If your dog suddenly gave birth to a litter of puppies, you might be surprised, but you'd know one thing for sure: Somehow, some way, she managed to rendezvous with a male dog. That, and the next thing on your to-do list would be calling all your friends, asking if they want a puppy.
This month a new paper published in the Nature journal Scientific Reports describes the unusual case of a zebra shark (Stegostoma fasciatum) named Leonie at the Reef HQ aquarium in Queensland, Australia. Leonie, despite not having had contact with a male shark for three years, laid three eggs that all hatched healthy baby sharks. And though Leonie's handlers were surprised, they didn't assume she somehow magicked her way into the tank of a male shark. No, the most reasonable explanation is even weirder — that she made some babies without the help of a male.
This nifty little trick is called parthenogenesis, or "virgin birth," in which a female reproduces without her eggs being fertilized. Although it isn't all that uncommon in the natural world among plants and invertebrates, scientists keep stumbling on instances of vertebrate parthenogenesis that are total shockers — like the reticulated python at the Louisville Zoo in Kentucky that had never been near a male in her life, but hatched out six healthy daughters back in 2012.
But what's odd about Leonie's case is that she had reproduced sexually once before, and then, a few years later, switched to asexual reproduction (genetic tests confirmed this second set of babies are the product of just one parent, not two). This is the first known case of this sexual-to-asexual switch happening in sharks, though it has also been observed in a spotted eagle ray, which is a close shark relative.
It's really handy to be able to produce viable embryos without the help of a partner, but scientists aren't sure why animals that usually reproduce sexually will simply clone themselves. It's a testament to how serious the business of reproduction is to all organisms that, when cut off from sexual partners, the female of some species will just get the job done herself — if she's able.