Two species of ants in the Arizona desert are at war -- and they're fighting their battles in the bedroom. The ant queens are employing devious tricks to steal the sperm of their rival species' males, in an ongoing inter-species battle of the sexes.

Usually sex is a competition between males of the same species. They fight to impress their target females, and the winner gets to pass on his sperm -- and genes -- to the next generation. That is, after all, the biological definition of success.

But in some cases, the competition gets murkier.

According to a new study published Wednesday in Proceedings of the Royal Society B, two species of Pogonomyrmex harvester ant engage in a complicated mating war, with males of one species butting heads with the females of another.

The males of one of these ant species will often mate with the queens of the other (and vice versa) producing sterile offspring. These sterile hybrids (or mules) aren't totally uncommon in other insects and animals. But in most cases, said Sara Helms Cahan, a University of Vermont biologist who co-led the study with her graduate student Michael Herrmann, these mules are a biological mistake. They're incapable of passing along the genes of either parent to a new generation, so they don't benefit either species.

But in species like these ants, who need a certain number of sterile colony members to work without reproducing, mules do just fine. There are at least four species of ants who commonly steal sperm from rival species.

"They're already producing lots of things that don't reproduce on purpose," Cahan said, "so it gives them a unique opportunity to turn into parasites. They can take those hybrids and put them to work. But it has the result of creating this huge conflict between males and females."

It's in every queen's best interest to get rival males to mate with her -- she gets her hybrid workers, and the rival queens have less sperm available to them. The males, on the other hand, would be wise to avoid these foreign queens. If they reproduce with their own queen, they may luck out and be the father of a reproducing insect. With the rival queen, they have no chance of continuing their genetic lineage -- and their limited supply of sperm is wasted.

Cahan and Herrmann wanted to better understand why these males mate with the wrong queens in the first place. Herrmann tracked down the ants mating swarms, which occurred in 0nce-a-year free-for-alls after summer monsoon rain.

In the hedonistic swarms, it seemed that the males just didn't know which queens where which -- until they were close enough to sense their chemical signals, at which point they were already mating. But Herrmann isolated pairs to take close observations of them in the act, and then froze them so he could take measurements of their sperm.

It seemed like the males mated exactly the same way with either queen -- they gave each the same amount of their precious sperm.

But that the null result was actually masking intense strategy on either side.

"The males were certainly doing something," Cahan said, "but you just couldn't see it overall."

As it turns out, the males slowed down when they realized they were mating with rival queens. It seems that they attempt to get away with a slower release of sperm, probably so they can pull away and use whatever's left somewhere else.

But the queens won't have it: In both species, queens held onto the slowing males until they'd released all of their sperm.

"Both males and females have power in this relationship, and they're working at cross-purposes," Cahan said. "So we saw what looked like nothing, but that 'nothing' was actually masking a lot of effort on either side."

We're only catching a current snapshot of the way these two species interact. Like any antagonistic relationship between species, Cahan said, this rivalry has driven evolution on either side. Every time a male of one species adapts to protect themselves from this sperm theft, the queens of the other will adapt to out-do them -- and so on and so forth, ad infinitum.