Mating games

Firefly courtship is a tale of passion, betrayal and empty hook-ups

One summer evening in North Carolina, while lazing in her backyard, Sara Lewis saw glimmers of her future career. The fireflies were putting on a magnificent show—the males madly flickering as they cruised the air, the females, less numerous and hidden in the grass, winking back. Lewis was then studying coral reef ecology at Duke University, and was fascinated by the extreme competition for mates. “I realized that the vast majority of fireflies I’d ever seen in my life were males, and that they were all vying to get access to just a few females,” she explains in her office at Tufts, where she is now a professor of biology.

Biologist Sara Lewis stalks Photinus. © DAN PERLMAN

That night, Lewis launched an impromptu research project. She combed the grass and brush in her yard for female fireflies, marking their locations as she went. Orpheus, a black Labrador pup, kept her company. “The dog figured out what I was looking for. He’d point—one paw up, tail out. After a while, I’d just follow him around.” Photinus, the genus of fireflies common in North America, became Lewis’s obsession. She has worked to unravel the intricacies of the insects’ courtship behavior for the past 15 years. It is a tale of passion, betrayal and empty hook-ups.

Firefly love is a matter of some urgency, Lewis says. After a two-year larval stage, the adults have just 14 nights in which to propagate before they die, and they go at it with enviable focus. “They don’t eat, they don’t sleep, they don’t watch TV,” Lewis says.

They must first make themselves known to potential mates. Because the world holds some 2,000 firefly species, many of them living side by side, the insects have developed distinctive flash patterns. The males of Photinus marginellus flash once every three seconds, those of P. consimilis twice a second. The females, who remain stationary in grass or low brush for the entire two weeks, have their own reponses: P. marginellus blinks back almost immediately, while coy P. consimilis waits almost a full two seconds, if she replies at all.

What do lady fireflies want? The flashiest males, of course. Using LEDs to mimic male flash patterns, Lewis and her colleagues found that females were more likely to respond to males who displayed faster pulse rates or longer flashes. The extra bling is one of those gaudy accessories—like the peacock’s tail—that impress females but make the guys more conspicuous to predators.

Female Photinus is not just being contrary. Lewis also found that flash patterns may signal a male’s fitness as a father. Upon mating, the male Photinus firefly presents the female with a nuptial gift, a package of sperm and protein called a spermatophore. The more protein dad donates, the better chance those newly fertilized firefly eggs have of hatching. And in P. ignites, there’s a direct correlation between the length of the flash and the size of the gift, so it behooves those females to be discriminating.

Fireflies illuminate a stream in Japan, where Sara Lewis will travel this summer to study the 20 to 30 species that live there. © HIDEAKI TANAKA/SEBUN PHOTO

But a sexy signal doesn’t always mean he’s a better provider; there’s no such correlation in P. greeni. “Over evolutionary time, males can develop ways of cheating,” Lewis says. “He may have great looking flashes, but deliver a lousy nuptial gift. There could be some other trade-off, or it could just be false advertising.” It’s a question that she and her colleagues continue to investigate.

This summer, Lewis will spend a month studying the 20 to 30 species that live in Japan. With so much variety in so small a nation, she says, it’s no surprise the firefly holds a special place in Japanese culture. The Japanese word for firefly, hotaru, also means harmony among living creatures. “They have words for watching fireflies on a clear summer evening versus a misty one,” says Lewis (she has been boning up on Japanese at Tufts). While abroad, she hopes to find out if males of Japanese species, like their American cousins, offer nuptial gifts. If so, she says, “it suggests we can’t look at the courtship and mating behavior of fireflies without taking into account this pretty expensive gift.”

Courtship? Mating? Gifts? Is it possible that fireflies could illuminate the human dating game? Alas, not very, Lewis says. “They can shed light only on very general principles about sexual selection and evolution.” Whether an evening spent watching fireflies can teach you anything about romance, then, still depends on who you’re watching with.

Jacqueline Mitchell is a senior health sciences writer in Tufts’ Office of Publications. She can be reached at This story ran in the Tufts Journal in July 2007.