Nature has concocted all sorts of deterrents for preventing different species from trying to breed. There are distinct mating calls, strange mating behaviors and reproductive organs that simply don't fit together. For the humble earthworm that tries to become intimate with the wrong worm species, however, the stakes are much higher than a bit of wasted time and energy. Mating with the wrong earthworm fella, scientists have discovered, can kill females.
As Wired UK reports, researchers were perplexed to find that, when female worms mated with different species in the lab, the females tended to die. If they did survive, on the other hand, many were rendered sterile. And if they did somehow manage to survive and the mating proved successful, their offspring tended to have shorter-than-normal lives.
Clearly, something was amiss with these pairings. Cross-species matings usually do not prove deadly. The researchers took a closer look at the worms to find out what was going on. The foreign males' sperm, it turned out, was eating through the females' uteruses, spilling into their ovaries and fertilizing all the eggs at once—even the underdeveloped ones, Wired describes. Hence, the sterility.
But, as Wired writes, "the vicious sperm would not stop there." The sperm often continued their rampage beyond the ovaries. As the researchers told The Verge, the sperm would "migrate to inappropriate parts of the body," burrowing through the females' and causing tissue damage. Essentially, they said, the females were "overrun by sperm."
Here's Wired on why the sperm would do such a thing:
The researchers believe that the answer lies in the divergence of sexual organ development in different worm species. Worm sperm is feisty at the best of times as it often has to compete with the sperm of multiple worms -- depending on how many males a female has mated with -- in order to gain access to the eggs. The females are usually able to cope physically with the sperm fight happening inside them, in order to go on to produce offspring. But this tolerance, as well as the aggressiveness of the male worms' sperm, seems to differ from species to species.
These females, it turned out, were not evolved to deal with this particularly aggressive sperm. However, mysteries remain. As the researchers told The Verge, "the biochemistry and genetic details controlling these mismatches are still a black box to us."