Fireflies use their light to attract mates. However, the downside of being visible to potential sexual partners is that it can bring you to predators' attention. Scientists have now noticed that fireflies have evolved what sounds like a solution.
When you stand out enough to launch a whole new tourism industry, it's a fair bet that things that want to eat you will catch on as well. Fireflies have dealt with this is becoming poisonous to insect-eating bats, which would otherwise pose the biggest threat to them until humans arrived. However, tasting really bad doesn't help a lot if no one knows this until after you're dead, so it pays to advertise just how unpalatable you are. Indeed, there is even a theory their beautiful light first evolved as a way of saying “hey I'm not good to eat”.
According to Professor Yossi Yovel of Tel Aviv University, fireflies may have done this a second time by hacking bats' not-particularly secret weapon.
Yovel came upon the discovery by accident. "We were wandering around a tropical forest with microphones capable of recording bats' high frequencies, when suddenly, we detected unfamiliar sounds at similar frequencies, coming from fireflies," he recalled in a statement. "In-depth research using high-speed video revealed that the fireflies produce the sound by moving their wings.”
Further investigation revealed the pitch is not only inaudible to humans, but also other fireflies – this is neither a mating call nor a way of claiming territory.
So if the fireflies don't make this sound for their own kind, and presumably not for researchers with ultrasonic detectors, who is it for? It could just be a side-effect of the way the insects move their wings without an evolutionary purpose of its own, but Yovel thought the possibility it was a bat deterrent was worth exploring. He found the sound is produced by fireflies from four genera native to locations at the opposite ends of Asia, so it's no oddity restricted to a single population. In at least two species, both sexes make the sounds.
Yovel calls the sounds “musical armor” and published his research in iScience. He admits he cannot prove it evolved as a mechanism for alerting bats that these insects are not for eating, but thinks there are several reasons to consider it likely. For one thing, the fact the sounds have a narrow peak around 40-50kHz, matching many bats' prime hearing range, seems unlikely to be a coincidence.
"The idea of warning signals that the sender itself cannot detect is known from the world of plants but is quite rare among animals,” first author Ksenia Krivoruchku noted.
It might seem that an insect famous for its visibility would not need a second way to alert predators to its identity. However, bats have notoriously bad eyesight, so the glow of the fireflies might be lost on them – particularly if they only detect it through echolocation between two flashes.