odds of emerging during a cold summer and having a massive die-off” which was the context?
Back to OP my question: that is very, very interesting - the use of natural cycles (possibly of the xylem) of deciduous trees. That would account for the mechanism for “discrete integer counting”. But what physical mechanism accounts for the 17 year cycle? What mechanism allows the cicada genes to respond to 17 distinct changes, and only 17 distinct changes, of the xylem (or whatever)?
Anyone who knows the answer to that, and can prove it, stands to be a major figure in the accumulation of scientific knowledge.
Indeed. It is reminiscent (in more ways than one) of the gap that is our understanding of the mechanisms that trigger puberty, i.e. something somehow keeps track of how old we are and then, somehow, sets puberty in motion. Other than that, it’s clear.
From some article about this I read once… (as usual)
Cicadas do occasionally mutate and emerge a year or to early. If so, then they are out of sync with their fellow cicadas and less likely to find a mate and reproduce; plus, they are more easily decimated by any predators since thier population is small.
The 17-year mechanism allows them to avoid continuously emerging during a bad season. If they emerged every year, then a sequence of bad summers could decimate them (el nino, major volcanic eruption sunspot cycles, dust bowl, etc. maybe affecting weather or food supply for a few years). If there is a climactic downturn, odds are it is over 17 years later.
The very long period means that there are not any significant predators that are mainly dependent on cicadas as primary prey and waiting for them to emerge. Predators would starve the other 12 years. When cicadas do emerge for a short time, it is in such large numbers that predators cannot make a significant dent before they compelete thier breeding cycle. Then those predators starve and decline for the next decade or more.
As mentioned above, any predator that did not already have the extremely long dormancy period of exactly 17 years would have trouble evolving to that number.
Is it possible for cicadas to have a 100-year cycle? Would there be a big advantage in such a long dormant period?
possibly 97 or 101 years, for the prime number.
Thank you. The idea that they were less likely to encounter a single bad summer with the cycle never made sense to me. The cardinality of both sets of infinity (all positive integers and all positive integer multiples of 17) are the same. It would only at best be a delaying tactic, and, over the timescales involved, this would be irrelevant.
But if it takes multiple bad summers–since bad summers do have some correlation to each other–it does make sense that the probability of being wiped out is lower.
Cicadas and prime numbers:
Also interesting is the fact that there is not just one 17 year brood in the U.S., but at least two, and for all I know, several. Are these different broods timed in relation to a whole different set of predators in different areas of the country? Doesn’t seem likely. So why is the east coast now having the 17 year emergence, and here in the midwest, we had it a few years ago?