A new explanation for how fireflies flash in sync

A similar scenario played out in the 1990s, when a naturalist from Tennessee was named Lyn Faust read the self-assured published claim of a scientist named Jon Copeland that there were no synchronous fireflies in North America. Faust knew then that what she had seen 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, hovering at about human height. Instead of blinking in a tightly coordinated manner, these fireflies emit a burst of rapid flashes within a few seconds, then go silent for a few times before losing another burst. (Imagine a crowd of paparazzi waiting for celebrities to appear at regular intervals, taking a volley of shots at each appearance, then twiddling their thumbs in the downtime.)

Copeland and Moiseff’s experiments demonstrated this in isolation P. carolinus fireflies actually tried to flash to the beat with a neighboring firefly – or a flashing LED – in a nearby jar. The team also placed highly sensitive video cameras at the edges of fields and forest clearings to record flashes. Copeland watched the footage frame by frame, counting how many fireflies were lit at any one time. Statistical analysis of this painstakingly collected data proved that all fireflies in the field of view of the cameras on a scene were indeed emitting flashes at regular, correlated intervals.

Two decades later, when Peleg and her postdoc, the physicist Raphael 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 captured 360-degree video, they were able to capture the dynamics of a firefly swarm from the inside, not just from the side. Instead of counting flashes by hand, Sarfati devised processing algorithms that could triangulate firefly flashes captured by both cameras, then record not only when each flash occurred, but also where it occurred in three-dimensional space.

Sarfati first brought this system to the field in June 2019 in Tennessee for the P. carolinus fireflies that Faust had made famous. It was the first time he had seen the spectacle with his own eyes. He had envisioned something like the tight scenes of firefly synchronization from Asia, but the Tennessee eruptions were messier, with bursts of up to eight rapid flashes lasting about four seconds that repeated about every 12 seconds. Still, that mess was exciting: as a physicist, he felt that a system with wild fluctuations could be much more informative than one that behaved perfectly. “It was complex, it was confusing in a way, but also beautiful,” he said.

Random but sympathetic strobes

In her undergraduate thesis on synchronizing fireflies, Peleg first learned to understand them through a model formalized by the Japanese physicist Yoshiki Kuramoto, building on previous work by theoretical biologist Art Winfree. This is the ur model of synchrony, the grandfather of mathematical schemes that explain how synchrony can arise, often inexorably, in everything from groups of pacemaker cells in human hearts to alternating currents.

At their most basic, models of synchronous systems must describe two processes. One is the inner dynamics of an isolated individual—in this case, a lone firefly in a jar, governed by a physiological or behavioral rule that dictates when it flashes. The second is what mathematicians call coupling, the way a firefly’s flash affects its neighbors. With accidental combinations of these two parts, a cacophony of different agents can quickly pull together into a neat chorus.

Yoshiki Kuramoto, a professor of physics at Kyoto University, developed the most famous synchronization model in the 1970s and co-discovered the chimera in 2001.

Photo: Tomoaki Sukezane

In a Kuramoto-esque description, each individual firefly is treated as an oscillator with an intrinsic preferred rhythm. Imagine fireflies having a hidden pendulum swinging steadily within them; imagine a bug flashing every time its sling sweeps through the bottom of its bow. Also suppose seeing a neighboring flash pulls a firefly’s rate-determining pendulum forward or backward a little bit. Even if the fireflies start out in sync with each other, or their preferred internal rhythms vary individually, a collective that adheres to these rules will often come together in a coordinated flashing pattern.

Over the years, several variations of this general scheme have emerged, each of which modified the rules of internal dynamics and coupling. In 1990, Strogatz and his colleague Rennie Mirollo from Boston College proved that a very simple set of firefly-like oscillators would almost always synchronize if you connected them together, no matter how many individuals you had. The following year, Ermentrout described how groups Pteroptyx malaccae fireflies in Southeast Asia could synchronize by speeding up or slowing down their internal frequencies. As recently as 2018, a group led by Gonzalo Marcelo Ramirez-Avila of the Higher University of San Andrés in Bolivia devised a more complicated scheme in which fireflies switched back and forth between a “charging” state and a “discharging” state in which they flashed.

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