News flash from the firefly
In our world, boys buy bling to fetch females. In their world, fireflies use a bioluminescent belly — the abdominal embodiment of beetle-bling. Male and female fireflies use their flashes for mutual attraction; each of approximately 2,000 species uses a distinct signal to attract each other and avoid mating with other species.
But why do the males in some firefly species flash in unison, and what does that say about the infinitesimal insect brain? Those questions weighed on the massive mammalian brain of neurophysiologist Andrew Moiseff, of the University of Connecticut. He used Photinus carolinus, a lightning bug from the Smoky Mountains, as a way to understand how fireflies process signals and distinguish “us” from “them.”
Although the behavior he studied concerns mating, “We are looking at this from a different perspective,” says Moiseff. “What strategies do they need to allow the communication to take place? This is a very convenient system for understanding signal processing.”
A flash in the dark
Normally, local males of P. carolinus flash in unison, timed by an internal body clock. After a brief interval, nearby females flash the response call appropriate to females of the species. This unmistakable “come hither” allows the guys to find and mate with the glowing gal.
Most firefly males flash sporadically. Do P. carolinus guys get an evolutionary advantage from flashing all at once?
Definitely, according to a study published today in Science by Moiseff and Jonathan Copeland, of Georgia Southern University. They found that females respond dramatically better to unison flashing. Using a digital gadget to display phony flashes to a lady lightning bug, they found that her response rate fell from 82 percent after unison flashes to as low as 3 percent after sporadic flashes.
That would virtually guarantee that he would die a virgin — would be unfit for survival, in evolutionary terms.
P. carolinus forms a carnivorous larva that typically overwinters in the soil for a year before changing into a pupa and then an adult. The adult’s only job is to mate, so it lives only two to four weeks. Although adults don’t seem to eat, they do drink nectar.
After mating, momma flies deposit their larvae and kick the bucket.
In the Smokies, P. carolinus usually emerges in the first week of June, depending on the weather.
We wondered if unison flashing could be confusing to the ladies, but Moiseff countered, “It depends on what you consider confusing. Yes, that does make it more difficult for her to chose a specific individual, but we don’t know if she chooses an individual.”
Changing the question to, “Can she see the pattern of the species she’s looking for?” illustrates the utility of unison illumination, Moiseff says. “Imagine, with a bunch of things flashing that may appear random, it may be difficult to detect that any one is the correct pattern, but if they are flashing simultaneously, the pattern will jump out at you.”
We can see why this would work for females, but why would greater competition for guys lead to more offspring, the test of evolutionary advantage? After all, most male sexual displays are designed to make one guy stand out in a crowd, not to make many guys look like a crowd. “It might be that if a male does not play along, he will not get chance to mate at all,” says Moiseff, hastening to note that he’s not tested this idea. “Under these conditions, you must cooperate and take your chance.”
The evolutionary question of unison flashing begs for an answer, but as neuroscientists, Moiseff and Copeland want to know about the mental equipment that the females need to recognize the signal of her own species. “She probably has to count, and to be able measure the time interval,” says Moiseff. “One major goal is to understand the brain circuits and sensory system that would allow the visual system to count, and detect a specific pattern from all the other things the visual system can see.”
The firefly brain contains just a few thousand neurons, and Moiseff notes that, using electrodes, it’s possible to measure their action, one by one.
One final note: The firefly offers something that the rat, fruitfly and nematode do not: a lovable research subject. “I’ve never met anyone who does not like them,” says Moiseff. “Even in places with no fireflies, there are firefly books and kid’s stories. Most people had the experience of catching them as a kid in summer. It’s such a pleasant childhood memory.”
And yes, as a kid, Moiseff did catch lightning bugs in a jar.
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Firefly Synchrony: A Behavioral Strategy to Minimize Visual Clutter, Andrew Moiseff and Jonathan Copeland, Science, 9 July 2010. ↩
- More on Moiseff’s research. ↩
- The Firefly Files. ↩
- National Geographic’s firefly facts. ↩
- The hidden cost of firefly flashes. ↩
- The Bioluminescence Web Page. ↩
- Weird animal courtship rituals. ↩
- Short films about animal attraction by Isabella Rosellini. ↩
- Another video of P. carolinus in action. ↩
- Great Smokey Mountains and their synchronous fireflies. ↩
- Great Smokey Mountain’s biodiversity inventory. ↩