A bat hawks and then ingests a palatable scarab beetle. The second interaction is of a bat capturing, then dropping a firefly. Finally, after several subsequent firefly interactions, a bat is shown approaching and avoiding another firefly. Credit: Barber Lab/Boise State University

Fireflies' glow may have evolved as a way to deter their main predator from devouring them, according to a new study.

What's new: The finding goes against conventional wisdom, which holds that fireflies' bioluminescence is for attracting a mate. It also shows how bats rely on multiple senses to hunt for prey, and avoid eating noxious meals, rather than simply using echolocation.

How they did it: Researchers from Boise State University, the University of Florida and Purdue University pit fireflies against big brown bats in a dark flight room for 1 to 4 days. They used high-speed photography to record interactions between the insects, which are a type of beetle, and the bats.

They found that toxic, bioluminescent fireflies transmit multi-sensory warning signals to echolocating bats, in an effort to warn them off.

  • After taking initial nibbles and then spitting out the fireflies, the bats seemed to learn over time not to try to eat the insects.
  • This also occurred with fireflies that did not light up at the time, though some of them were eaten. In these cases, the researchers hypothesized, the bats were relying on one sense to discriminate between meals, including fireflies' flight patterns.
"We found that bats can learn to avoid fireflies when using sonar or vision, but learn almost twice as fast and more completely when using both — providing fundamental evidence that combining information across senses can increase the power of warning signals."
— Jesse Barber, Boise State University

Interestingly, the study cited previous work that found when fireflies first emerged about 65 million years ago, they relied on pheromones to attract mates, rather than light signals, and most flew during the day. As they shifted to being nocturnal, the study concludes, they may have faced "heavy selective pressure" from bats, and developed their ability to light up.

"For fireflies on the wing in the night sky, the predators to avoid have been, and continue to be, bats. We predict that future evolutionary work will reveal that bioluminescence in beetles emerged with bats and that, indeed, bats may have invented fireflies," says Barber.

What they're saying: While praising the study in general, Nick Dowdy, a postdoctoral fellow at the Milwaukee Public Museum who was not involved in this research, says he questions the conclusion that bats may have driven bioluminescence in fireflies.

"There simply isn't enough data to make that claim right now," Dowdy said. "That might be true, but we have just as much if not more data suggesting that ancestral fireflies shifted to nocturnal activity and evolved bioluminescent signaling as an effective means of locating mates in the dark."

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