Math & Money

Why Are Cicadas So Good At Math?

Awake my children Our day is nigh. We have waited long enough, crawling beneath the world. Rise, and meet your destiny. Sing your song! EMERGE!” They may look weird and scary, crawling out of the ground with their red, beady eyes, but this is no zombie plague. It’s the rise of the periodical cicadas. Cicadas are nothing to be afraid of. They’re not locusts, they’re not gonna wipe out your crops or eat your baby. Cicada mouthparts are more like straws than jaws, specially adapted for sucking up plant juices, and that’s exactly what they’ve been doing, underground, for more than a decade. Some cicada species show up every year, giving a buzz to summer afternoons, but a genus known as Magicicada, or the periodical cicadas, is special. Magicicada have the longest confirmed life cycle of any insect, only emerging every 13 or 17 years.

You might notice something special about those numbers. They’re primes. The U.S. is home to 15 of these geographically distinct populations, or broods. 12 are on a 17 year cycle, and 3 follow a 13 year cycle. There’s usually several species in a brood, but in any one place they’ll all emerge on the same schedule, which is what’s going on this year in the midwestern United States. [CICADA NOISE] Wow, we have some periodical cicadas watching you’re probably feeling kind of out of the loop. So, let’s see. When you were last awake, we were playing Golden Eye, throwing away all those AOL trial CDs, and preparing for the certain and that year 2000 computer bug was a thing. Computer bug, get it? [CICADA NOISE] Anyway, so since then, we built that international space station thing all movies are about superheroes, oh and invented the Smart Phones… pretty much so we can watch YouTube videos about cicadas in the bathroom.

Don’t worry, not everything has changed. Our presidential candidates are still named Bush, Clinton and we’re all really excited about the next Star Wars trilogy. Another one. How was the last one? Don’t ask. No one is precisely sure how our cicada friends know it’s time to rise, but in the late spring on their scheduled year, when the soil temperature hits 64 degrees fahrenheit, juvenile cicadas emerge from the ground, climb upward into the trees, and achieve their final form. There, the males will spend a couple weeks singing their deafening love song. [CICADA NOISE] Now this synchronized emergence is part of cicadas’ survival strategy. They come out in force, millions per square mile in some areas. Pretty much everything eats cicadas–birds, possums, squirrels, reptiles, even dogs and people, it’s a feeding frenzy, crunchy death on a massive scale.

But hungry predators can only make a tiny dent in the population by the time they’ve eaten their fill, and since every female can lay hundreds of eggs at a time, the cicadas persist. We see this strategy of predator satiation throughout nature, huge herds of wildebeest, salmon swimming up river, even plants do it by producing all their fruit at once. But why prime numbers? Why not 4 or 8 or 14-year cycles? This is where it gets interesting. Predator populations also move in cycles, depending on how much food and competition is around, and that cycle will often shift to fit the pattern of their prey. Now if cicadas had settled on, say, a 12 year pattern, then any predator with a 2, 3, 4, or 6 year cycle could also adapt to depend on the feast.

But by emerging every 17 years, cicadas minimize these deadly coincidences. A predator whose population cycles every 5 years would only align with the cicadas every 85 years! 2015 is a special year for periodical cicadas, because a 13 year brood and 17 year brood are both emerging at the same time. These cicadas also use prime numbers to protect themselves from one another. We know there’s something in the cicadas’ genes that keeps each brood on its 13 or 17 year cycle. Broods are usually geographically separated, but if they happen to overlap and swap genes, it could throw off the whole synchronized cycle of each group. So the cicadas use their prime number patterns to minimize that chance, only showing up to the same party every 221 years.

The broods emerging this year won’t sing simultaneously again until 2236! The big question is how did they come to calculate prime numbers? How do cicadas know how to do math? Well… they don’t. Stephen Jay Gould once compared evolution’s strategy of synchronized satiation to Adam Smith’s invisible hand in economics. Each cicada aims to help itself, to pass on its genes to the next generation. No individual makes a choice to fall in with the group, but any who don’t follow the cycle and emerge alone, well they’re easy prey, and, sorry, no reproduction for you! In the end, this accidental cooperation, based on individual choices, ends up helping the population as a whole. As Gould wrote, “It is sometimes advantageous to put all your eggs in one basket, but be sure to make enough of them, and don’t do it too often.” Stay curious. Now, even with their prime number strategy that doesn’t mean cicadas are immune from all threats.

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