One of the affected worms, with sperm in all the wrong places. (Gavin Woodruff)

When two different species try to mate, they’ll almost always fail. Some, like horses and donkeys, will get lucky and produce hybrid offspring — but those mules are usually sterile.

But for some worms, a new study published in PLOS Biology reports, the price of inter-species breeding is no less than a painful death.

Researchers mated some species of nematode, and they weren't surprised when the worms didn't produce viable offspring. But curiously, the females who’d mated with the wrong species were often sterile after the fact, and had shorter lifespans than their fellow females.

In the affected worms, sperm didn't stop where they were supposed to. (Janice Ting) In the affected worms, sperm didn't stop where they were supposed to. (Janice Ting)

The reason is kind of horrifying: When researchers observed the transparent worms under a microscope, they saw that the foreign sperm had broken through the uterus and into the ovaries. After fertilizing eggs still held in the ovaries, which couldn’t develop into offspring, the sperm had destroyed the organ and traveled further into the female’s body. The internal injury was sterilizing the female worms, and sometimes even killing them.

These worms may look almost indistinguishable even to experts, study author Asher Cutter, associate professor of ecology and evolutionary biology at the University of Toronto, said, but they’ve evolved to be as different as humans and mice. This experiment shows one major divergence in their evolution. Every species needs to solve the same problem: Groups of sperms, whether they belong to one male or several, need to compete with one another to fertilize an egg. These species of worm have formed radically different strategies.

“It’s interesting, even beyond the ick factor of nematode sperm crawling around in strange places,” Cutter said. When two different species attempt to mate, scientists usually see the sperm from foreign males being ignored, or out-competed by sperm of the correct species. “The unusual thing here is that they weren’t ignored or neglected,” Cutter said. “In fact, they seem sort of over aggressive within the reproductive tract.”

Cutter and his colleagues don’t know how the aggressive sperm manages to infiltrate the body cavities of the gentler species. Perhaps, he said, there are compounds in the seminal fluid that relaxes the musculature of the reproductive tract, making it easier for the sperm to break through a weak one.

A uterus that evolved alongside this aggressive sperm would have toughened up to survive it. “It can turn into kind of an arms race between males and females,” Cutter said. “And it causes this chain reaction of evolutionary change, which can go in really different directions for different species.” For another great example of battling animal genitalia, just look to ducks.