Has anyone come up with an evolutionary basis for the cicada to appear once every 17 years, live for a week or two, and the die, only to repeat the cycle in another 17 years?
As a survival strategy, I guess it works for the cicada, but it seems like a radical step to take to evade predators…(which eat millions of them anyway every 17 years)
This particular cycle of life makes no sense to me, especially if you believe in evolution and natural selection. Why not pop up every year? Even if their life boiled down to have sex, then die, which it does now, they could do that on an annual basis and still have the same result for their species
Any theories as to what the significance of their 17 year cycle pertains to, and are their any other organisms that have similar cycles?
It doesn’t have to make sense to you. All that matters is that this strategy was reproductively successful for this species. It’s just as simple as that.
Yes: it’s a prime number. Really. The less predictable their cycle is, the harder it is for a predator to happen on the same cycle. If they came out every 16, 12, 4 or 2 years a biannual predator would sync to them. Similarly any number divisible by 3. Leaving those out there is only 5, 7, 11, 13, 17, 19 and so on … why 17 out of those I don’t have an explanation for.
Their cycle doesn’t have to make sense to you; it only has to enable them to reproduce, which it does. Imagine that in the past there were other hypothetical types of cicadas with other cycles:
Annual cicadas . . . who were bright red, and very tasty to predators.
Two-year cicadas . . . who were deaf, and couldn’t hear the mating signals.
Three-year cicadas . . . who were indifferent to mating.
Four-year cicadas . . . who were sterile.
In short, the cicadas with 17-year cycles may actually have the best chance at reproducing, for other factors entirely. Natural selection doesn’t need to find the “best” solution, only the one that’s better than the other ones available.
Yeah, I know it doesn’t have to make sense to me. :rolleyes: That’s no answer, though. A LOT of things don’t make sense to me. (surprise) However, this one is a bit different to me than just the “that’s the way it is” answer.
For instance, is there any physical changes going on in the external environment that might stimulate the cycle? Perhaps they correspond to sunspot activity, or the earth’s electromagnetic properties?
I know it doesn’t have to make sense to me… that doesn’t mean there is no explanation as to the why!
This is actually the most interesting explanation I’ve ever heard on this. I don’t know about its validity, but it presents an interesting theory.
No such environmental factors follow strict prime number cycles. And IIRC the cicadas are locally synced, so in a different region you’ll have culmination in a different year.
Specifics of timing aside for a moment, massive, infrequent reproduction makes sense here because although the predators have a party when it happens, it’s too massive an event for the predators to make a meaningful dent in it, and it’s sufficiently rare not to induce a dependency (the predators can’t rely on a food source that only appears occasionally), or to effect a long-term increase in the population of predators (any increase in predator population enabled by the last event has had 15 or 16 years to settle back down again)
After casting doubt on the predator-population-pressure explanation,
[QUOTE=excerpt]
Some scientists think their emergences every 13 or 17 years (depending on the species) is a way to keep predators from increasing their own populations after exploiting them as a food source, thus avoiding a cycle when predator numbers eventually escalate enough to decimate cicada numbers. This seems an unlikely explanation to me for 2 reasons: cicadas are only available for a few weeks a year which is not enough time to have a significant long term impact on predator populations. More bird nestlings may survive at first due to the abundance of cicadas, but then for the rest of the year, they must adapt to the normal supply of food. Moreover, the average wild bird only lives 2 years–far shorter than 13 or 17 years. Cicadas could avoid upswings in predator populations with much shorter periods between emergences.
