For humans, there is something soothing and cozy about the meandering flickers of light that come from fireflies on a warm summer evening. But what do those little green flashes mean? Researchers had thought that these signals developed as a part of firefly mating and courtship. However, new research by Brian Leavell and colleagues at Boise State University and the University of Florida indicates that these seemingly friendly pops of light are also a grave warning to nearby predators: these little insects are not a tasty meal. In fact, they are downright toxic. However, there is one catch. Bats, which are notorious insect predators, are certainly not famous for their visual abilities, although they can perceive light. Could they learn that firefly flashes should be approached with caution?
The research team began their work by confirming that firefly-naïve big brown bats (collected from an area with no fireflies) can learn to avoid these little glowing insects. The researchers put a bat and a firefly in a dark flight room and recorded their interactions using a high-speed camera. The bats learned quickly: over the course of a couple of days, they began spitting out and avoiding the fireflies altogether. Next, the researchers tested whether it was the firefly's flashes that bats were learning to avoid. They painstakingly painted the abdomens of fireflies with opaque paint to conceal their glow and repeated the experiment. Bats interacting with these ‘blacked-out’ fireflies also learned to avoid them, but they learned at a slower rate than with the fully illuminated fireflies.
The scientists reasoned that the bats must be detecting some other signal produced by the insects when their abdomens were painted over in order to learn that they are toxic. A scent cue might be a logical guess; however, the authors discounted this because the bats always tasted the fireflies before learning to avoid them, indicating that there is no scent that indicates toxicity. Could the bats be using a sound-based cue? Amazingly, bats can distinguish different types of insect prey based on their wing-beat rates and flight patterns. The authors returned to the flight chamber. This time, they tethered the fireflies by delicately strapping down their wings to prevent any flight-associated sounds, but still allowed their abdomens to glow. The researchers now ‘swung’ the flightless fireflies on their tethers in front of the bats to mimic flight. Again, the bats learned to avoid the bitter insects, but at a slower rate that was similar to the rate of learning when the fireflies were blacked out. They concluded that the bats learned best when they could both see and hear the fireflies.
This study is the first to show that predators learn most effectively with multisensory cues emitted from their prey; in other words, two cues are better than one when it comes to avoiding a poisonous meal. Leavell's team also points out that only a few firefly species glow as adults, and they propose that the insects may have evolved to light up as a way to deter predators. Because other non-glowing fireflies can use scent cues to attract their partners, the team suspects that the pleasant green twinkles of glowing fireflies were later co-opted for attracting mates. Indeed, it seems that a firefly's neon flicker can simultaneously signal ‘danger!’ and beckon ‘come hither’, depending on who is watching.
