For modelers, the race is now on to encapsulate the observed firefly patterns in new and improved frameworks. Ermentrout has a paper under review that offers a different mathematical description of Photinus carolinus: Suppose that instead of waiting a purely random amount of time beyond the compulsory minimum for recharging, the bugs are just noisy, irregular oscillators? The fireflies might then start to act like neatly periodic flashers only when gathered together. In computer simulations, this model also matches the Peleg group’s data. “Even though we didn’t program it in, things like the waves emerge,” Ermentrout said.
In the Smoky Mountains, thousands of fireflies flash in unison; researchers want to know how
During typical summers in the southeastern U.S., streams of visitors travel to Great Smoky Mountains National Park to witness one of nature’s most spectacular displays of light: thousands of male fireflies, all flashing together in near-perfect harmony.
“This is the most beautiful biological phenomenon that I’ve ever witnessed,” said Orit Peleg, an assistant professor in the Department of Computer Science and BioFrontiers Institute at CU Boulder.
In a study published today in the Journal of The Royal Society Interface, she and her lab members shed new light on this beautiful phenomenon—striving to understand how relatively simple insects manage to coordinate such feats of synchronization.
The team discovered that the light shows may be more complicated than scientists realized: Rather than flash according to some innate rhythm, the fireflies seem to observe what their neighbors are doing, then adjust their behavior to match.
Peleg’s team hopes the research will inspire citizen scientists around the world to get out and help to protect populations of these charismatic creatures. Some of these insect species, which use their glow to attract mates, have found themselves competing for attention with human sources of light.
“So many people have had positive experiences with fireflies,” said Raphaël Sarfati, lead author of the new study and a postdoctoral researcher at CU Boulder. “They’re also very fragile. Many species are on the decline around the world because there is more and more light pollution.”
One in a swarm
As Peleg describes it, the firefly display is over almost as soon as it begins. Males belonging to the species Photinus carolinus only flash for about two weeks every June, and then just for a few hours a night.
Studying them “is a constant race against time,” Peleg said.
Still, the sight is a must-see: During their mating displays, swarms of males stay low to the ground so as to better show off for females hiding in the leaf litter below. They also flash with a distinct rhythm: a few quick bursts of light followed by a several-second pause, then more bursts. In person, the display looks like a wave of light passing over the hillside.
To date, scientists have struggled to explain how this synchronization works.
“Is it something hardwired in fireflies that makes them want to synchronize?” Sarfati said. “Or is it something more context dependent, maybe based on their environment?”
To find out, the researchers drove to Great Smoky Mountains National Park in June 2019. There, they set up two 360-degree cameras in a wooded area—a new technological approach that allowed the team to map out the locations of the bugs flashing in their vicinity. The group also assembled a pop tent on site and introduced a few fireflies at a time to the isolated environment.
“It was, basically, like we were one of the fireflies in the swarm,” Sarfati said.
Follow the leader
A composite image of fireflies flashing in a forest in the Smoky Mountains. (Credit: Peleg Lab)
The researchers-turned-fireflies found what they were looking for: Male P. carolinus fireflies, the team reported, don’t behave the same when they’re alone versus in a big group.
When the researchers, for example, put a single male into the pop tent all on his own, that bug would flash without a good sense of rhythm—a few bursts here, a few bursts there. Increase the number of fireflies, however, and things began to change.
“When you start putting 20 fireflies together, that’s when you start observing what you see in the wild,” Sarfati said. “You’ve got regular bursts of flashes, and they’re all synchronized.”
The fireflies, in other words, likely aren’t hardwired to flash with a particular pattern. Instead, their light displays seem to be more social. Bugs watch what their neighbors are doing and try to follow along. The group’s findings, Peleg said, could help researchers learn more about a range of other synchronous behaviors in nature—and maybe one day design swarming robots that act in tandem.
“This kind of synchrony occurs in many natural systems,” Peleg said. “The cells in our hearts all flex and contract at the same time. Neurons in our brains also synchronize.”
Firefly populations have also proved tricky for researchers to monitor in the wild, she said. Her team thinks that its 360-degree camera approach could provide a solution, perhaps allowing anyone to head out into the woods to keep an eye on their local fireflies. Peleg, at least, doesn’t want to see the glow from these insects disappear.
“There always was this last half an hour at night where we were tired and sitting in the forest, waiting for the fireflies to stop flashing,” she said. “It was the most relaxing part of this work.”
Other coauthors on the new study include Julie Hayes, a former post-baccalaureate student at CU Boulder, and Élie Sarfati of the Supinfogame Rubika in France.
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Introduction
In Japanese folk traditions, they symbolize departing souls or silent, ardent love. Some Indigenous cultures in the Peruvian Andes view them as the eyes of ghosts. And across various Western cultures, fireflies, glow-worms and other bioluminescent beetles have been linked to a dazzling and at times contradictory array of metaphoric associations: “childhood, crop, doom, elves, fear, habitat change, idyll, love, luck, mortality, prostitution, solstice, stars and fleetingness of words and cognition,” as one 2016 review noted.
Physicists revere fireflies for reasons that might seem every bit as mystical: Of the roughly 2,200 species scattered around the world, a handful have the documented ability to flash in synchrony. In Malaysia and Thailand, firefly-studded mangrove trees can blink on beat as if strung up with Christmas lights; every summer in Appalachia, waves of eerie concordance ripple across fields and forests. The fireflies’ light shows lure mates and crowds of human sightseers, but they have also helped spark some of the most fundamental attempts to explain synchronization, the alchemy by which elaborate coordination emerges from even very simple individual parts.
