Sharks are natural born predators. Turns out, that's because they get plenty of practice in the womb.

Researchers have discovered that sharks don't wait to be born before they start hunting prey. They actually swim around among their mother's uteruses looking for siblings to dine on, according to a new study out of Okinawa Churaumi Aquarium in Japan.

This isn't the first research to reveal that unborn sharks can become cannibalistic, but it's the first time that researchers have witnessed embryos actively swimming around between uteruses to find a meal. It's a fascinating, albeit macabre discovery, which reveals the unseen competitive world that young sharks emerge from. No wonder they come out so bitey.

For the study, researchers put on diving gear and braved shark-infested (aquarium) waters with ultrasound technology in hand, to get scans of pregnant tawny nurse sharks. This gave them an unprecedented look at the goings-on inside a mom shark's belly. The most immediate surprise for researchers was seeing how mobile the embryos were, not just within the uterus, but between uteruses (sharks have more than one).

"Our data shows frequent embryonic migration between the right and left uteri, which is contradictory to the "sedentary" mammalian fetus," writes the study authors.

Over the course of the study, more than 40 ultrasound clips were captured from three pregnant sharks, each displaying a varying amount of movement in utero. In one case, researchers witnessed as many as 24 embryo migrations between uteruses.

These migrations started off innocently enough, until researchers starting seeing the embryos picking each other off. In one case, a mother shark began her pregnancy with four pups, but ended up giving birth to only one. The others, its victims.

Researchers can't say whether this is behavior that is common across all species of sharks, or if its just something that happens among tawny nurse sharks. But given that sharks lack yolks of a placenta to sustain them during development, this might be an important part of their strategy for getting sustenance under gestation.

"It seems likely that in this mode of reproduction, the active swimming ability of the embryo may allow it to effectively search and capture nutritive eggs in the uterine environment," suggest researchers.

Baby sharks hunt down their siblings while still in the womb
They actually swim between uteruses (sharks have more than one) to chomp on their siblings.