Sometimes, scientific discoveries are made accidentally. So it was with a new study at Tel Aviv University (TAU) that discovered a possible defense mechanism developed by fireflies for protection against bats that might feed on them.
The idea for this study came up unintentionally during a study that tracked the echolocation – finding objects using reflected sound allowing the animals to move around in pitch darkness, navigate, hunt, identify friends and enemies and avoid obstacles, recalled Prof. Yossi Yovel, director of TAU’s Sagol School of Neuroscience and a member of the School of Mechanical Engineering and the School of Zoology at the university’s Faculty of Life Sciences.
“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 said. According to the study, fireflies produce strong ultrasonic sounds – soundwaves that the human ear, and more importantly the fireflies themselves, cannot detect.” The researchers hypothesize that these sounds are meant for the ears of bats, keeping them away from the poisonous fireflies, and thereby serving as a kind of “musical armor.”
He and his team conducted the research in collaboration with the Vietnam Academy of Science and Technology (VAST) and published the paper in the open-access journal iScience under the title “Fireflies produce ultrasonic clicks during flight as a potential aposematic anti-bat signal.”
According to TAU’s Steinhardt Museum of Natural History, not much is known about fireflies in Israel. “All we know about these amazing beetles is that their numbers are decreasing, and even this is based on non-systematic observations. Its scientists sought to improve their knowledge about various firefly species in Israel and their distribution and their understanding of the factors behind their decline.
Fireflies are insects known for their unique glow that they use as a mating signal. Since their bodies contain poison, the light flashes probably also serve as an aposematic signal (a warning to potential predators). This signal is also the firefly’s weakness, simply because it makes it an easy target for predators. Sexual signaling often increases predation risk. Although salient communication signals promote higher mating success, they might also assist predators or parasites who learn to eavesdrop on these signals to locate their prey or host. Bats are among the fireflies’ most prevalent potential predators, and some bats have poor vision, making the flashing signal ineffective.
This led the researchers to check whether fireflies had some additional layer of protection against bats. “In-depth research using high-speed video revealed that the fireflies produce the sound by moving their wings and that the fireflies themselves can’t hear this frequency. As a result, the scientists suggested that the sound is not intended for any internal communication within the species,” added Ksenia Krivoruchku, the doctoral student who led the study. The researchers said that the clicks were synchronized with the wingbeats and that they increase detectability of fireflies by bats.
Following the accidental discovery, the team at Yovel’s lab examined three different species of fireflies that are common in Vietnam (Curtos, Luciola and Sclerotia) plus one Israeli species (Lampyroidea) and found that they all produce these unique ultrasonic sounds but cannot hear them.
Can it be concluded that fireflies have developed a special defense mechanism specifically for bats? Yovel stressed that this claim was not proved in the study, but several features do point to this conclusion. First of all, there is the fact that the fireflies themselves can’t hear the sound while bats can both hear it and use it to find the fireflies – so it’s more likely that it serves as a warning signal.
Krivoruochku added that the discovery of ultrasonic sounds in fireflies is in itself an important contribution to the study of predator-prey relations. The idea of warning signals that the sender itself cannot detect is known from the world of plants but is quite rare among animals. Our discovery of the ‘musical battle’ between fireflies and bats may pave the way for further research and possibly the discovery of a new defense mechanism developed by animals against potential predators.
Fireflies are known for emitting light signals for intraspecific communication. However, in doing so, they reveal themselves to many potential nocturnal predators from a large distance. Therefore, many fireflies evolved unpalatable compounds and probably use their light signals as anti-predator aposematic signals. Fireflies are occasionally attacked by predators despite their warning flashes. Bats are among the most substantial potential firefly predators. Using their echolocation, bats might detect a firefly from a short distance and attack it in between two flashes. We thus aimed to examine whether fireflies use additional measures of warning, specifically focusing on sound signals. We recorded four species from different genera of fireflies in Vietnam and Israel and found that all of them generated ultrasonic clicks centered around bats’ hearing range. Clicks were synchronized with the wingbeat and are probably produced by the wings. We hypothesize that ultrasonic clicks can serve as part of a multimodal aposematic display.