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Fireflies use social cues to synchronize their lights

Every summer in the Great Smoky Mountains National Park, male fireflies put on a spectacular synchronized light show to capture the attention of potential mates. But how do thousands of fireflies manage to flash their lights simultaneously? 

In a new study, researchers at UC Boulder have found that firefly synchronization seems to be driven by social cues rather than by a hardwired biological response.

For about two weeks early in the month of June, male fireflies of the species Photinus carolinus flash their lights in synchrony for a few hours at a time.

According to Professor Orit Peleg, the firefly display is over almost as soon as it begins, so studying it in detail “is a constant race against time.”

During the incredible display, swarms of males fly low to the ground to show off for females hiding in the leaves below. The fireflies flash with a distinct rhythm: a few quick bursts of light followed by a several-second pause, then more bursts. 

“This is the most beautiful biological phenomenon that I’ve ever witnessed,” said Professor Peleg.

Despite the attention that it draws, however, scientists have struggled to explain how this synchronization works. Study lead author Raphaël Sarfati is a postdoctoral researcher at CU Boulder.

“Is it something hardwired in fireflies that makes them want to synchronize? Or is it something more context dependent, maybe based on their environment?”

To investigate, the researchers set up a field experiment in the Great Smoky Mountains National Park in June of 2019. They set up two 360-degree cameras in a wooded area to map out the locations of bugs flashing in their vicinity. 

The experts also assembled a pop up tent on their site and introduced some fireflies to this isolated environment. “It was, basically, like we were one of the fireflies in the swarm,” said Sarfati.

The study revealed that male P. carolinus fireflies do not behave the same when they are alone as they behave in the swarm. 

For example, when a single male was placed in the tent all on his own, the bug flashed without a good sense of rhythm and displayed spontaneous bursts of light. 

“When you start putting 20 fireflies together, that’s when you start observing what you see in the wild,” said Sarfati. “You’ve got regular bursts of flashes, and they’re all synchronized.”

The results suggest that the light shows are more complicated and less predictable than scientists had previously expected. The fireflies seem to observe what their neighbors are doing, and then adjust their behavior to match, rather than flashing in rhythms that are preprogrammed or innate.

Professor Peleg noted that the findings may ultimately help experts understand more about a range of other synchronous behaviors in nature.

“This kind of synchrony occurs in many natural systems. The cells in our hearts all flex and contract at the same time. Neurons in our brains also synchronize,” said Professor Peleg.

The researchers also hope that their work will inspire citizen scientists to help protect fireflies, who are increasingly competing with human sources of light to attract light. 

“So many people have had positive experiences with fireflies,” said Sarfati. “They’re also very fragile. Many species are on the decline around the world because there is more and more light pollution.”

The study is published in the Journal of The Royal Society Interfac.

By Chrissy Sexton, Staff Writer

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