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Firefly Light Shows Don't Just Dazzle. Swarms Can Also Synchronize Their Flashes

A processed image showing the lights of firefly swarms in the Smoky Mountains.
A processed image showing the lights of firefly swarms in the Smoky Mountains.

Updated July 9, 2021 at 5:09 PM ET

Add this to your summer to-do list: next time you see the fireflies outside, try paying attention to the rhythm of the flashes.

In some species, swarms will flash in sync.

The fireflies' light show, of course, is a mating ritual, as the males create a chemical reaction to get the attention of females.

"The phenomenon of fireflies in sync goes back hundreds of years," says Cornell University mathematician Steven Strogatz, "going back to the 1500s when Sir Francis Drake and his crew were traveling and they saw these spectacular displays of male fireflies on the riverbanks in Thailand and Malaysia gathered in mangrove trees, all flashing in unison all night long."

It's an example of what's called synchronous behavior, a subject that's been of growing interest to mathematicians and physicists.

"Synchrony is all over the place in nature, and fireflies are one of the most spectacular and dramatic examples of it," Strogatz tells NPR's All Things Considered. "And mathematically, it's very mysterious because in math terms, we're studying an enormous system of so-called oscillators, things that have a rhythm, that go through a cycle, and in this case a cycle of flashing periodically."

In 1990, Strogatz developed the first mathematical model to explain fireflies' synced-up light show. Physicists from the University of Colorado Boulder wanted to investigate some of the mystery behind that effect, so last June, they packed up their cameras and headed to the Smoky Mountains. For seven nights, they documented a species of firefly called Photinus carolinus, which put on one of the more intricate light shows. The displays are a series of synchronized bursts that look a little like visual Morse code.

The researchers then turned that data into a 3D reconstruction of the swarm. What they found was that the fireflies need to reach critical density before the rhythm kicks in.

"The flashes tend to move pretty rapidly but not instantly from one point of the swarm to another," says Raphael Sarfati, one of the researchers. "So there's really a propagation effect very similar to a wave of flashing information."

Maybe you will notice something that is unexpected, or maybe you'll feel like they are indeed on a dialogue sometimes and responding to one another.

Their findings were published this week in the journal Science Advances.

"Fireflies that are further away start flashing into bursts, and then a little bit closer to you and a little bit closer to you, and then they pass by and you can really experience this wave that's just beautiful," says Orit Peleg, another of the study's authors.

What they're describing is a relay, almost, where the fireflies sync themselves with those nearby, sending out waves of light. While you might not be able to see this exact phenomenon, as it's rare in American firefly species, Sarfati suggests taking some time to observe your nearby fireflies anyway.

"Just try to watch and see if you notice something particular, like something that catches your eye about how they're flashing — not so much the one firefly but how they're collectively flashing," Sarfati says. "Maybe you will notice something that is unexpected, or maybe you'll feel like they are indeed on a dialogue sometimes and responding to one another."

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