Why cicadas stay underground for 13 or 17 years

HostMost of the bugs we see in our yards are only around for a few weeks or months. They show up when the weather gets warm, they do their thing, and then they're gone. But there's this one type of cicada that stays hidden underground for almost twenty years, which feels like a really long time to just sit in the dark. What's actually going on down there while they wait?

GuestIt does seem like a long time, but they aren't just sleeping or hibernating like a bear would. They're actually quite busy, just in slow motion. Think of it like a really, really long childhood. When they first hatch, they're tiny, about the size of a grain of rice. They crawl down into the dirt and find a tree root to latch onto. For the next thirteen or seventeen years, they just sit there and drink. They live off the fluid that moves through the tree roots. It's not very rich in food, which is why it takes them so long to grow up. They have to wait until they're big enough and strong enough to make the climb to the surface.

HostBut seventeen years of just drinking root juice seems like a stretch. I mean, plenty of other bugs eat things that aren't very good for them and they still manage to grow up in a single summer. Why do these specific ones take the slow path?

GuestWell, the slow path is actually a clever way to stay alive. If you're a bird or a squirrel, a big, crunchy cicada is a great snack. If these bugs came out every year in the same small numbers, the birds would just eat every single one of them. By staying underground for a long time and then all coming out at the exact same time, they overwhelm the neighborhood. There are suddenly billions of them. The birds eat until they're stuffed, but they can't possibly eat all of them. Most of the cicadas survive simply because there are too many of them to handle.

HostOkay, I get the strength in numbers thing, but why those specific numbers? Why thirteen or seventeen? Why not just wait ten years or fifteen years?

GuestThat's where the math gets really interesting. Thirteen and seventeen are what we call prime numbers. That means you can't divide them evenly by two or three or four. Now, imagine you're a bird and your population has a big boom every two or three years. If the cicadas came out every twelve years, they would hit that bird boom every single time because twelve can be divided by two and three. It would be a disaster for the bugs. But by picking thirteen or seventeen, they rarely ever line up with the cycles of the animals that want to eat them. They're basically using math to hide in time.

HostWait, these are bugs. They don't have brains that can do math. How can a bug that lives in the dirt know the difference between a twelve-year cycle and a thirteen-year cycle?

GuestThey don't know the math in their heads, but the ones that came out on the wrong years simply didn't survive to have babies. Over a long, long time, the ones that happened to wait for those prime numbers were the only ones left. It's more about the bugs that didn't get eaten than the bugs knowing a trick. But they still have to know when it's time to come up. They need a way to count the years while they're stuck in the dark.

HostYeah, how do you keep track of time when you can't see the sun or the seasons changing? Is there some kind of internal timer that just goes off after a decade?

GuestThey actually use the tree as a clock. Every year, when the tree wakes up in the spring and starts growing new leaves, the fluid inside the roots changes. It gets a different mix of sugars and chemicals. The cicadas can taste that change. So, they just sit there and count the pulses of the tree. One taste of spring, two tastes of spring, and so on. Scientists once tried to trick them by making a tree bloom twice in one year by messing with the lights and heat. The cicadas counted both blooms and came out a year early. They really are just keeping a tally of the seasons by how the roots taste.

HostThat's wild. So they're basically counting to seventeen in the dark. But it still feels like a huge risk. If the weather stays cold or the tree dies, they're just stuck. Does this long wait ever backfire on them?

GuestIt can. If a forest is cut down, the cicadas attached to those roots die out. And sometimes the timing gets a bit messy. You might have a few rebels that come out four years early. We're seeing more of that lately, and some people think it might be because the longer growing seasons are confusing their internal clocks. But for the most part, the system works. It has worked since the ice ages. In fact, that might be where this whole thing started. When the world was much colder, it was harder to grow up fast, so they had to wait for those rare, warm windows.

HostSo they're essentially living in the past, following a survival plan that was made for a much colder world.

GuestIn a way, yes. They're like little time capsules that stay buried until the count is finally up. When they finally emerge, they only have a few weeks to find a mate and lay eggs before they die. They spend ninety-nine percent of their lives waiting for that one short month in the sun. It's a very strange way to live, but it's one of the most successful tricks in nature.

HostThey spend nearly two decades tasting the roots of a tree just to make sure they all show up to the party at the exact same moment.

GuestThose bugs are down there right now, tapping into the roots and waiting for the year that belongs to them.