The Long Sleep: Why Do Cicadas Stay Underground for 17 Years in the USA?
If you've lived in certain parts of the United States, you might have experienced the overwhelming, deafening chorus of cicadas. These fascinating insects are known for their incredibly long life cycles, particularly the periodical cicadas, which spend a staggering 17 years underground before emerging en masse. This isn't a random quirk of nature; it's a highly evolved survival strategy. Let's dive deep into the reasons behind this remarkable 17-year slumber.
Understanding Periodical Cicadas
First, it's important to distinguish between annual cicadas, which emerge every year, and periodical cicadas. The latter are the ones that follow strict, synchronized life cycles of either 13 or 17 years. In the USA, the most famous and widespread are the 17-year cicadas, belonging to the genus Magicicada. These are not the same as the cicadas you might see buzzing around every summer.
The Primary Reason: Predator Avoidance
The most widely accepted scientific explanation for the extraordinarily long underground life of periodical cicadas is **predator satiation**. This is a clever strategy where a massive number of individuals emerge simultaneously, overwhelming and thus saturating the local predator population. Think of it like a buffet for birds, mammals, and other insectivores.
- Overwhelming Predators: When billions of cicadas emerge at once, predators can only eat so many. Even though the sheer number of cicadas is immense, each individual has a much higher chance of surviving and reproducing because the predators are quickly overwhelmed.
- Breaking Predator Cycles: By having such long, synchronized cycles, periodical cicadas ensure that their predators do not evolve to specifically time their breeding cycles to coincide with cicada emergences. If cicadas emerged annually, predators that feed on them would have a consistent food source and could develop specialized hunting strategies. The 17-year gap makes it difficult for predators to rely on cicadas as a consistent food source.
Life Underground: A Slow and Steady Growth
For the vast majority of their lives, these cicadas are in their nymphal stage, living underground. This subterranean existence is crucial for their development and survival.
What do they do underground?
- Feeding on Sap: Cicada nymphs feed on the xylem sap of trees and woody shrubs. They have specialized mouthparts to pierce the tree roots and suck out the nutrient-poor sap. This is a slow process, and it's why their development takes so long.
- Growth and Molting: Over the years, the nymphs grow, shedding their exoskeletons multiple times in a process called molting. Each stage of growth is called an instar. The final instar is when they are ready to emerge.
- Protection from Harsh Conditions: Living underground also protects the nymphs from extreme weather conditions, such as harsh winters, droughts, and intense heat, as well as from a wide array of terrestrial predators that would easily prey on them above ground.
The Synchronized Emergence: A Biological Marvel
The synchronized emergence is one of the most awe-inspiring aspects of periodical cicadas. All the individuals of a particular brood (a group that hatches in the same year and emerges in the same cycle) will emerge within a few weeks of each other, even if they are geographically dispersed over a large area.
How do they know when to emerge?
While the exact biological triggers are still being studied, scientists believe it's a combination of factors:
- Temperature Cues: Soil temperature is a significant factor. When the soil at a depth of about 8 inches reaches a consistent temperature of around 64 degrees Fahrenheit (17.8 degrees Celsius), it signals to the nymphs that it's time to come out.
- Internal Biological Clock: There's likely an internal biological clock that keeps track of the years, synchronized by the environmental cues.
Once they emerge, their above-ground life is remarkably short – typically only 4 to 6 weeks. During this brief period, their sole purpose is to mate and reproduce before they die. The males sing their mating songs, the females lay their eggs on tree branches, and the next generation begins its long journey back into the earth.
Why 17 Years? The Evolutionary Advantage
The 17-year cycle is thought to be an evolutionary adaptation to avoid "brood-switching" or "hybridization." If two broods with different cycle lengths were to emerge at the same time, they could potentially interbreed. This could dilute the length of their cycle, making them more vulnerable to predators. Prime numbers, like 13 and 17, are thought to be the most effective at minimizing overlap and potential hybridization with other broods.
The math behind this is fascinating: If a predator has a 3-year cycle of abundant food and a 5-year cycle of abundant food, and cicadas emerged on a 3-year cycle, they would overlap with the predator's abundant food years every 15 years (3x5). However, if cicadas emerged on a 5-year cycle, they would also overlap every 15 years. If they emerged on a 7-year cycle, they would overlap every 21 years. By using prime numbers, the potential for overlap with predator cycles is minimized.
The Cicada Song and Its Purpose
The iconic buzzing sound of cicadas is produced by the males. They have specialized organs called tymbals on their abdomen that vibrate at high speeds to create a sound. This "song" serves as a mating call to attract females.
"It's just loud!"
Yes, the sound can be incredibly loud, reaching levels of 90-100 decibels, comparable to a lawnmower or a motorcycle. This volume is necessary to be heard over the chorus of other males and to reach females across their habitat.
What Happens When They Emerge?
When the time is right, the nymphs will tunnel to the surface and undergo their final molt, transforming into winged adults. They will then seek out trees and other woody plants to feed and mate.
The Feast for the Eyes (and Ears):
- Massive Numbers: Expect to see billions, even trillions, of cicadas in affected areas.
- Short-Lived Adults: The adults live for only a few weeks.
- Egg Laying: Females will lay hundreds of eggs on twigs and branches.
- Predator Buffet: Birds, squirrels, chipmunks, snakes, and even household pets will likely feast on the cicadas.
Despite their intimidating numbers and loud noises, periodical cicadas are harmless to humans and pets. They do not bite or sting, and their primary defense is their sheer abundance.
Frequently Asked Questions (FAQ)
How do cicada nymphs survive underground for so long?
Cicada nymphs survive by feeding on the xylem sap of trees and woody shrubs. This sap is nutrient-poor, requiring a long time to accumulate enough energy for development. They also benefit from the stable environment underground, protected from extreme weather and many predators.
Why are there different cicada broods with different cycles?
The different broods of periodical cicadas (e.g., Brood XIII and Brood XIX) emerge on different 13- or 17-year cycles. This helps to prevent hybridization between broods and to minimize overlap with the life cycles of potential predators, ensuring their survival through evolutionary prime numbers.
How can you tell if cicadas are coming to your area?
You can often find information from local universities, agricultural extension offices, or entomology websites that track the emergence cycles of periodical cicadas. If you live in an area known for periodical cicadas and it's a year designated for a specific brood's emergence, you can expect them.
What is the purpose of the cicadas' loud singing?
The loud singing, or stridulation, is produced by the male cicadas and serves as a mating call to attract females. The sheer volume is necessary to be heard over the noise of other males and to communicate effectively for reproduction.
