Blinky, blinky in love!
Why do fireflies flash? “All the better to mate with, my dear,” is the standard response. Males flash to attract females, and mating begins when the flash carries the right rhythm.
But now we read that female fireflies are far pickier than that. The flashing, so attractive to humans and fireflies alike, is necessary but not sufficient for mating, at least in Photinus greeni, a member of a common genus of North American firefly.
According to two researchers at Tufts University, females of these fireflies take nutrition into account as they choose who will father their offspring.
That sexy flashing signal begins, but does not end, the selection process, says Sara Lewis, a professor of biology, who guided Adam South, a recent Ph.D. from Tufts, on the project.
Lewis and South found that females are more likely to mate with males who give them a larger “nuptial gift.” And those males are also more likely to be the proud dad when a female has had multiple mates.
Many insects inseminate with a spermatophore, a protein-rich structure that contains sperm — and nutrients for the eggs that develop after fertilization. And the spermatophore “is not just a really cool-looking coiled structure,” says Lewis, but rather represents a “huge investment. If you open up a male Photinus , there is nothing in there except the reproductive glands.”
Research strategy: Divide and conquer
Many scientists — like children and everybody else — have paid attention to firefly flashing, says Lewis. “It’s very obvious, in courtship, that females prefer males based on the flash. End of story? Not so fast.”
Twenty years ago, while staying up night after night, Lewis and colleagues found that “like almost all animal taxa, female fireflies mate with a bunch of different males.”
The new study, published online in April and funded by the National Science Foundation, probed whether an alluring flash could, by itself, allow one guy to celebrate father’s day, while a failed flasher would die without descendants to strew flowers on his grave.
In an experiment chamber, Lewis and South simulated flashing with a light-emitting diode, then compared the effects of flashing and the size of the nuptial gift.
The study involved perhaps the first all-night-long look at the complicated, eight-hour firefly mating process, and revealed such oddities as antenna drumming.
Although an attractive flash (determined, in this species, by the interval between flashes) did put the guy on the gal’s short list for mating, what really helped him pass on his genes was the size of the nuptial gift. “This was really surprising; we thought the flashing would have some effect [in helping him reproduce his genes], but it had no effect at all,” says Lewis. “The major thing that seems to be affecting his paternity share is the size of the nuptial gift.”
Male fireflies, like many other insects, invest in both courtship and a nuptial gift, says Lewis. The proteins in the non-sperm portion of the “gift” wind up in the eggs, so a big gift gives an evolutionary advantage to the female — and to its donor.
The study provides another peephole into the evolutionary struggle to reproduce. Mate choice — the fights between males — has been known since Darwin, says Lewis, “But for a century after Darwin, researchers focused on mate choice because it was not known that so many females mated with multiple males.”
In a monogamous female, only one male’s sperm could reach the egg. But for polyandrous females, sperm competition and processes inside the female also play a role.
Because so much attention has focused on the initial stage of male competition, subsequent stages have been concealed, especially for fireflies, which mate in the dark. “People have gotten to the point of watching the pair begin to mate, and turned off the headlamp and walked away,” Lewis says.
In contrast, she and her fellow night-owls have spent hours watching the slow process of mating following the fabulous flashing. “It’s taken a while for people to start looking at this second part of sexual selection. From the beginning of courtship to the offspring hatching or emerging, there are a lot of processes that can bias things for or against any particular male.”
So how does the female know that she’s gotten a large spermatophore? “It’s a big mystery,” says Lewis.
One thing is clear, however. Fellows, wise up. Flashy bling is a good start, but don’t be a cheapskate. Buy her a nice, high-protein meal!
— David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Molly Simis, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- Determinants of reproductive success across sequential episodes of sexual selection in a firefly, A. South and S. M. Lewis, Proc. R. Soc. B, doi:10.1098/rspb.2012.0370, 2012. ↩
- Firefly basics ↩
- Spermatophore in insects ↩
- Evolution 101: Sexual selection ↩
- Nuptial gifts, explained ↩
- The evolution of polyandry ↩
- Fireflies: Glowing, going, gone ↩