Tufts University

Bright Ideas

Bright IdeasTufts Biology Professor Sara Lewis' groundbreaking research - performed in conjunction with colleagues, students, and even her husband - has shed much light on fireflies and people alike.

For many people, watching fireflies light up the summer night is a magical part of growing up. Children and adults alike delight in the after-dinner treat of "lightning bugs" flickering against the backdrop of grass, trees and sky. Kids swarm to them, clasping them in their hands and collecting them in jars.

But despite 15 years of landmark firefly research, Tufts' Sara Lewis claims the illuminating insects played no role in her Connecticut youth.

"Fireflies are a very important cultural icon to a lot of people. Many people admire them just because it reminds them of their childhood. But I never really played with fireflies as a kid," swears Lewis, an associate professor of biology, who first became interested in fireflies while earning her PhD in zoology from Duke, where she had a backyard full of them.

Now, the Tufts evolutionary biologist is making up for lost time. While you wouldn't quite call her interaction with fireflies ‘playing,' you can tell from the number of times she enthusiastically describes her work as "really, really cool" that she thoroughly enjoys her research.

For more than a decade, Lewis and her colleagues and students - Tufts Team Firefly - have been turning fields west of Boston into their own personal laboratory playground for several weeks every summer. Covered from head to toe in long pants and hooded sweatshirts ("There's a high correlation between mosquito density and firefly density," she laments good-naturedly), Lewis and her team go out into the night armed with their headlamps, bait (LED lights), and clipboards (equipped with blue-illuminated pens undetectable to fireflies). Many of the questions they ask can only be answered by long hours spent in the dark watching firefly mating behavior. For other research, they'll need to leave the field with captive fireflies, ready to light up the lab.


"We're interested in studying evolutionary processes, and we're interested in studying them in ecological time," Lewis explains. "We want to know how the interactions between living organisms play out in terms of how things change over time."

Specifically, Lewis is interested in the evolutionary process of sexual selection. Whereas natural selection focuses on differences in survival, sexual selection deals with differences within a species concerning their reproductive success.

"How successful are they at getting mates? If males mate successfully, how successful are they at siring offspring? Ultimately, we'd like to know what it is that makes some individuals better than others at passing genes on to the next generation" Lewis asks.

Lewis, who attended Harvard as an undergraduate, has also studied flour beetles and seahorses. But she finds fireflies ideal for studying sexual selection.

"The Photinus fireflies we mostly observe spend two years as larvae, eating, growing, eating, growing, eating, growing - and then they pupate in the spring and come out as adults and live for two weeks. And during that time they do nothing but devote themselves to reproduction. So it's a very clean system as far as studying the process of sexual selection, because it's essentially uncontaminated by other behaviors, such as looking for food, looking for a place to sleep, watching TV, that stuff," she jokes. "If you can follow an individual every single night of their adult life, you know exactly what they're doing as far as passing genes onto the next generation."

And of course, fireflies' courtship signals are highly visible, allowing researchers to observe the flirtation that leads to mating. Males fly around flashing a signal, trying to attract a female on the ground amidst a crowd of suitors. If she likes what she sees, she'll respond with a flash of her own, the male firefly will come calling, and voila! Romance in the summer dusk. By the next evening, they'll both be back at it again - only with different partners.

It may sound a little mysterious, but Lewis has uncovered a lot of the science behind fireflies' courtship and mating behaviors. After coming to Tufts in 1991, she and a graduate student discovered that when mating, male fireflies give the female not only sperm but also a "nuptial gift" which includes a protein package beneficial to fertilization and reproduction.

Lewis considers the discovery the most exciting find in her firefly research because of "the really interesting work that has been fired up as a result." The breakthrough helped scientists understand that for male fireflies the job isn't over after the deed is done: after mating, females can still decide which male's sperm to fertilize her eggs with.

"People have been studying sexual selection pretty much since Darwin, but there's a second stage of sexual selection which happens during and after mating, called postmating sexual selection, that people are just starting to look at. If a male mates with a female and doesn't get to fertilize any eggs, he could have just given up in the beginning - he's not going to have any offspring!" Lewis exclaims. "So when people focus on just mating it's very much just the tip of the iceberg, the beginning of the story. It's important to put the whole process together from start to finish."

Bright Ideas

"People are very cooperative at Tufts," Sara Lewis notes. "In contrast to many other institutions, there's a lot of collaborative research that goes on here, and that's really, really exciting and fun."

