The Deadly Light of Sex

Dancing between sex and death

They may look like little living lanterns that bestow a fairytale atmosphere on rural landscape during summer months, but a new research made at Tufts University has found a dark side behind the magic show of the fireflies.

The researchers found that even if fireflies waste little energy to produce the flash signals, something stops males from getting flashier, even if this would mean more sex: predation.

Photinus firefly males broadcast their bioluminescent flashes during the flight while looking for females. Females hidden in the grass look at passing males and, if interested, will respond by flashing.

Previous studies made in various firefly females showed they are more attracted by males that emit longer or faster frequency flashes, thus more conspicuous.

"Since females so clearly prefer the flashier males, one thing that's been puzzling scientists is what's keeping these males from evolving longer and longer, faster and faster flashes. In theory, there might be some hidden costs to more conspicuous flashes, but what are they?" said lead researcher Sara Lewis, professor of biology at Tufts.

In the quest for the answer, the researchers first assessed the energy wasted by fireflies while producing their bioluminescent flashes. Tiny respirometry chambers calculated how much carbon dioxide each firefly expelled when flashing, as compared to the resting state. The measurements revealed that fireflies consume surprisingly little energy for emitting their magical flashes, even less than for walking.

After that, the team made a simple field test to assess the potential predation trade-up of firefly flash signals. Even if Photinus fireflies produce toxins that keep off most predators, yet they are heavily hunted by the larger Photuris fireflies.

In field tests, Photuris fireflies were much more attracted by the fake firefly signals compared with non-flashing but identical control Photinus fireflies. Moreover, the predators were increasingly attracted by more frequent flash signals. Thus, even if more conspicuous flash signals mean for male fireflies more female sex partners, they also increase the possibility of ending up as meal for a Photuris.

"Every single night, male fireflies are out there flying a fine line between sex and death. For us, it definitely rivals the most exciting television thriller! So, next time you're outside on a summer night take a moment to admire the firefly romance and risk that's playing out all around you," said Lewis.

"The importance of these two conflicting forces could easily shift in different firefly populations." she added.

This evolutionary shift could produce completely new firefly species with their own specific flash types.

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