The Motors of August Cicadas

(reprinted from “Germtales” 8/16/06)

I am being serenaded by cicadas and it is glorious. They are the sound of summer, the neon hum to the flicker dance of lightning bugs on warm humid nights. Cicadas are everywhere and nowhere. How can something that loud and large be so hard to spot?

Their suits from a past life pile up, empty shells abandoned near trees, sometimes in mid-climb. Each is perfect in every exquisite detail, with a slit along the back where its owner wriggled out to take on a new identity complete with wings, its long subterranean childhood forgotten in the rush to meet the future. 

Dinosaurs listened to cicadas. And before them, lizards, amphibians and other insects as far back a quarter of a billion years ago during the Permian period when even Pangaea wasn’t quite Pangaea yet. Cicadas have survived global extinctions, ice ages and the asphalt tombs of urban sprawl. Summer after summer they deftly navigate a gauntlet of hungry predators in a daring dash to the treetops for a few brief weeks of uncorked noisy revelry, a blow-out party years in preparation.

Scientists know quite a lot about cicadas, from the meanings of their songs  to their diva-worthy requirements: a soil temperature of at least 64°F to emerge and an air temperature of at least 70°F to sing. They have documented the tragic/comic cicadian ardor for lawn mowers and leaf blowers and analyzed the male’s tymbals (abdominal ridges) used to compose love songs. Though wings are for mostly for flying, females also use them for signaling. “Over here, honey!”

Still, despite so much research, cicadas have managed to keep more than a few secrets.

For starters, each fertilized female lays hundreds of eggs in tree branches, which means the first order of business for newly hatched larvae is literally to take a flying leap into the unknown. It is the fastest way down and they have no time to lose. They must dig into the ground and start feeding on tree roots before the weather turns frosty. Just like Carl Sagan’s stars, there are billions upon billions of larvae, yet I don’t think I have ever seen a single one in mid-leap. Maybe they leap in the dark. Or maybe they disappear in the glint of the sun. Or maybe, just maybe, they magically turn themselves invisible. They are, after all, in the genus Cicada Magicada. 

Even more of a mystery, though, is why a few species in North America emerge only once every 13 or 17 years. The most popular theory explaining this unique prime number preference involves predator defense. Most predator species—birds, bats, squirrels, raccoon, skunks, possums—have reproductive cycles of one or two years. A cicada emergence on this scale is a luck-of-the-draw surprise feast and when it is over, bulked up predator populations quickly crash back to more manageable levels. There are always far more cicadas than predators. 

But why 13 and 17 years specifically? Why not 5 or 7 or 19? Most of the hundreds upon hundreds of other cicada species in the world, including species native to the very same areas of North America, manage to survive just fine on a two-year cycle. Also, spending too much time underground is not without risk. A forest might be ripped up to make way for a highway or parking lot, its root-dependent nymphs lost as collateral damage. Trees can also die of disease (since 2002, the Emerald Ash borer, for example, has killed tens of millions of trees). Fire, farming, urban sprawl—each takes a toll.

These, however, are comparatively recent hazards to which our cicadian heroes have had little time to adapt. To what, then, could a 13 or 17-year cycle be adapted? Is it possible that these broods are a kind of time shadow, vestiges of a changing climate at the end of the Pleistocene? As North America warmed up and glaciers melted, cicada populations expanded into new areas, but it was a long process spanning millennia. A population—or brood—of cicadas might have found itself stuck underground for an extra season or two or more waiting for the soil to heat up to that critical 64°F degrees. Perhaps they continued to feed on roots while biding their time. What if a cold spell lasted for several years and the cicadas that survived emerged with their internal clocks reset? Would the new cycle continue since there would be no environmental pressure for it to change? 

The mystery goes even deeper: How exactly do insects with a brain the size of a speck count at all? It turns out they take their cue from trees. In a very clever experiment, a team of researchers at the UC-Davis tricked orchard trees into two foliage cycles per season. The 17-year cicada nymphs sucking on roots emerged at the 17th cycle, even though only 8 ½ years had passed (abstract).

That still doesn’t quite explain how cicadas count to a specific number, which is thought to be hard-wired into their biology. In fact, 13 and 17-year cicadas could be counting by fours altogether, with a one year add-on: 

(3 x 4 ) + 1 = 13 and (4 x 4) + 1 = 17.

On a molecular level, it turns out there is not much difference between 13 and 17-year cicadas. If a 17-year cicada emerges early, it is often by four years (though sometimes by one), which means that it is possible that the 13-year broods developed as a sub-population of early-emerging 17 year cicadas. No one really knows. 

Head-spinning. 

Next year, Brood XIII—which ironically happens to be a 17-year brood—will emerge here in Chicago, as well as parts of Michigan, Wisconsin and Iowa. Perhaps a few popped out early because it has been a pretty thunderous season. The motors of August cicadas, so loud, so summer, so right now, but also a sound of the deep past, of patience and of time itself. This is just the warm up band for the chorus to come. 

I can’t wait.

— J.A. Ginsburg  / @Trackernews

* Brood XIII won’t bee seen again until 2024. If you can’t wait that long, here is a schedule of all the North American 17 and 13-year broods. 

** video credit: Amazing Cicada life cycle - Sir David Attenborough’s Life in the Undergrowth - BBC wildlife