Talk about sibling rivalry. Shark embryos actually cannibalize their littermates while still in the womb; the largest one eats all but one of its siblings. Now, new research reveals why sharks are such bad brothers.

In order to find out why this phenomenon occurs, researchers analyzed shark embryos found in sand tiger sharks which, despite their name and their in utero behavior, are a non-aggressive species. They are only known to attack humans when bothered first. In order to better understand these embryos, the scientists examined them at various stages of gestation. They discovered that the later the pregnancy is, the more likely the remaining shark embryos had just one father.

So what does that mean exactly? Before now, researchers weren't sure whether females mated with just one partner or with multiple partners. After a bit of DNA testing, researchers discovered that litters that possessed five to seven embryos had at least two fathers. It's possible that females mated with even more males, though; at the start of gestation, there can be as many as 12 littermates. It could be that the other littermates with different fathers had already been eaten.

The cannibalization itself is actually a useful strategy for the sharks. It allows the two remaining babies to grow large enough to be relatively unbothered by predators once they're actually born. What is more surprising, though, is the fact that the two sharks are usually full siblings as opposed to half siblings. This suggests that the largest embryo actually targets other embryos that are from other fathers.

"Basically, that loser father ultimately provided food for a rival male," said co-author Demian Chapman, a marine biologist at Stony Brook University, in an interview with LiveScience.

Currently, it's still a mystery as to what makes one father's embryos successful over another's. Yet the researchers do have some theories. It's possible that the embryos from the first male to fertilize the female simply get bigger first and devour their littermates.

The strategy could actually encourage females to select good mates. Since shark mating involves violent biting, this cannibalism may allow females to avoid resisting and being too picky about what males she mates with while still ensuring high-quality offspring.

The findings are published in the journal Biology Letters.