A new study by biologists at TuftsUniversity has translated what male fireflies are saying to femaleswhen they flash their lights - and it looks like the males are bragging.
The National Science Foundation-funded research found that femalefireflies are strongly attracted to males who give longer flashesbecause it indicates they are able to be better fathers by providingmore of the essential pre-natal nutrition for their offspring.
"Humans have been fascinated by fireflies for centuries, but we'rejust beginning to decipher the meaning behind their spectacularcourtship displays," said Sara Lewis, associate professor of biology atTufts. "This study is the first to translate the hidden meaning behindtheir flashes."
Lewis and her then-doctoral student Christopher Cratsley examinedthe link between a male firefly's flash the nutritional gift he is ableto give to his mate. Their findings, published in the January/Februaryissue of Behavioral Ecology, focused on a common firefly species(Photinus ignitus) native to New England. These fireflies can be seenflashing in open fields shortly after sunset in late June through July.
The Tufts research is part of a broader effort in the field ofbehavioral ecology to understand how diverse systems of communication -ranging from the firefly flash to human speech - have providedevolutionary advantages in certain species.
Fireflies have long been used by scientists for health relatedresearch and to answer basic biological questions. Other recentresearch has used chemicals from fireflies to test bacteria forantibiotic resistance, giving hope for human health in the battleagainst drug-resistant tuberculosis in developing countries.
Firefly courtship relies on detailed flash "codes" that help toidentify the hundreds of different firefly species. This way the flashcodes help males to court potential mates of their own species.
Lewis said that male fireflies "advertise" their availability withcarefully timed light flashes, and females on the ground flash back ifthey're interested.
"Previous firefly research focused on flash pattern differencesbetween firefly species," Lewis added. "But this study is one of thefirst to examine how and why flash patterns differ within a species."
Cratsley, who graduated with a Ph.D. from Tufts in 2000, and is nowan assistant professor of biology at Fitchburg State College, explained"fireflies have an adult life of only two weeks, and during that timeall of their energy is devoted to courtship and mating. At the verymost, males have only about 10 opportunities to mate, so they need tostand out in the frenzied crowd of male competitors, and communicate tofemales that they're worthy of consideration for mating."
In their research, Cratsley and Lewis carefully recorded male flashsignals, and found that some males produced longer flashes while othersproduced shorter ones. They used computer-generated flashes and light-emitting diodes to simulate male firefly behavior and see which flashtypes the females responded to most by tracking the females' responseflashes. They found that females were much more responsive to thelonger light flashes.
"We were curious to know why females should care so much about theduration of any male's flash," said Lewis. "We were surprised to findthat flashes appear to be a male's way of bragging about what he canoffer to a potential mate."
After the lights go out and mating begins, she noted, male firefliesprovide females with a 'nuptial gift' that accompanies their sperm.This gift, also known as a spermatophore, is a high-protein nutritionalpackage that females digest and use to provision their eggs. Bymeasuring the duration of a male's flashes and comparing them to thespermatophore size of the same males, Lewis and Cratsley discoveredthat the length of a male firefly's flash is a good predictor of thenuptial gift he's capable of delivering.
"Because Photinus fireflies don't eat once they become adults, malenuptial gifts provide a key source of nutrition for a female and hereggs," says Cratsley.
Lewis plans to continue her firefly research to examine whether thesize of nuptial gifts affects the success of fertilization. A femalefirefly mates with multiple males, but she controls the number of eggsthat each male will fertilize. Competition for fertilization continuesafter mating in a process called "post-copulatory female choice."Lewis' continued research will focus on whether spermatophore sizeinfluences this choice.
"When Darwin talked about sexual selection, he spoke primarily ofmate choice," said Lewis. "In recent years we have begun to realizethat choice of mate isn't everything. Sexual selection continues aftermating."
Lewis' previous research on how fireflies use nitric oxide to turntheir flashes on and off was published in the June 2001 issue ofScience.
Tufts University, located on three Massachusetts campuses in Boston,Medford/Somerville, and Grafton, and in Talloires, France, isrecognized among the premier research universities in the UnitedStates. Tufts enjoys a global reputation for academic excellence andfor the preparation of students as leaders in a wide range ofprofessions. A growing number of innovative teaching and researchinitiatives spanning all Tufts campuses and joint degree programs areavailable for both undergraduate and graduate school students inliberal arts, sciences and engineering, and the University's eightgraduate and professional schools.
A new study by biologists at Tufts University has discovered a darkside lurking behind the magical light shows put on by fireflies eachsummer. Using both laboratory and field experiments to explore thepotential costs of firefly courtship displays, the biologists haveuncovered some surprising answers.
The research, to be published in the November 2007 issue of AmericanNaturalist revealed that it's energetically cheap for fireflies toproduce their distinctive flash signals, but that flashier males aremore likely to end up on the dinner table.
On summer evenings, male Photinus fireflies lift off into the air tobroadcast their bioluminescent flashes in search of females.
Femalesperched in the grass sit and admire passing males and, if they'reinterested, will flash in response. Previous research on many differentfirefly species has shown that females respond more readily to malesthat give longer flashes, as well as those with faster flash rhythms.This female choice favors firefly males that produce more conspicuousflashes.
"Since females so clearly prefer the flashier males, one thingthat's been puzzling scientists is what's keeping these males fromevolving longer and longer, faster and faster flashes," says SaraLewis, professor of biology at Tufts and leader of the research teamthat included postdoctoral researcher William Woods and twoundergraduate students. In theory, there might be some hidden costs tomore conspicuous flashes, but what are they"
To answer this question, the researchers set out to look at twopotential costs of firefly flash signals. First they measured theenergy that fireflies expend while they're producing theirbioluminescent flashes. In carefully controlled laboratory experiments,the team used tiny respirometry chambers to measure how much carbondioxide each firefly produced when they were flashing compared withwhen they were resting.
"Basically, we're in the business of measuring bug breath," notesWoods. These respirometry results demonstrated that fireflies requiresurprisingly little energy to produce their magical flashes, even lessthan what it takes them just to walk around.
Evolutionary balancing act could generate new species
Once the Tufts team established that flashing had such a low energycost, they tried a simple field experiment to measure the potentialpredation costs of firefly flash signals. Photinus fireflies are knownto produce noxious chemicals that deter most predators, yet make themthe top menu choice for the larger predatory fireflies known asPhoturis. Using basic materials that included electronic fake fireflies(manufactured by Firefly Magic), plastic toy-dispensing capsulesdesigned for vending machines, and sticky glue, the researchers madetwo startling discoveries.
In the field, predatory fireflies were attracted significantly moreoften to the fake firefly signals compared with non-flashing butotherwise identical controls. In addition, when flash signals were morefrequent, they were much more likely to attract predators. So eventhough more conspicuous flash signals provide male fireflies with anevolutionary leg up in terms of attracting females, they also have apotentially fatal downside because they are more likely to attractpredators in search of their next meal.
"Every single night, male fireflies are out there flying a fine linebetween sex and death. For us, it definitely rivals the most excitingtelevision thriller!" says Lewis. "So, next time you're outside on asummer night take a moment to admire the firefly romance and riskthat's playing out all around you."
According to Lewis, the importance of these two conflicting forcescould easily shift in different firefly populations. Therefore, it'spossible that this evolutionary balancing act might generate entirelynew firefly species with their own distinctive flash codes.
Funded by a National Science Foundation program called ResearchExperiences for Undergraduates, the Tufts research could ultimatelyhelp us to better understand the evolution of communication in manyorganisms, including humans.