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SUNDAY SCIENCE: CICADAS ARE EMERGING NOW. HOW DO THEY KNOW WHEN TO
COME OUT?  
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Carl Zimmer
April 24, 2024
New York Times
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_ Scientists are making computer models to better understand how the
mysterious insects emerge collectively after more than a decade
underground. _

The United States is home to a dozen cicada broods that have a
17-year cycle, and three with a cycle that takes 13 years., Will
Dunham/Reuters

 

Earlier this month, millions of Americans looked up at the sky to
witness a total eclipse. Now, another cyclical marvel has arrived,
this time at our feet. Trillions of noisy, red-eyed insects
called cicadas
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emerging from the earth after more than a decade of feeding on tree
roots.

The United States is home to 15 cicada broods, and in most years at
least one of them emerges. This spring, Brood XIX, known as the Great
Southern Brood, and Brood XIII, or the Northern Illinois Brood, are
emerging simultaneously.

Cicada watchers have spotted the first insects coming out of the
ground, reporting their sightings to apps such as iNaturalist
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Safari [[link removed]]. The
Great Southern Brood, which emerges across the South and the Midwest
every 13 years, has been seen at sites scattered from North Carolina
to Georgia. The Northern Illinois Brood, which appears every 17 years
in the Midwest, is expected to appear in the next month, as
temperatures there warm.

How cicadas manage to rise en masse after spending so long underground
remains largely a mystery. “There’s surprisingly little
information about cicadas that you’d like to know,” said Raymond
Goldstein, a physicist at the University of Cambridge.

Once a brood climbs out of the ground, the cicadas crawl up trees to
mate, and the females lay eggs in tree branches. After hatching, the
young insects drop to earth and burrow into the soil. Then, each
cicada spends the next 13 or 17 years underground before emerging to
mate and repeat the cycle.

That means that trillions of insects have to track the passage of time
in the soil. It’s possible that they detect annual changes in tree
roots. But how can cicadas add up those changes to divine when 13 or
17 years have passed? Scientists cannot say.

Chris Simon, a cicada expert at the University of Connecticut,
suspects that some answers will be found in the insects’ DNA. “Is
there a consistent difference between something that has a 13-year
cycle and a 17-year cycle?” she asked.

Dr. Simon and her colleagues recently sequenced
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first time. They caught the insect, which belonged to a brood with a
17-year cycle, in Tennessee in 2021. They hope to sequence the genes
of insects from other broods as well, and compare their DNA.

Once cicadas recognize — somehow — that they’ve reached their
special year, they need a way to emerge together. Evolutionary
biologists have proposed that cicadas come out in vast numbers as a
survival strategy. Their enemies, such as birds and parasitic wasps,
can attack only a small fraction of them, leaving the rest free to
reproduce.

One crucial signal is the temperature of the ground. The soil needs to
pass a threshold of about 64 degrees before broods start to appear.

But cicadas cannot surface together simply by sensing the warming
soil. An immature cicada that happens to be a couple feet underground
will experience cooler temperatures than one just a few inches below
the surface. If cicadas paid attention only to the temperature they
felt nearby, they would come out in small groups and be quickly wiped
out by predators.

Dr. Goldstein and his wife, Adriana Pesci, a mathematician at
Cambridge, recently became intrigued by this paradox. “We’re
attuned to mysteries,” Dr. Goldstein said.

Working with their Cambridge colleague Robert Jack, Dr. Goldstein and
Dr. Pesci created a mathematical model
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cicada brood based on observations of real insects. Then, they played
with the different variables in their model to get the simulated
cicadas to emerge together like real ones.

The scientists speculated that cicadas base their decision to come out
not just on the rising temperature of the soil, but also on the
actions of neighboring cicadas. The researchers allowed their virtual
insects to eavesdrop on each other. If their neighbors were getting
noisy as they prepared to climb out of the ground, the insects were
more likely to emerge as well.

It turned out that the model worked only if the scientists let the
cicadas communicate this way. The combination of temperature and
communication caused broods to emerge in a rapid series of bursts —
which is exactly what happens in the real world. Each burst
included cicadas
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were in soil that had not yet reached 64 degrees. Once they left, it
took a few days for the soil to warm enough to prompt more cicadas to
prepare to leave.

Dr. Goldstein acknowledged that he and his colleagues simply added a
hypothetical communication channel to their model to make it work.
They have no direct evidence that cicadas actually listen to each
other underground.

“Nobody’s ever tried to figure that out,” Dr. Simon said. “It
would be very difficult to do.”

As bizarre as cicada emergences may seem, Dr. Goldstein sees them as
part of a broader pattern in biology. Many animals, from flocking
birds to herds of wildebeests, have to make collective decisions based
on noisy, unreliable signals. Even cells in a developing embryo have
to coordinate their growth.

“It’s the essence of life,” he said.

_CARL ZIMMER [[link removed]] covers news
about science for The Times and writes the Origins column
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_Subscribe to the NEW YORK TIMES.
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__

Where Do Elbows and Knees Come From? Biologists Track Them Back to Our
Boneless, Sharklike Ancestors
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Elizabeth Pennisi
Joints in jaws and limbs, once thought to have evolved for life on
land, turn out to date back much earlier, to sharks and their
relatives
SCIENCE MAGAZINE
April 25, 2024

* Science
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* biology
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* reproduction
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* cicada
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