Brood X
There has been a lot of talk about cicadas lately, and they have finally started to come out. These are periodical cicadas that emerge on a 17 year cycle. The cicadas that are emerging this year are from brood X and last appeared in 1987. Since then they have been sitting underground, tapped into the roots of trees, slowly maturing. During the last few days the mature nymphs have started to emerge in this area. They climb a vertical surface: tree, lamp post, blade of grass, you name it, then molt, and you have a winged adult. The adults move up into the tree tops and begin wooing, which pretty much sounds like one of alien space ships from any of the old 1950's sci-fi movies.
For some reason there aren't any in my yard, but I took my kids to a park about a mile away yesterday and they were all over the place. The song is pretty cool and amazingly loud. Even though there aren't any in our yard I can hear them calling pretty clearly from the woods that surround our subdivision.
In any case, they come to mind here because it strikes me that they have taken the whole arribada thing to an extreme. Turtles that nest en masse probably do so as a mechanism to ensure that as many hatchlings survive as possible when they emerge, also en masse. Ideally predators become sated picking off the hatchlings that emerge early allowing more of the late ermergents to survive. And the cicadas do this on a 17-year cycle!
It makes me wonder how many other taxa, or what proportion, have adapted similar strategies to increase the survival of their young? Coral come to mind which spawn syncronously, as do many fish species, like grouper that have spawning aggregations.
Click on the image (couresy of J. Stein Carter from his website) or follow this link to learn more about cicadas.
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Comments
There is something strange about the predator-satiation hypothesis and the 17 year cicada cycle. Of course, I don't have a better explanation, but it seems extreme that they would adopt such a long delay before emergence (i.e. wouldn't 7 years be just as efficacious?). Still, they are really fascinating insects.
Posted by: matthew | May 17, 2004 06:06 AM
Yeah, I'm not sure how much of a role the 17-year cycle plays in the predator-satiation process. Certainly the synchronous emergence does and the fact that they only remain above ground for one month each year. The 17-year cicadas are present every year, it's just a different brood, and this one (brood X) is supposed to be the biggest. I think people might get the sense that the species only occurs on a 17-year interval because this brood is so large and individual broods do not necessarilly overlap geographically. There's a nice map here http://www.ummz.lsa.umich.edu/magicicada/Periodical/compositebroods.html of the distribution of each of the broods of the species that just emerged.
I do find the 17-year cycle a bit puzzling. Not sure what the adaptive advantage would be. I believe I heard that these are also the biggest in size, so perhaps that plays into it. There are also 13-year cicadas, interestingly there are 13-year species that are morphologically and behaviorally similar to each of the three 17-year cicada species. Also interesting that the 17-year cicadas tend to be distributed in the north, while the 13-year's are in the south. Perhaps the 17-year cicadas need more time because of the cooler weather?
Posted by: Michael | May 17, 2004 11:37 AM