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This spring, cicadas are buzzing across the Southeast and Midwest, thanks to the simultaneous emergence of two major broods. Their 17-year and 13-year cycles are coinciding for the first time since 1803—when Thomas Jefferson was in the White House. The result is expected to be more than one trillion cicadas singing in a hard-to-miss spectacle.
Where have all these insects been? They’ve been out of sight underground—and they are not alone. Many species have adopted survival strategies that see them spend most of their lives lying low and biding time until the moment is right to re-emerge. We’ve put a spotlight on six of the most incredible species that prefer to “go dark,” including those noisy bugs.
Desert-dwelling frogs
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Australia’s deserts are an unlikely home to hundreds of thousands of frogs usually found below the surface of the ground. They spend the dry months, or even years, in a state of dormancy there. Short-lived wet periods spark a frenzy of activity. Jodi Rowley, curator of amphibian and reptile conservation biology at the Australian Museum and the University of New South Wales, notes that many Australian frogs evolved in wet ancient environments and then adapted to today’s dry conditions. “I think it’s quite remarkable that an animal so reliant on fresh water can persist in such seemingly hostile environments,” she says.
Frogs from several families, including tree frogs, use periods of estivation, a sort of summer hibernation, to survive the arid environment without losing too much water from their bodies. They do this by digging underground and cocooning themselves with fatty skin secretions that retain precious water until the rains come again. “Just how long they can survive in this way we don’t know,” Rowley says.
When the rains arrive, the frogs have little time to dig themselves up, gorge themselves on food and find mates. “Most of the species are ‘explosive breeders’: They have such a short time that it’s a mating frenzy,” Rowley says. Afterward, another race is on—tadpoles must develop into frogs and bury themselves to await the next rains. “For this reason, most desert frogs have a really short larval duration,” she says. “Speed is of the essence.”
“Rip Van Winkle” orchids
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More than 100 different plants, including many species of orchids, enter states of underground dormancy, skipping spring emergence for up to 20 years. Some have dubbed them “Rip Van Winkle” orchids, after Washington Irving’s fictional character who missed the American Revolution during a 20-year slumber.
Richard Shefferson, an evolutionary ecologist at the University of Tokyo, says this strategy has evolved many times. Most often, dormancy seems to be an adaptation that allows the plant to survive without the costs and risks associated with sprouting and flowering each year, he explains. When orchids sprout, they run the risk of being eaten, and in some species, including many lady’s slippers, the effort produces pollination and reproduction rates of an abysmal 10 percent. That’s why many orchids have adopted other ways to survive and even to reproduce.
Dormant orchids can get the nutrition they need by digesting fungi that live among their roots, and some fungi-fueled species don’t use photosynthesis at all, which enables extreme dormancy. For example, the coralroot orchid is usually dormant in four out of every five years, living completely underground. “When they sprout, they flower but have evolved to become self-pollinators, and that gives them 100 percent pollination,” Shefferson says. “So, this orchid has evolved a life history that is typically dormant, because why would you waste resources sprouting when you don't photosynthesize and can’t flower until you are big and full of stored resources?”
Dozing dormice
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The European edible dormouse, a squirrel-like nocturnal mammal, is perhaps the world’s most dedicated hibernator. The animal is able to slumber in underground burrows for more than 11 months of the year—under some conditions. The body fat that edible dormice pile on prior to this marathon combination of winter hibernation and summer estivation made them especially appealing to Romans. They devoured the small mammals dipped in honey or rolled in poppy seeds, a practice that gave the species its name.
Dormice typically spend more of any given year hibernating than they do awake, but in some years they stretch the slumber to last for nearly an entire trip around the sun. Why? Some studies suggest these years have a scarcity of seeds from European beech, a major source of dormouse sustenance that the trees produce on a cycle of boom and bust. When these seeds are in short supply, dormice don’t mate, perhaps because their young, born only three months before hibernation time, wouldn’t be able to fatten up quickly enough to survive. In these non-reproductive years, dormice apparently don’t feel like it’s worth waking up much at all. But other research suggests a different drive has pushed dormice to spend most of their time underground: It keeps them from being eaten from predators like nocturnal owls.
Indestructible tardigrades
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Tardigrades are amazing microscopic animals, also called water bears, that can survive all types of extreme conditions. They can shrivel up due to almost complete dehydration and slow their metabolism to almost nothing. In this state, they can survive for more than 30 years without food and water. They can endure blasting temperatures of more than 300 degrees Fahrenheit, chills near absolute zero and even radiation in the frozen vacuum of space. Tardigrades have been on Earth for 600 million years, which means they’ve already survived all five of the planet’s prior mass extinctions.
In their inactive state, tardigrades appear to be dead, and exactly how they emerge isn’t completely understood. However, they will likely still be doing so if most of the planet’s other species, including humans, have vanished. A study by Oxford University and the Harvard-Smithsonian Center for Astrophysics, examining the resiliency of life on Earth to apocalyptic catastrophes from supernovas to asteroid impacts, found that the tiny creatures are likely to survive any catastrophe until our sun dies.
African lungfish
When waterways dry up, the African lungfish has an amazing way of adapting—one so successful it has enabled the species to survive unchanged for an astounding 400 million years. These animals look more like eels than typical fish, and they have thin limbs that enable them to travel on land. Lungfish have gills, but they also have true lungs, which means they can breathe air through their mouths, by which they get most of their oxygen.
In dry periods, lungfish cocoon themselves in the mud and their own mucus, which hardens to provide a water-retaining shelter, protecting them from bacteria and pathogens and leaving an airway so that they can breathe. They enter a state of torpor during which they neither eat nor drink, instead living off stored energy and slowing their heart rate to two beats per minute.
For months, or even years, the lungfish await the signal that water has returned; they know it is time to re-emerge when their mouths, open for breathing, begin to fill with water as the rivers and wetlands are renewed.
Cicadas
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The periodical cicadas, whose “singing” males produce the iconic sound by vibrating membranes over their hollow abdomens, are ringing out this year from Oklahoma to Wisconsin to Maryland. Cicadas spend nearly all their lives living quietly underground. However, unlike many other species, they are not dormant. “The nymphs underground are actively digging, moving around and feeding on tree roots,” Smithsonian entomologist Floyd Shockley writes in an email. “They just stay as nymphs for a LONG time.”
The nymphs’ life cycle is regulated by their genes, and among the roots they also pick up cues to the passage of seasons from their deciduous tree hosts. During the year of their scheduled emergence, they wait on the weather. “Soil temperature rising above 64 degrees Fahrenheit 12 to 18 inches below ground is their trigger to start digging their way out,” Shockley writes.
After perhaps 13 or even 17 years as nymphs, the insects spend only a few short weeks above ground, during which they mate and females lay eggs in the trees. When those eggs hatch, after six or seven weeks, a new generation of nymphs falls to the ground and burrows in to begin the cycle all over again—nearly all of which will play out underground.
Top Image Credit: Illustration by Emily Lankiewicz / Auscape / Universal Images Group via Getty Images / Eric Hunt via Wikipedia under CC By-SA 4.0 / Marco Simonini / REDA&CO / Universal Images Group via Getty Images / Kiosya Y, Vončina K, Gąsiorek P (2021) Echiniscidae in the Mascarenes: the wonders of Mauritius. Evolutionary Systematics 5(1): 93-120. Via CC By-SA 4.0 / MediaNews Group / Reading Eagle via Getty Images
