Deciphering the Strange Mathematics of Cicadas [Video]

In the Hampton Roads area, the plague of locusts predicted for this year hasn’t really started yet–and it probably won’t, since we’re not in the direct path.  I used to discuss locusts and mating cycles in class–a previous textbook offered it as an example.

This year is a good year to think about them, too.  Many populations of locusts have weird life cycles and we only see them every 17 years.  For others, it’s every 13 years.  These different species rarely coincide for mathematical reasons.  This is a temporal prezygotic mechanism of reproductive isolation.  That means that because they come out at different times (hence the temporal), they rarely if ever encounter each other: every 221 years or so.  The last time a 13-year cicada species met a 17-year cicada species was in 1998; the next time this same pair of species should encounter each other will be in 2219.  That’s not a lot of opportunity for mating and gene-swapping, which is what makes it prezygotic: the eggs and sperm simply don’t meet up. 

Moreover, these cicadas also have different mating songs, so they wouldn’t be attracted to each other even if they do meet in 2219.  This is a behavioral isolation mechanism, and it’s also prezygotic, since the different species won’t find each other sexy. 

Over time, these differences in timing and song probably led to the cicadas forming different species.

Only the seven (7) Magicicada species have these types of life cycles.  There are other cicada species, including common annual ones.  Apparently, they’re all edible and supposedly tasty–I’ve never had the chance to try one (though I totally would.  I’m adventurous about food).

Want to learn more?  Two good resources about general periodic cicada life cycle can be found at Wikipedia and Magicicada.org.