Sara Lewis

Firefly flash dance illuminates mating prowess

When male fireflies flash their lights at females, it appears that they're bragging about their prowess as a mate, according to a new study by biologists at Tufts.

"Humans have been fascinated by fireflies for centuries, but we're just beginning to decipher the meaning behind their spectacular courtship displays," said Sara Lewis, associate professor of biology and the lead investigator on the study, which was published in the January/February issue of Behavioral Ecology. "This study is the first to translate the hidden meaning behind their flashes."

The research, funded by the National Science Foundation, found that female fireflies are strongly attracted to males who light up for longer periods of time because it indicates they are able to be better fathers by providing more of the essential prenatal nutrition for their offspring.

"We were curious to know why females should care so much about the duration of any male's flash," said Lewis. "We were surprised to find that flashes appear to be a male's way of bragging about what he can offer to a potential mate."

After the lights go out and mating begins, Lewis said, male fireflies provide females with a nuptial gift that accompanies their sperm. That gift—a spermatophore—is a high-protein nutritional package that females digest and use to provision their eggs. By measuring the duration of a male's flashes and comparing them to the spermatophore size in the same males, Lewis and her former doctoral student, Christopher Cratsley, discovered that the length of a male firefly's flash is a good predictor of the nuptial gift he's capable of delivering.

Their findings focused on a firefly species, Photinus ignites, which is native to New England. "Because Photinus fireflies don't eat once they become adults, male nuptial gifts provide a key source of nutrition for a female and her eggs," said Cratsley, who received his Ph.D. in biology from Tufts in 2000 and is now an assistant professor of biology at Fitchburg State College.

The research is part of a broader effort in the field of behavioral ecology to understand how diverse systems of communication—ranging from the firefly flash to human speech—have provided evolutionary advantages in certain species.

Firefly courtship relies on detailed flash "codes" that help to identify the hundreds of different firefly species. The flash codes help males to court potential mates of their own species.

Lewis said that male fireflies "advertise" their availability with carefully timed light flashes, and females on the ground flash back if they're interested. "Previous firefly research focused on flash pattern differences between firefly species," Lewis said. "But this study is one of the first to examine how and why flash patterns differ within a species."

"Fireflies have an adult life of only two weeks, and during that time, all of their energy is devoted to courtship and mating," Cratsley said. "At the very most, males have only about 10 opportunities to mate, so they need to stand out in the frenzied crowd of male competitors and communicate to females that they're worthy of consideration for mating."

In their research, Cratsley and Lewis recorded male flash signals and found that some males produced longer flashes, while others produced shorter ones. They used computer-generated flashes and light-emitting diodes to simulate male firefly behavior and determine which flash types the females responded to most frequently. They found that females were much more responsive to the longer flashes.

Lewis plans to expand her firefly research to investigate whether the size of the nuptial gift affects the success of fertilization. A female firefly mates with multiple males, but she controls the number of eggs that each male will fertilize. Competition for fertilization continues after mating in a process called "post-copulatory female choice." Lewis will focus on whether the size of the spermatophore influences this choice.

"When Darwin talked about sexual selection, he spoke primarily of mate choice," said Lewis. "In recent years, we have begun to realize that choice of mate isn't everything. Sexual selection continues after mating."

Lewis' earlier research on how fireflies use nitric oxide to turn their flashes on and off was published in the June 2001 issue of Science.