In leafcutter ants and honeybees, it’s survival of the fittest sperm. Biologist Boris Baer, for a study out this week in Science, investigated these two species because of their peculiar sexual practices: In one day, the queen acquires all the sperm she’ll need to fertilize her eggs over the course of her lifetime. But in the race to be the top genetics-spreader, the males have evolved a dirty trick. Their seminal fluids actually do battle within the female’s reproductive tract.

To test out the idea, Baer and colleagues exposed the sperm of the bee and ant males to their own seminal fluid, and also to that of other males of the same species. The seminal fluid killed more than 50 per cent of the rival sperm within 15 minutes. “The males seemed to use the seminal fluid to harm the sperm,” says Baer [New Scientist]. When the team studied other organisms whose lifestyle didn’t depend on this kind of polyandry, they didn’t see the same effect.

However, in an interesting twist, it turns out that the queen is onto these devious males. In her sperm storage area, the spermatheca, the queen has a fluid she can deploy at the time of her choosing to put a stop to the seminal competition. “We basically show that there are two wars going on at the same time,” says Dr Baer. “The male would actually like to kill sperm from other males, but the female has other ideas” [Australian Broadcasting Corporation]. Baer suspects that she lets the competition run on long enough to eliminate the weakest candidates, then halts it before too much has been destroyed.

Given that we humans aren’t the most faithful lot, is it possible that we evolved something similar? Unlikely. “To my knowledge women do not copulate with 90 mates in half an hour, so whether there is much room that this has evolved in humans as well, I have my doubts,” says Baer [New Scientist].

And if you haven’t had your fill of sperm news, fear not—there’s more. Baer’s team studied semen warfare in the lab, but in a separate Science study, researchers show that they could observe it happening inside the insect, thanks to glowing sperm. While scientists first created such a thing a decade ago, now Scott Pitnick says his team has found a way to track them in real time. “It turns out that they [the sperm] are constantly on the move within the female’s specialised sperm-storage organs and exhibit surprisingly complex behaviour,” Prof Pitnick said. “It far exceeds our expectations, in that we can essentially track the fate of every sperm the female receives” [BBC News].

Related Content:
DISCOVER: The Most Incredible Things Ants Can Do (photo gallery)
DISCOVER: Sperm Cells Demonstrate Some Brotherly Solidarity
80beats: Revealed: The Secret of the Sperm’s Wild Dash to the Egg
Discoblog: In Competitive Sex, Male Butterflies Employ “Dipstick Method”

Image: Wikimedia Commons / Christian R. Linder