Orit Peleg remembers when she first encountered the mystery of synchronous fireflies as an undergraduate studying physics and computer science. The fireflies were presented as an example of how simple systems achieve synchrony in Nonlinear Dynamics and Chaos, a textbook by the mathematician Steven Strogatz that her class was using. Peleg had never even seen a firefly, as they are uncommon in Israel, where she grew up.
“It’s just so beautiful that it somehow stuck in my head for many, many years,” she said. But by the time Peleg began her own lab, applying computational approaches to biology at the University of Colorado and at the Santa Fe Institute, she had learned that although fireflies had inspired a lot of math, quantitative data describing what the insects were actually doing was scant.
She set out to fix that. In the last two years, a series of papers from Peleg’s group have opened a fire hose of real-world data about synchrony in multiple firefly species at multiple study sites, and at a much higher resolution than previous modelers or biologists had managed. “Pretty astonishing” is how the mathematical biologist Bard Ermentrout at the University of Pittsburgh described the team’s results to Quanta. “I was blown away,” said Andrew Moiseff, a biologist at the University of Connecticut.
These papers establish that real firefly swarms depart from the mathematical idealizations that flitted through journals and textbooks for decades. Nearly every model for firefly synchrony ever concocted, for example, assumes that each firefly maintains its own internal metronome. A preprint that Peleg’s group posted in March, however, showed that in at least one species, individual fireflies have no intrinsic rhythm, and it posited that a collective beat emerges only from the spooky synergy of many lightning bugs gathered together. An even more recent preprint, first uploaded in May and updated last week, documented a rare type of synchrony that mathematicians call a chimera state, which has almost never been observed in the real world outside of contrived experiments.
Firefly biologists hope the new methods will reshape the science and conservation of fireflies. Mathematicians devising theories of synchrony like the ones that Strogatz described in his textbook, meanwhile, have operated without much experimental feedback from messy real-world synchronizers. “That’s the big breakthrough,” said Strogatz, a professor of mathematics at Cornell University. “Now we can start closing the loop.”
The Elusive Proof of Synchrony
Reports of fireflies flaring in unison in Southeast Asia filtered back to Western scientific discourse for centuries. Thousands of fireflies, called kelip-kelip in Malaysia — their name is a sort of visual onomatopoeia for their twinkling — can settle on riverside trees. “Their light blazes and is extinguished by a common sympathy,” a British diplomat touring Thailand wrote in 1857. “At one moment every leaf and branch appears decorated with diamond-like fire.”
Not everyone accepted these reports. “For such a thing to occur among insects is certainly contrary to all natural laws,” one letter to the journal Science complained in 1917, arguing that the apparent effect was instead caused by the viewer’s involuntary blinking. Yet by the 1960s, visiting firefly researchers confirmed through quantitative analysis what local boatmen in mangrove swamps had long known.
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Introduction
A similar scenario played out in the 1990s, when a Tennessee naturalist named Lynn Faust read the confident published assertion of a scientist named Jon Copeland that there were no synchronous fireflies in North America. Faust knew then that what she had been watching for decades in the nearby woods was something remarkable.
Faust invited Copeland and Moiseff, his collaborator, to see a species in the Great Smoky Mountains called Photinus carolinus. Clouds of the male fireflies fill forests and clearings, floating at about human height. Instead of blinking in tight coordination, these fireflies emit a burst of quick flashes within a few seconds, then go quiet for several times that long before loosing another burst. (Imagine a crowd of paparazzi waiting for celebrities to appear at regular intervals, snapping a salvo of photos at each appearance, and then twiddling their thumbs in the downtime.)
Copeland and Moiseff’s experiments showed that isolated P. carolinus fireflies really did try to flash on beat with a neighboring firefly — or a blinking LED — in a nearby jar. The team also set up high-sensitivity video cameras at the edges of fields and forest clearings to record flashes. Copeland went through the footage frame by frame, counting how many fireflies were illuminated at each moment. Statistical analysis of this painstakingly gathered data proved that all the fireflies within the cameras’ view at a scene really did emit flash bursts at regular, correlated intervals.
Two decades later, when Peleg and her postdoc, the physicist Raphaël Sarfati, set out to collect firefly data, better technology was available. They designed a system of two GoPro cameras placed a few feet apart. Because the cameras took 360-degree video, they could capture the dynamics of a firefly swarm from within, not just from the side. Instead of counting flashes by hand, Sarfati devised processing algorithms that could triangulate on firefly flashes caught by both cameras and then record not just when each blink happened but where it occurred in three-dimensional space.
Sarfati first brought this system into the field in Tennessee in June 2019 for the P. carolinus fireflies that Faust had made famous. It was his first time seeing the spectacle with his own eyes. He had imagined something like the tight scenes of firefly synchrony from Asia, but the Tennessee bursts were messier, with bursts of up to eight quick flashes over about four seconds repeated roughly every 12 seconds. Yet that messiness was exciting: As a physicist, he felt that a system with wild fluctuations could prove far more informative than one that behaved perfectly. “It was complex, it was confusing in a sense, but also beautiful,” he said.
