The National Science Foundation-funded research found that female fireflies are strongly attracted to males who give longer flashes because it indicates they are able to be better fathers by providing more of the essential pre-natal nutrition for their offspring.
"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 at Tufts. "This study is the first to translate the hidden meaning behind their flashes."
Lewis and her then-doctoral student Christopher Cratsley examined the link between a male firefly's flash the nutritional gift he is able to give to his mate. Their findings, published in the January/February issue of Behavioral Ecology, focused on a common firefly species (Photinus ignitus) native to New England. These fireflies can be seen flashing in open fields shortly after sunset in late June through July.
The Tufts 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.
Fireflies have long been used by scientists for health related research and to answer basic biological questions. Other recent research has used chemicals from fireflies to test bacteria for antibiotic resistance, giving hope for human health in the battle against drug-resistant tuberculosis in developing countries.
Firefly courtship relies on detailed flash "codes" that help to identify the hundreds of different firefly species. This way 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 added. "But this study is one of the first to examine how and why flash patterns differ within a species."
Cratsley, who graduated with a Ph.D. from Tufts in 2000, and is now an assistant professor of biology at Fitchburg State College, explained "fireflies have an adult life of only two weeks, and during that time all of their energy is devoted to courtship and mating. 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 carefully 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 see which flash types the females responded to most by tracking the females' response flashes. They found that females were much more responsive to the longer light flashes.
"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, she noted, male fireflies provide females with a 'nuptial gift' that accompanies their sperm. This gift, also known as 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 of the same males, Lewis and Cratsley discovered that the length of a male firefly's flash is a good predictor of the nuptial gift he's capable of delivering.
"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," says Cratsley.
Lewis plans to continue her firefly research to examine whether the size of nuptial gifts 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' continued research will focus on whether spermatophore size 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' previous research on how fireflies use nitric oxide to turn their flashes on and off was published in the June 2001 issue of Science.
Tufts University, located on three Massachusetts campuses in Boston, Medford/Somerville, and Grafton, and in Talloires, France, is recognized among the premier research universities in the United States. Tufts enjoys a global reputation for academic excellence and for the preparation of students as leaders in a wide range of professions. A growing number of innovative teaching and research initiatives spanning all Tufts campuses and joint degree programs are available for both undergraduate and graduate school students in liberal arts, sciences and engineering, and the University's eight graduate and professional schools.
Journal
Behavioral Ecology