Research shows a new reason for why fireflies glow

Updated: September 6, 2018

a-firefly-in-the-night Photo by anko70/Shutterstock

Watching the ephemeral flash of a firefly’s abdomen pinprick the night sky is always a summer highlight at the cottage. For years, biologists assumed the blinking lights were used only for mating, but recent research out of Boise State University suggests the lights might also ward off nocturnal predators. “That’s what we initially set out to test,” says Jesse Barber, a biologist at Boise State University. “Do bats avoid fireflies using their bioluminescent signal?”

The hypothesis sounds counterintuitive. Isn’t the flashing light a beacon for nocturnal predators? It turns out that’s not the case as “[Fireflies] are very chemically protected,” Barber says. They contain a steroid known as lucibufagin that makes them distasteful, even toxic, to predators. “The toxins have been shown to kill some animals with just one firefly ingested.” So, rather than a beacon, the flashing light of a firefly is more of a warning sign to predators.

Barber and his team tested this theory by releasing a set of naïve bats — bats who had never experienced fireflies before — in a dark room with a group of fully intact, free-flying fireflies. After a few encounters, the bats quickly learned to avoid the fireflies. Barber and his team then used fine paintbrushes to apply black paint — the kind you’d use on model airplanes — to the abdomens of the fireflies, concealing their bioluminescent signal. “This was a really tedious operation,” Barber says. “It would take sometimes 45 minutes per firefly under a dissecting scope, bracing one hand off the other, to cover every single piece.”

Barber and his team then introduced a new set of naïve bats to the painted fireflies. Although the bats did eventually learn to avoid the insects, it took them twice as long. “They did so not as well and not as quickly,” Barber says, “but they were able to use characteristics that they extracted from the sonar to be able to tell that these pray were not the ones they wanted to catch.” Without the flashing light acting as a warning signal, the bats relied on their echolocation, identifying and familiarizing themselves with the fireflies’ flight patterns.

It turns out that along with their flashing lights, fireflies have a distinct flight pattern. “They fly this very level, very slow path,” Barber says. The combination of their bioluminescent signal and distinct flight pattern alert predators like bats to their toxicity. “When you combine the flight pattern and the bioluminescence, flight and light, you get about twice as fast a learning.”

So, next time you’re at the cottage enjoying the flashing lights show, you’re actually witnessing some high-stakes sabre-rattling.

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