'They're Cannibals Before Birth': Sand Tiger Sharks Eat One Another in the Womb

A new film has revealed the disturbing and violent details of how sand tiger sharks begin their lives.

The Nat Geo WILD documentary Cannibal Sharks features researcher Demian Chapman from Florida International University who has been studying the species for more than six years.

"In the 1970s when scientists first began looking at reproduction in sand tigers they noticed that they always gave birth to two offspring," Chapman said in the documentary. "But early in pregnancy they would actually find 12 or 14 embryos. So the big question was, what was happening to all those other embryos?"

"One big clue was one of the researchers actually stuck his fingers into the womb of a sand tiger that was dead and actually got bitten by one of the embryos," he said.

As part of his research on the sharks, Chapman took DNA samples which revealed that the embryos often had different fathers because the females mated with multiple partners. The baby sharks develop in two separate uteri, each containing up to seven embryos inside.

"What happens is the oldest of those embryos is actually a little bit ahead in terms of development of the others—develops teeth and eyes a bit earlier," Chapman said. "And once it's got those things it actually starts hunting and killing all of its siblings. So they're cannibals before birth which is really wild."

Once the older embryo—which may measure just 4 inches long—has killed its siblings it then consumes them, causing it to grow exponentially bigger and stronger, according to a study authored by Chapman which was published in the journal Biology Letters. This process is referred to as "intrauterine cannibalization," meaning taking place in the womb. The cannibalization leaves just two embryos remaining from the initial 12 or 14—one from each uterus.

The new documentary also features a horrific video clip which shows one large great white shark taking a huge chunk out of another one of its species during a fight. The footage was captured by divers off Neptune Island, South Australia.

The film also shows a photograph captured in Australian waters—off North Stradbroke Island in Queensland—of another great white with huge bite marks that have left it almost severed in half.

"What an amazing photo," Mark Meekan, a fish biologist at the Australian Institute of Marine Science, said in the film. "I mean, this is an enormous shark, it's 12 feet long but look at the size of that bite. It's absolutely massive. That's an immense amount of power you need to take a bite out of another shark like that. You have to be pretty big yourself."

Experts think that shark-on-shark attacks such as these are more common than previously thought.

sand tiger shark
A shoal of 50,000 sardines swim around a sand tiger shark at the Hakkeijima Sea Paradise Aquarium in Yokohama, Japan. YOSHIKAZU TSUNO/AFP/Getty Images