Sara Lewis stalking Photinus

Strangers in the Night

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, and was fascinated by the extreme competition for mates. “I realized that the vast majority of fireflies I’d ever seen were males, and that they were all vying for just a few females,” she explains in her office at Tufts, where she is now an associate professor of biology.

That night, Lewis launched an impromptu research project. She combed 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 been unraveling 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.”

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 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 favored 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 eggs have of hatching. And in P. ignitus, there’s a direct correlation between the length of the flash and the size of the gift, so it behooves females to be picky.

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.”

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.