Lewis started by looking at firefly mating behavior, and her research revealed that the length of the male flash signal was a key difference-maker in the courtship process. Lewis - again working with a graduate student - observed differences in flash lengths within the Photinus firefly species (most of the previous research in the field dealt with differences between the hundreds of firefly species rather than within a species). Lewis and her student then simulated the different male flashes and discovered that females preferred longer flashes, supposedly because they were a fairly reliable indicator of the size of the male's nuptial gift.

Each new discovery leads Lewis and her team to explore new questions. One of her PhD students recently performed an experiment showing that males with the most attractive flash signals were often garnering the lowest paternity rates. The reason?

"It turns out that the male flash signals are not always an honest indicator of the size of a male's nuptial gift," Lewis suggests. "Males in this particular species seem to be giving a false signal, essentially exaggerating the size of their nuptial gift, and then they turn out not to be so great. Females may be later biasing paternity in favor of certain males, based not on the courtship signal but based on the actual nuptial gift size."

Surprise: some male fireflies, much like their human counterparts, are less than truthful when trying to woo females. However, unlike fireflies, male humans aren't usually at risk of being eaten when they play the mating game. One of Lewis' many projects this summer involves measuring the ecological cost to a Photinus male firefly of flash signaling, and according to her, that cost can be high: larger predatory Photuris female fireflies often mimic the smaller Photinus female flashes, seducing an unsuspecting male who they then devour alive - not quite the treat the male was expecting.

Between Photinus males exaggerating the size of their nuptial package to females of their species, and Photuris females trying to trick them into being dessert, "you could say there's a very complex information interchange going on," says Lewis, noting that firefly research has helped transform the way evolutionary biologists have thought about sexual selection in the last 20 years.

"The conceptual change that's been happening in our field is seeing sexual selection not as a cooperative venture but as coevolutionary sexual conflict. Males are advertising one signal, and females are hoodwinked into thinking this is a male with a great nuptial gift, but maybe it's really not that good," she says. "So it's an evolutionary game that's being played out between the sexes."

Thought these issues might sound like they lie in the domain of Dr. Phil or Dr. Ruth, Dr. Lewis has a firm grasp of the evolutionary consequences of this behavior.

"We used to think, ‘Well, in order to continue the species males and females need to cooperate, pass signals, mate and produce progeny. And then people began to realize that there's actually a conflict of interest between males and females. Males are actually trying to mate with as many females as possible and convince those females [to fertilize with their sperm], but that might be counterproductive to the females' evolutionary interest, which is producing the greatest number of offspring."

This summer, Lewis' team is continuing its work. "We're focused on tying together these two aspects: what happens after mating has already taken place - with fireflies, we call it ‘after the lights go out' - with what happens beforehand - trying to tie together the courtship signals with eventual reproductive output," she says.

As she does with all her research, Lewis is taking an interdisciplinary approach, joining forces with experts in multiple fields to figure out the science behind the flashes.

"People are very cooperative at Tufts. In contrast to many other institutions, there's a lot of collaborative research that goes on here, and that's really, really exciting and fun."

Lewis may spend a lot of time focused on tiny bugs and quick flashes, but she doesn't lose sight of the big picture.

"All of life, everyone in every generation, tries to pass their genes on to the next generation. That's basically how we look at biology. Everything, all adaptations, the way organisms look and act, have gotten to be that way because they're really only focused on getting genes to the next generation. So if they survive but don't mate: nothing. If they survive and mate but don't get to fertilize any eggs: from a male point of view, nothing. It's an evolutionary dead end. That's how we think about things all the time," she says. "Every generation always dies, so it's only the contribution to the next generation that makes a difference. Otherwise we wouldn't be here. We'd be fossils."

Yet Lewis knows that people are often fascinated with fireflies for reasons having nothing to do with the furthering of science or evolution.

"There's something very magical about things that fly around lighting up at night," she admits. "It's definitely cool. People don't like insects in general but [fireflies] are probably the most charismatic insect. Many people think insects are really creepy, especially when they're out in vast numbers. But no one gets freaked out by a huge field of fireflies."

Profile written by Ben Hoffman, Class of 2006

Ben Hoffman, a native of Harrisburg, Pennsylvania, is an English major and communications and media studies minor. Ben has been a sports editor at the Tufts Daily for the past two years, and last fall he served as the head of the sports department. He also interned for the Boston Globe in the fall before studying abroad in Prague in the spring.

Photography by Melody Ko and Brian Loeb '06.

This story originally ran on Aug. 8, 2005