by A similar scenario played out in the 1990s, when a Tennessee naturalist named Lynn Faust read the confident published claim by a scientist named Jon Copeland that there were no synchronous fireflies in North America. Faust knew then that what she had been observing for decades in the nearby woods was something extraordinary.
Faust invited Copeland and Moiseff, his collaborator, to see a species in the Great Smoky Mountains called Photinus carolinus. Male firefly clouds fill forests and clearings, hovering at approximately human height. Instead of blinking in close coordination, these fireflies emit a burst of rapid flashes in a few seconds, then go silent several times before losing another burst. (Imagine throngs of paparazzi waiting for celebrities to appear at regular intervals, taking a series of photos at each appearance, then twiddling their thumbs in the downtime.)
The experiments of Copeland and Moiseff showed that the P. carolinus the fireflies actually tried to blink in time with a neighboring firefly, or a blinking LED, in a nearby jar. The team also installed high-sensitivity video cameras at field edges and forest clearings to record the flashes. Copeland went through the footage frame by frame, counting how many fireflies were lit at each moment. Statistical analysis of this carefully collected data showed that all fireflies within view of the cameras in a scene actually emitted bursts of flashes at regular and correlated intervals.
Two decades later, when Peleg and his postdoc, 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 were able to capture the dynamics of a swarm of fireflies from the inside, not just the side. Instead of counting the flashes by hand, Sarfati devised processing algorithms that could triangulate the firefly flashes captured by both cameras, and then record not only when each flash occurred, but also where it occurred in three-dimensional space.
Sarfati first introduced this system in Tennessee in June 2019 for the P. carolinus fireflies that Faust had made famous. It was the first time he had seen the show with his own eyes. He had envisioned something like the tightly-synchronized firefly scenes of Asia, but the Tennessee bursts were messier, with bursts of up to eight rapid flashes for about four seconds repeated about every 12 seconds. Yet 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, confusing in a way, but also beautiful,” he said.
Random but sympathetic flashers
In his student experience with synchronized fireflies, Peleg first learned to understand them through a model formalized by Japanese physicist Yoshiki Kuramoto, based on earlier work by theoretical biologist Art Winfree. This is the ur model of synchrony, the granddaddy of mathematical schemes that explain how synchrony can arise, often inexorably, in anything from clusters of pacemaker cells in the human heart to alternating currents.
In their most basic form, synchronous system models need to describe two processes. One is the internal dynamics of an isolated individual, in this case a lone firefly in a jar, governed by a physiological or behavioral rule that determines when it blinks. The second is what mathematicians call coupling, the way a flash from one firefly influences its neighbors. With haphazard combinations of these two parts, a cacophony of different agents can quickly become an orderly chorus.
In a Kuramoto-esque description, each individual firefly is treated as an oscillator with an intrinsic preferred beat. Imagine fireflies as having a hidden pendulum that is constantly swinging inside of them; Imagine that an insect blinks every time your pendulum passes the bottom of its arc. Suppose also that seeing a nearby flash pulls the pendulum of a pacing firefly a little forward or backward. Even if the fireflies start out out of sync with each other, or if their preferred internal rhythms vary individually, a collective governed by these rules will often converge on a coordinated pattern of flashes.
Several variations of this general scheme have emerged over the years, each modifying the rules of internal dynamics and coupling. In 1990, Strogatz and his colleague Rennie Mirollo of Boston College showed that a very simple set of firefly-like oscillators would almost always be in sync if you interconnected them, no matter how many people you included. The following year, Ermentrout described how groups of Pteroptyx malaccae fireflies in Southeast Asia could synchronize by speeding up or slowing down their internal frequencies. Recently, in 2018, a group led by Gonzalo Marcelo Ramírez-Ávila of the Universidad Superior de San Andrés in Bolivia devised a more complicated scheme in which fireflies alternate between a “charge” state and a “discharge” state during the which shone
