Ecology

Cicadas – Why every 17 years?

Amanda Markle
Strawberry Hill Nature Preserve

(5/2021) If you have a pet, you probably already know that animals, though lacking what we consider to be the traditional tools, can be surprisingly accurate in their perception of time. Often, they appear seemingly better at it than those of us who do have the benefit of a wristwatch. Forget to fill your cat’s food dish by the regularly appointed hour, and she will not hesitate to let you know of your folly.

Biological timekeeping can be observed in animals, plants, fungi, even organisms as simple as cyanobacteria. Circadian clocks, the central mechanisms that allow for internal timekeeping, allow living things to anticipate regular changes in their environment and adapt their behavior accordingly. They help animals know when to search for food, when to sleep, and when to look for a mate. These internal clocks sync up with Earth’s solar day with astonishing accuracy, even without external cues. Lock a human in a lab with no clocks, no social interaction, and a constant low level of light, and with time, his sleep-wake cycle will shift only slightly, averaging about 24.2 hours.

These internal clocks can be affected by external or environmental cues, known as zeitgebers, a German word translated as "time-giver." Zeitgebers help calibrate the biological clock when there are changes in the environment. The most obvious, and most powerful, is light. Researchers can induce off-season migration or mating in birds by artificially controlling the amounts of light and dark they are exposed to. Natural light therapy is one of the best ways to combat jet lag when travelling between time zones. Light is not the only external cue that can affect internal clocks, however. Lunar cycles, feeding patterns, tides, and temperature all have an impact on the way organisms track the passage of time. Even scent can be used as a timekeeper.

One study found that dogs can predict the moment of their owner’s return from work by judging how human scent dissipates over time in their absence. These factors work together to give animals the ability to track the passage of time and engage in cyclical behaviors that most humans would need a calendar to remember. This year, we are lucky enough to witness one of the most fascinating of those behaviors–the emergence of the 17-year cicadas.

By now you have likely heard about the storied Brood X–the massive cohort of periodical cicadas set to emerge this year in 15 states across the Eastern United States. Though different broods and species of cicada erupt from the ground every year, Brood X will appear above ground hundreds of billions strong, in a mass of lacy wings and bulging eyes; a brood the size of which has not been seen since the last time they emerged back in 2004. Which leads to the question: how does a cicada, buried alive underground, keep accurate track of time for seventeen years? How do billions of insects coordinate such a dramatically grand debut, and why?

Though out of sight, and mostly out of mind, cicadas do not spend their time underground hibernating; they have a lot of work to do before they emerge. Cicadas first dig underground as nymphs, after hatching from eggs laid in tree branches and falling to the ground. Once under the dirt, cicadas attach themselves to a tree root and feed on sap. Here, they undergo five distinct developmental stages known as instars. With each instar, the developing cicada molts its exoskeleton and grows one step closer to its adult form. Periodical cicadas can be divided into 13-year and 17-year groups, with the time it takes for each to complete its second instar phase determining which group they fall into, but external factors can throw off these cycles of growth.

Evidence strongly supports the theory that annual changes in the flow of nutrients through tree roots signals their countdown to emergence. In the rare instances that trees produce an extra leaf set in a year, typically caused by a warm spell in winter, followed by a cold snap and then a normal spring, cicadas can be found emerging a year too soon. The spikes in the nutritional composition of the sap on which they feed becomes a zeitgeber, altering the normal internal clock that controls their growth and development.

Despite the many predators they face, the sheer number of cicadas that emerge over a relatively short period of time ensure that enough insects survive to mate, lay eggs, and continue the species. That is why precise timing is so important to the cicada population. A precise internal clock, calibrated with external stimuli, is key to their survival.

By prolonging their underground development and growth phase their predators cannot rely on them as a consistent food source. As a result, predators likely will not have a consistently large population because they have staggered their emergence years. By overwhelming predators with enormous numbers and delayed emergence the probability of species survival significantly increases.

Many unknowns remain when it comes to the exact reason why periodical cicadas have evolved to follow the schedules they do. One question being studied is why their developmental cycles are so long. Some scientists theorize that cicadas evolved to avoid spikes in predator populations that occur after a cicada brood appears above ground. The biomass added to the food chain in years of large cicada populations can have significant effects on the survival of predators and their offspring. Studies have shown survival increases of 10% or more in bird species that feed on cicadas in these years, and the effects on population size can be seen for generations. By returning above ground so infrequently, cicadas can wait out these populations spikes until predator numbers return to typical levels.

A closely related theory looks at number of years periodical cicadas divide themselves into, 13- and 17-year groups–both prime numbers. Some scientists believe that evolving to follow a time schedule not easily devisable makes it harder for predators to evolve to match the cicadas’ periodic population increases, helping them maintain the advantage of appearing in enormous quantities. Still another theory postures that these 13- and 17-year cycles evolved to prevent the mixing and hybridization of different broods during times when periodic fluctuations in climate have limited the physical scope of cicada populations. One interesting observation is that of brood stragglers. These are cicadas from a certain brood that miss their cue, emerging typically one or four years before or after the rest of their cohort. The reasoning why this occurs is not known for sure, but it is theorized that a particularly successful group of stragglers in any one year may be able to establish their population enough to become a brand-new brood all their own.

The arrival of periodical cicadas may seem like an alien invasion, and a noisy one at that. But it is also an amazing display of biological timekeeping and evolutionary survival strategies; a phenomenon that only occurs in one tiny corner of the world. So, while they are filling your backyard with their boisterous chorus this summer, take a moment to appreciate all the complex systems that have come together to bring them here at this exact moment. You will not see these ones again for a long, long time.