How did Mary the stickleback fish gave birth to already-fertilized eggs? Scientists have solved how the fish had a “virgin birth,” and the answer is simpler than one might expect.

'Virgin Birth'

Mary is a female stickleback fish that researchers gathered from the wild for a series of scientific research. She looked like an ordinary egg-carrying female, so the researchers were surprised to find that Mary was carrying nearly completely developed embryos in her ovaries, something that is quite unheard of in egg-laying species.

Typically, when three-spined sticklebacks reproduce, the male builds a nest and performs a zig-zagging dance to attract a female who will lay her eggs in the nest. Once she has done so, the male will chase her away, and he will fertilize the eggs and take care of them until they hatch about two weeks later.

In Mary’s case, it was clearly unusual for her to be carrying already fertilized eggs. Because she was already close to death when the researchers discovered her condition, they saved her offspring by conducting a Caesarean section and delivering 54 embryos, all of which were healthy and not deformed in any way. In fact, 20 of her offspring were still alive after three years and were even able to breed normally.

Accidental Pregnancy

So what happened to Mary?

Researchers surmised that there were two possibilities regarding Mary's condition: one is if she cloned herself, and another possibility is if she had both male and female sex organs. In those instances, the clone should be genetically identical to a mother, while offspring from hermaphroditism should have only the genetic material of the mother. However, both possibilities were shot down because genetic testing revealed that the offspring had genes that Mary did not have and could therefore be the father’s.

According to researchers, it’s possible that Mary went to lay her eggs in a nest that already had sperm-covered eggs in them, so some of the sperm might have entered Mary and fertilized them even if she never got to lay them.

The researchers are now actively looking for other stickleback females in the wild with the same phenomenon to see if it really was accidental or if it is a sign of evolutionary or genetic change in the species.