A rare look at fireflies that blink in unison, in a forest without tourists

Forests are darker and quieter due to coronavirus, giving a photographer and researchers the chance to document a special moment.

Fireflies known as snappy syncs (Photuris frontalis) illuminate the bottomlands of Congaree National Park in May each year, creating an astonishing pulsing display with their rapid-fire, coordinated flashes.

Photograph by Mac Stone

In Congaree National Park, South Carolina, fireflies have a seemingly magical talent: Lighting up in a synchronized display, pulsating in near-unison in the dark forest during a brief window in late spring.

These fireflies are known as snappy syncs (Photuris frontalis) for their fast, coordinated signals. Out of 125 firefly species in North America, they are one of a handful that can display in a synchronized fashion. The males blink out their signals while perched in short vegetation or while flying low, emitting quick bursts of bioluminescent light in the hopes of attracting females. But the behavior remains poorly understood.

In 2019, more than 12,000 people came to watch this incredible biological phenomenon, says David Shelley, chief of resource stewardship and science at Congaree. But this year, due to the coronavirus pandemic, the park's annual festival was cancelled.

While the public may be disappointed, the absence of people offers researchers a rare look at the insects in Congaree and beyond, providing data in a pristine state that might not be possible again.

Moreover, fireflies can breed this summer without interference from light pollution and disturbance in forests around the country—likely serving as a boon to the insects, many of which are declining across their range.

To record and study the bioluminescent creatures during this unusual moment in time, a team of researchers and National Geographic Explorer Mac Stone spent more than a week in Congaree in mid-May.

“We’re pretty hopeful that it is some of the most natural data we can get in terms of them not being [influenced] by people,” says Julie Hayes, a researcher in ecology and computer science at University of Colorado-Boulder.

Stone was attracted to the project because the fireflies live among stands of old-growth cypress, a rare and imperiled type of forest that he’s photographing around the Southeast. The pandemic had canceled many of his events and trips, but the visit to Congaree kind of “took the sting out of it for me.” Stone says. “It was an honor”—and a unique opportunity to take night-time photos to share publicly without the interruption of visitor’s flashlights.

Exploring the mystery

In May, Hayes and University of Colorado-Boulder colleague Raphael Sarfati traveled to Congaree to take 3-D video recordings of the insects as they displayed.

Shelley says he was happy that the researchers could take advantage of this moment. “You get one shot at this per year.”

Exactly how the fireflies coordinate their signalling and achieve synchrony remains a mystery. The insects can signal at roughly the same time across a wide area, even though they can only see the individuals within a short distance of themselves.

“Somehow that’s enough to have everybody on the same page,” Sarfati says. “They somehow set their own personal clock at the same pace as the rest of the group.”

Sarfati, Hayes, and lab leader Orit Peleg have not yet published research from their May expedition, but they got clues about how dense the insects must be before they start synchronizing and how their signals propagate over long distances.

As to what the purpose of the synchrony is, “we ask ourselves that every day,” Peleg says. (Learn more: How do these mysterious fireflies synchronize their dazzling light shows?)

But the answer promises to be revelatory, researchers say.

Synchronous phenomena are vital to life as we know it, from the coordinated contractions of the heart to the firing of neurons in the brain.

All three scientists have an interest or background in physics and computer science, and work to square mathematical models of synchronous or emergent behavior with what they observe in the wild.

Models have been created by physicists such as Steven Strogatz, who studied the insects and marveled at their “bewildering complexity,” as he put it.

“We’re wading through that bewildering complexity,” Hayes says, which is difficult. “It’s a very different process to fit the model to the behavior you’re seeing in the field, versus using a pre-existing theoretical model” based on previous observations, she says.

None alike

One aspect of the complexity: Every species is quite different.

For example, Peleg, Hayes, and Sarfati are all currently studying the synchronous signaling of Photinus carolinus, another firefly species in the Smoky Mountains of Tennessee and North Carolina. Unlike snappy syncs, these insects emit a burst of signals before going dark for about six to eight seconds, depending on the temperature and humidity. Then they start up again.

The peak for these fireflies is happening right now, in early June. “I’m running on four hours of sleep, rushing around to see the fireflies,” says Lynn Faust, who wrote the book Fireflies, Glow-worms, and Lightning Bugs, a guide to the insects in the central and eastern United States. “It’s intense because everything comes out at once.”

Faust says that the two species, in Congaree and the Smoky Mountains, are very distinctive, not just in their signals—the former being rapid fire, the latter being syncopated with coordinated pauses.

Female snappy syncs have also been shown, like other members of their genus, to sometimes cannibalize males of their own and related species. That’s why they’re sometimes called “femme fatales,” for their ability to lure in and kill unsuspecting males, Faust says.

“Heartbeat of the swamp”

The respite from people in these locations not only affords research opportunities—it may actually benefit the insects themselves.

Firefly larvae live in the topsoil and leaf litter for a couple years before emerging as adults. Having a year without a lot of human visitors trampling the larvae will “allow more to make it to adulthood,” Faust says. “I see it as a positive thing.”

Stone says it was still hectic running about Congaree in the near-dark trying to focus cameras and using long exposures that are sensitive to the slightest movement and light. But the lack of people definitely made Stone’s job easier, and the chance to see and document the insects was priceless.

“You feel a big responsibility” to do it well, Stone says. “It looks like you're seeing the heartbeat of the swamp … it's transformative to see.”

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