By John Upton
Evenings are sparkling across great stretches of the northern hemisphere, where pulses of nitric oxide are being produced in fireflies’ abdominal cells to trigger harmonies of alluring green-yellow flashes. The nitric oxide briefly prevents parts of the cell that turn sugar into useful energy from consuming oxygen, allowing that oxygen to penetrate deeper into the cell, where it fuels a light-producing reaction involving a compound devilishly dubbed luciferin.
The constellations are cacophonies of courtships – the beetles have emerged from years of larval life spent underground, where they munched on worms and similar prey – and now they have mere weeks to procreate. Every night of their short adult lives is dedicated to procreation: Males fly and flash their wares, while females prop themselves on grass blades and invisibly judge their suitors. When a female likes what she sees, she flashes a response that draws him near.
While these unheard mating calls bring males and females together, new research published in Proceedings of the Royal Society – Biological Sciences reveals that something far more substantive determines the male’s success as a breeder.
The male wraps his sperm in a sustenance-rich package called a spermatophore, which is offered to the female. Adult fireflies do not eat, and spermatophores help provide females with energy needed to produce, bury and lay their eggs.
Tuft University researchers Sara Lewis and Adam South discovered that the size of the spermatophore offered by a common North American firefly is more influential than the flashiness of his bioluminescence in determining whether he will sire young. The larger a firefly’s spermatophore, the more likely it is that a female will invite him to mount her — and, once they have copulated, the more likely it is that she will lay eggs bearing his young.
That means the virility of youth is crucial in helping a male firefly pass on his genes to the next generation.
“Our previous work has shown that making those spermatophore gifts is costly to males,” Lewis told me. “The gifts get smaller across sequential matings, and males’ mating rates slow down.”
The researchers also discovered hitherto unknown steps in the hours-long copulation of Photinus greeni, the species of firefly studied, which they explain in this video: