For springtails, sex can be an Easter egg hunt. Many males of the tiny soil organisms sustain their species by leaving drops of sperm glistening here and there in the landscape in case a female chooses to pick one up.
“The male never meets the female,” says Zaira Valentina Zizzari of VU University Amsterdam, who studies a species of these extreme loners called Orchesella cincta. Just about every degree of mating intimacy, from unseen sperm donors to elaborate courtship and internal insemination, shows up in springtails. That makes the ancient group — which may not belong to the insects but to another set of six-legged arthropods on its own evolutionary trajectory — a treasure trove for biologists studying sex.
In O. cincta, little brown-and-white males roam the leaf litter making no apparent effort to find a female. Instead, males pause here and there for a few seconds to leave behind white stalks topped by a shiny-coated globes, each holding more than 1,000 sperm.
Sperm on a stalk is still viable after sitting two days in the lab. Outdoors it may not last so long. “You have a lot of rivals searching for sperm just to destroy it,” Zizzari says. Males eat rivals’ sperm.
Given the rivalry, it wouldn’t be surprising if males engaged in an arms race to produce more sperm stalks than their competitors. But Zizzari was surprised to discover that male O. cincta make fewer sperm packages when a competitor is sperm-dotting the neighborhood. Maybe he’s enhancing the few he makes with extra sex appeal, Zizzari mused. To test the idea, she offered lab females a choice of globes from males with rivals or those made by an uncrowded guy.
Given the two options, “some females just sit and wait — and suddenly you see a female running to a sperm droplet and picking it up,” Zizzari says. Others make the rounds of droplets on offer, gently touching them with their antennae. “Then she’s not convinced, and she goes for another round.”
As Zizzari peered through a microscope watching the deliberations, only 24 of 40 females made up their minds within 15 minutes. Those that reached a decision preferred, 3-to-1, to touch their reproductive tract openings to a droplet from a male vexed with a nearby rival, she and her colleagues reported December 4 in Biology Letters. Even in a species that evolved anonymous sperm-banking, it seems, competition inspires special effort.