If you're in the Midwest, your summer might have a very crunchy soundtrack: The sounds of hordes of cicadas emerging from the Earth, chirping and whizzing and sexing and dying after more than a decade underground. That isn't to say that people around the rest of the United States aren't hearing the same thing on a smaller scale.
That's because cicadas spend most of their long, long lives (the longest of any known insect) underground, and some species emerge periodically -- once every 13 or 17 years. When broods sync up, things get loud -- which is what's happening right now in the Midwest.
In the latest video from It's Okay to be Smart, Joe Hanson explains why you should be more in awe of the insects than annoyed with them: They're actually doing some pretty impressive math -- though it's not on purpose -- and it's a really smart evolutionary tactic.
By coming out in literal droves, cicadas are able to survive -- er, at least for the handful of weeks it takes them to reproduce -- even though basically everything wants to eat them. Birds can go to town on the emerging cicadas and still leave plenty over to lay eggs.
That's where the prime numbers come in. Why 13 and 17 years? By living life in the prime, cicadas can minimize the number of times their big debut coincides with the birth year of a predator. If they'd "picked" a number divisible by another number, they'd link up with predators who were born at those intervals way more often -- and the predators could adapt to follow cicada lifecycles even more closely, shifting so that they had their babies right when cicadas showed up to provide an endless food source.
But 17 years (and even 13 years) is a long time for just about any bird or lizard, so they've got no chance of hitching their carts to the cicada's brood. And by coming out every 17 years instead of every 16, the cicadas also avoid sync-ups with predators who have 2-year and 4-year reproductive cycles, too.
Very cool.