The first time it happened, scientists thought it might be a fluke. A female hammerhead shark residing at a zoo in Omaha, Neb., had not been in contact with male sharks for at least three years and yet experienced a “virgin birth.” She delivered a single pup.

But it has happened again, according to today’s issue of the Journal of Fish Biology. This time, a blacktip shark… had spent nearly her entire eight years at either the Virginia Aquarium without any male companionship from her kind.