For many, the warm glow of fireflies in the night air is a sure sign that summer has arrived. After dark, these bioluminescent beetles are generally visible only when they emit flashes of yellow, green or pale red from their lower abdomen as part of their mating ritual. Some species of firefly have found their own key to successful coupling— synchronous flashing patterns, a phenomenon that has attracted the attention of a team of researchers studying what pattern recognition tells us about how the brain is wired.
To better understand how the brains of humans and other animals process visual signals, Andrew Moiseff, a professor of physiology and neurobiology at the University of Connecticut in Storrs, and Jonathan Copeland, a biology professor at Georgia Southern University in Statesboro, over the past four summers have studied the role that synchronized flashing plays in the mating of the Photinus carolinus species of firefly found in Tennessee's Smoky Mountains National Park.
In synchronous flashing by P. carolinus fireflies, many males produce flashes simultaneously, rhythmically and repeatedly, according to the researchers, who published their findings in the July 9 issue of Science. These patterns consist of a burst of several flashes (typically six) followed by a period of no flashing that lasts about six-to-eight seconds. During these pauses, the female responds with two flashes in rapid succession, with the second flash beginning almost immediately after the first one is finished. The female may produce one to four of these "doublets" while perched on leaves or branches, says Moiseff, the study's lead author.
"This presents some interesting questions that we have not yet answered about whether the dark period, as well as the flashes, play important behavioral roles," Moiseff says.
In the laboratory, Moiseff and Copeland exposed female P. carolinus fireflies to groups of light-emitting diodes (LEDs), meant to mimic male fireflies. Each individual LED produced the species-specific pattern of flashes for P. carolinus, but the researchers varied the degree to which the flashes were in synch. Their results showed that females responded more than 80 percent of the time to flashes that were in perfect unison or in near-perfect unison. However, when the flashes were out of synch, the females' response rate was 10 percent or less. The study tells the researchers the following about the firefly brain: it is able to count, measure time differences and pause while awaiting a response.
"Our real interest is understanding brain circuitry," Moiseff says. "The method they're using to do this is through pattern recognition, something that is important to all animals." Moiseff and Copeland have studied other animals as well, including owls and bats, to better understand how brains in general are wired. "We want to understand how the brain can process these visual signals," he adds.
A firefly's bioluminescence is the result of a chemical reaction involving a protein called luciferase. About one percent of known firefly species demonstrate this synchronous flashing behavior, Marc Branham, an associate professor in the department of entomology and nematology at the University of Florida in Gainesville, explained in an e-mail to Scientific American. It's important to note, however, they are not synchronous all the time and only appear synchronous when in high densities and even then "drift" in and out of synchrony, he adds. "It is believed that in high male densities, males receive more female replies when the males are synchronous—as this cuts down on the photic 'noise' which may interfere with the females processing of the visual stimulus/signals," according to Branham, who didn't participate in Moiseff and Copeland's research.
Another outcome of Moiseff and Copeland's research could be a better understanding of what happens when a brain is injured or ceases to function properly, Moiseff says, adding, "In order to understand what's happening when the brain has problems with sensory systems you need to be able to understand how sensory systems normally work."
All images of the Photinus carolinus courtesy of Andrew Moiseff