[/QUOTE]
…there’s a statistical analysis of why longer periodicity helps cicadas survive cold summers [the bolding is mine]:
[QUOTE=excerpt]
Periodical cicadas depend specifically upon deciduous forests. During the coldest stages of Ice Ages, deciduous forests north of the southern Appalachians were rare relics outnumbered by other environments such as spruce forests and prairies. The bulk of deciduous forests then occurred south of the Appalachian mountains. Even here, summer temperatures occasionally were too cold for cicadas in the Magicicada family. They require temperatures above 68 degrees F for a period of 3-4 weeks for flight and mating. Drs. Cox and Carlton assumed that during the coldest stadials (which lasted on average 1500 years) 1 in 50 summers failed to reach this temperature, and cicada reproduction failed. Using a statistical formula, they estimated that over a 1500 year stadial, cicadas emerging every 6 years had a 4% chance of avoiding unusually cool summers; cicadas emerging every 11 years had a 51% chance of avoiding unusually cool summers; but cicadas emerging every 17 years had a 96% chance of avoiding unusually cool summers. Cicadas emerging after shorter periods were eventually eliminated from the gene pool, while those with genes for longer cycles became dominant.
[Map of Magicicada ranges from the below referenced paper. The distribution of 13 and 17 year periodical cicadas supports the paleoclimatic explanation for their high prime number emergences.]
17 year cicada species tend to live north of 13 year cicada species, even though the shorter cycle is a dominant gene. Summers too cool for breeding would’ve occurred more frequently in the norther parts of their range, so those with 17 year cycles would’ve had a greater chance of avoiding them than those with the 13 year cycle.
[/QUOTE]
Speaking just as a poster, I find posts like this very unhelpful. The characteristics or organisms are not simply random. While it’s quite correct that not all characters are adaptive, or optimal solutions, or maintained by selection, simply waving your hands and saying we don’t know doesn’t contribute anything useful to the discussion.
Even if a character did not originate or is not being maintained through natural selection, there is still some reason for it. Developmental constraints, for example, can maintain characters even when they may be selected against (because it is too difficult to change to another adaptive peak). Identifying the reasons a character that appears to be detrimental is maintained is of great interest to evolutionary theory, and shouldn’t be dismissed with comments that boil down to “just because.”
Think of the organism as the critter living underground.
To say they live for a week or two than die isn’t quite right.
They live for 17years then go through reproductive stage for a couple of weeks then die.
The majority of their life is very successful evading predators and eating. By waiting so long they spend a very short amount of their time being vulnerable.
We had oodles and oodles of cicadas every year when we lived in Oklahoma - I assumed they were just a different 17-year-cicada cohort emerging every summer.
You know, I thought about it like this, but it didn’t strike me as a good way to look at it. Even though what you say is true, it just doesnt seem like much of a life for 17 years of waiting. I get it. They are bugs. What do they know? But it bothers me from a live long and prosper perspective. These guys are more like hibernate long then wake up to mate and die. What a ripoff!
The prime number hypothesis sounds logical.
-avoids predators expecting a regular feast. It’s a moveable feast; or rather, less predictable.
-avoids predator dependency
-very long period prevents predator “learning”; mating call/racket is also different from other possible food sources.
-very short breeding cycle combined with infrequent occurence also prevents predators from “learning” to associate their call with food.
-very long period also allows them to miss multi-year climate problems - a drought or cold spell of a few years, or even a dozen, is over by next cycle.
Mutations that “wander” (apparently it happens from time to time) are less successful because there are almost no mates around. I remember an article, Scientific American I think, that mentioned that occasionally mutants that only go 16 years, but the most successful breeders stay with the pack - self-reinforcing cycle. Breeders that somehow get out of sync but then do 17 years also tend to find fewer mates.
My personal WAG - it has more to do with the trees upon which they depend than on them. I read somewhere that the trees in the areas where they are periodically trimmed by cicadas have far more leaves than close areas with no large cicada population. It stands to reason that the the bug/tree pairing would eventually fall into the cycle that best supports large amounts of nutritious sap being created byt he tree.
In short, the question is not why do the cicadas come so infrequently? But Why do the cicadas come out at all? They have a safe happy home down there in the tree roots - why risk a trip to the surface and all that messy hardening and flying around? Answer: Creating offspring and guaranteeing their food supply.
