inter species sex
This image depicts an instance of cross-species breeding gone awry. Fluorescence microscopy reveals sperm, in red, invading a female worm's body.Gavin Woodruff

Two creatures of different species do not generally mate with each other. A team of Canadian scientists believe they have found the reason for this, and claim that the sperm from the males of a different species can wreak havoc within the body of the female.

Scientists say that interspecies sex can lead to the sperm destroying the ovaries, and can - in some cases - kill the female. This research seems to have answered Charles Darwin's longstanding question of why different species do not mate amongst each other.

The study, named Intense Sperm-Mediated Sexual conflict Promotes Reproductive Isolation in Caenorhabditis Nematodes, was published in the PLOS Biology journal. In this study, the scientists made female worms of one species mate with male worms of another species. They also had a few other female worms of the species mates with male worms of their own species.

They found that the females which mated with their own species went on to breed properly, and had normal lives. However, the females who mated with different species, showed quite different results.

The sperms in the interspecies fertilization were too strong, and destroyed the ovaries, leaving the females sterile. In some cases, these sperms were so strong that they even killed the females.

"We found that hermaphrodites can sense, and try to avoid, males of species that can harm them," said Eric Haag, Associate Professor of Biology at the University of Maryland, in a press release.

However, there are cases when interspecies fertilization has been possible. For example, horses and donkeys have been bred together, to make mules. However, most of these mules were born sterile, and natural progeny has been rendered impossible within them.

Haag also said that there exists a reaction between the female reproductive tract and the sperms of different species, and that was the main reason for the failure of fertilization amongst the worms.

"The findings may be worth investigating in other species as well," said Haag. "Because similar coordination problems may be relevant to infertility in other organisms."

These researchers reverted to Darwin's Theory of Evolution, and believe that this happens so that the species can retain their distinct identity. They say that females, who belong to a species with less active sperms, may not be able to withstand the reaction caused by the sperms of the more active species' males.