Fireflies repel bat attacks with bioluminescence and slow, predictable flight. | Stephen Marshall
The glowing light of fireflies, used by the insects to signal mates, also warns predators that the bugs will leave a bad taste in the mouth, according to a new study.
The findings published in the August 22 issue of Science Advances challenge researchers' understanding of bioluminescence in fireflies, which has commonly been thought to be a method for mate attraction.
Bats in the study learned to avoid fireflies almost twice as quickly using a combination of vision and sonar that helped track firefly movements, compared to when they used only one of the senses alone.
"This provides fundamental evidence that combining information across senses can increase the power of [prey] warning signals," said Jesse Barber, coauthor of the study from Boise State University. "Bats using both bioluminescence and biosonar-derived movement patterns may be particularly important for firefly survival if bats interact with fireflies between flashes, or if the light environment reduces the flash's contrast against the background."
It is not easy to detect whether signals like a firefly's glowing abdomen provide multiple advantages, making it difficult to understand the role of bioluminescence in fireflies.
"Getting a natural predator and its prey to interact in a way where components of the warning signal can be isolated and dissected is inherently challenging and requires novel methods and experimental design," said Barber. "In fact, this study took us two years to refine methods and two additional years of data collection."
As the first part of their experiment, the scientists pitted flashing fireflies against three big brown bats to test whether and how bats know to stay away from the toxic glowing bugs. They discovered that the bats captured at least one firefly on the first day of the experiment. After that, the bats avoided the fireflies, prompting the researchers to wonder how they had learned their lesson.
Since at least the 19th century, naturalists have proposed that fireflies warn bats of bad taste with their glowing abdomens. "In our work," Barber said, "we … stood on the shoulders of giants and tested these ideas."
To evaluate whether it was the fireflies' flashing abdomens that kept bats away, the researchers painted over firefly bioluminescing organs to block all light production. "We had to meticulously hand-paint each beetle under a dissecting scope … bracing one hand against the other, being diligent not to cover over a spiracle [an opening on the body of an insect] and inhibit the animal's ability to breathe," said Barber.
They then provided these "darkened" insects to two bats that had previously learned to avoid fireflies. One bat captured all the darkened fireflies as well as other types of nontoxic bugs, demonstrating that it needed bioluminescence to avoid the fireflies.
But the second bat avoided all darkened fireflies and captured all the other bugs, suggesting that it was using another source of information to differentiate noxious fireflies. To evaluate what that second source might be, Barber and his colleagues used ultrasonic microphones to investigate the bats' sonar behavior. Bats use sound waves to locate their food in the dark, a phenomenon known as echolocation. In this study, the bats used echolocation to figure out the movement patterns of the fireflies.
To control the fireflies' movements, the researchers tethered the bugs and swung them gently, while they were still producing light. The recordings showed that the bats emitted distinct rapid sonar cries right before a capture attempt. But after a few nights, as they learned what to expect in the way of movement patterns from the fireflies, the bats often did not "buzz." Bats increase their buzzing when they want to glean more information from potential prey, so the fact that the buzzing stopped further reinforces the idea that the bats wanted nothing to do with these prey, explained Barber.
The lightning bugs' bioluminescence and flight patterns are an example of a multisensory warning display from prey — a process that, while it might seem costly for prey to execute, greatly accelerates the speed at which predators learn to avoid them.
"Perhaps one of the more remarkable realities of life on Earth is that most structures, processes and behaviors serve multiple functions simultaneously," said Barber.