Sexual conflict between two species can drive an evolutionary “bedroom battle royal,” leading to some crazy adaptations. Harvester ant queens steal sperm away from males of another species during sex—without even producing fertile offspring. The work, published in Proceedings of the Royal Society B this week, is a rare example of reproductive conflicts-of-interest between males and females of different species.

In the southwestern U.S., mating between queens and males from Pogonomyrmex barbatus and Pogonomyrmex rugosus result in sterile hybrids. These go on to become workers in the queens’ colonies, but they’re evolutionary dead ends for the males. Daughter queens are only produced with sperm from males of her own species. To test if hybridization can drive sexually antagonistic coevolution, a University of Vermont duo observed mating between the two harvester ant species in the desert near the border between Arizona and New Mexico. They found that both sexes manipulate their “copulatory investment” to work against each other.

Once a year in the summer, a day or two after a heavy monsoon rainstorm, harvester ants have a crazed, free-for-all mating swarm. The males can’t distinguish queens of their own species from queens of another... at least not until they’ve already started copulating. Once he realizes his mistake, he lowers his sperm transfer rate to preserve some for a better investment later on. 

But the queens have evolved a counter-adaptation. “They lock slow males in copula significantly longer, until they eventually deliver the same amount of sperm that they normally would have,” UVM’s Sara Helms Cahan says in a news release. “Essentially, they are sperm parasites.”

In that jumble of ants pictured above, the giant (mostly red) queen and the male she’s mating with on the right are locked in place by their copulatory organs. Even if he lets go completely, he’d still be firmly attached to her. Back in the lab, the team found no difference in the total quantity of sperm transferred between same-species and inter-specific pairs. 

These findings are an example of a larger biological phenomenon called the Red Queen Hypothesis. In Through the Looking Glass, the Red Queen says: “Now, here, you see, it takes all the running you can do, to keep in the same place.” Every organism's competitors, predators, and parasites are always evolving, so everyone needs to evolve as fast as possible just to keep up. 

“In this harvester ant system there really needs to be some sort of stalemate,” study author Michael Herrmann of UVM says, “because if the males actually were able to tell what type of female they were mating with, they would cut off the sperm to the queens that need it.” These queens must mate with both species of males: They can’t produce workers with sperm from males of their own species. To prevent an entire system collapse, neither queens nor males should have too many evolutionary advantages.

Images: Sara Helms Cahan (top), Joshua Brown (middle)