Why do fireflies flash in time? Their rhythmic, bioluminescent displays are extraordinary phenomena, sometimes lighting up entire forests with bright pulses of light. But why it happens is one of nature’s great mysteries – there are lots of ideas, but until now no one has experimentally tested any of them out.
Now Andrew Moiseff from the University of Connecticut and Jonathan Copeland from Georgia Southern University in the US have done just that. Their study, in the journal Science, suggests that swarms of male fireflies flash simultaneously so that females can recognise a potential mate from their own species.
Male fireflies fly around giving off a characteristic pattern of flashes, a different one for each species, like a system of Morse code. If a female spots a male of the same species, she will flash back during one of his pauses.
For around 1% of the 2000 firefly species – which are in fact a type of beetle – males will synchronise their flashes over large areas.
To test out their ideas of why this happens, Moiseff and Copeland created a virtual firefly world in the lab. They collected female Photinus carolinus fireflies, a synchronous species from the Smokey Mountains National Park in Tennessee, and flashed an array of LED lights at them, to mimic males of the same species.
When all the LEDs flashed together, females were duped into thinking they were real males, and they responded by flashing back around 80% of the time.
When the LEDs flashed out of sync, the females hardly responded at all – they only flashed back 10% of the time or even less.
It seems that when fireflies are crowded together, females can only make out individual males when they all flash together – otherwise it just looks like a jumble of flashing lights to them.
Flashing males are constantly on the move, which means that if a female focuses too narrowly she could easily miss parts of his characteristic flash pattern as he flies in and out of view. Instead, she probably needs to look over a larger area of space so she can pick out these moving patterns. But if there are lots of males, all flying around and flashing out of time, the patterns quickly become muddled and confused.
The next step of the research will be to work out whether female fireflies’ tiny brains are wired up in a way that means they can’t detect asynchronous flashes, and ultimately, whether they are driving the evolution of males that flash in time and put on these extraordinary night time firework displays.