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Next time you run out of food in the fridge ask yourself this, would you eat a human? If you were in a plane crash in the middle of a desert, would you eat the person who sat next to you, or would you brave the supply of frozen muffins and green omelettes they serve as sky “food”? For your own survival, would you kill and eat your brother or sister?

Some species of shark do indeed feast on their siblings, not just out in the deep blue sea, but while still inside the uterus. Now that’s taking sibling rivalry to a whole new level.

Sharks reproduce in three ways – they either species lay eggs and leave them to hatch, or they lay eggs and let them hatch in the uterus and then give birth to them, or they have a live births. Feeding a growing embryo while it develops in the womb is quite a challenge for sharks, but they have some crazy methods to do it.

Some species, such as the lemon shark, turn their yolk sac into a placenta by attaching it to the wall of the uterus. Salmon shark embryos eat a stream of unfertilized eggs while their in the womb, providing them with plenty of nummy nutrients. Nurse sharks and sand tiger sharks, at the tender age of not even born yet, swim about and eat their siblings. From the 20 or so pups that start life, only two remain – natural selection starting early *they grow up so fast*

This video from the documentary Animals in the Womb has incredible footage of sand tiger shark embryonic cannibalism.

Why do two pups survive, and not just one? Sharks have two wombs, at least, their uterus has two separate branches that keeps the two pups apart. Apparently having their own room helps curb their aggression, who’d have thought?