How Do Fireflies Flash in Sync? Studies Suggest a New Answer. - Quanta Magazine

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.

Orit Peleg remembers when she first encountered the mystery of synchronous fireflies as an undergraduate studying physics and computer science.

Peleg had never even seen a firefly, as they are uncommon in Israel, where she grew up.

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.

Nearly every model for firefly synchrony ever concocted, for example, assumes that each firefly maintains its own internal metronome.

Fireflies of the species Photinus carolinus are one of a handful of species known to flash in sync.

carolinus fireflies really did try to flash on beat with a neighboring firefly — or a blinking LED — in a nearby jar.

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.

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.

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.

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 determines when it flashes.

The second is what mathematicians call coupling, the way the flash of one firefly influences its neighbors.

In a Kuramoto-esque description, each individual firefly is treated as an oscillator with an intrinsic preferred rhythm.

Even if the fireflies start off out of sync with each other, or their preferred internal rhythms vary individually, a collective governed by these rules will often converge on a coordinated flash pattern.

But when Peleg and Sarfati’s cameras began capturing three-dimensional data from the burst-then-wait Photinus carolinus fireflies in the Great Smokies in 2019, their analyses revealed new patterns.

One was the confirmation of something that Faust and other firefly naturalists had long reported: A burst of flashes would often start in one place and then cascade through the forest at about half a meter per second.

carolinus firefly in a tent, it emitted bursts of flashes randomly instead of following any strict rhythm.

But once the team dumped in 15 or more fireflies, the entire tent lighted up with collective flash bursts spaced about a dozen seconds apart.

Imagine an isolated firefly that has just emitted a burst of flashes, and consider the following rules.

This firefly is also susceptible to peer pressure: If it sees another firefly starting to flash, it will flash too, as long as it physically can.

As the number of fireflies increases, it becomes increasingly likely that at least one will randomly choose to flash again as soon as it’s biologically possible, and that will set off the rest.

In Congaree National Park in South Carolina, Peleg noticed something odd when her team trained their equipment on the synchronizing firefly Photuris frontalis.

“I remember seeing out of the corner of my eye that there is this little firefly that is really not on beat.

Peleg suggested that studies looking at the behavioral patterns of female fireflies and not just the males might be informative.

The fireflies might then start to act like neatly periodic flashers only when gathered together.

Fireflies are difficult to study in the wild because telling species apart by their flashes is hard for all but the most dedicated researchers and hardcore hobbyists.