Eating Your Siblings in the Womb
In sand tiger sharks, embryos eat their siblings in the womb — only the two largest (one per uterus) survive to be born.
Sand tiger sharks practice intrauterine cannibalism — embryos eat each other inside the mother's womb. It's survival of the fittest before they're even born.
A female sand tiger shark produces many eggs, which are fertilized by potentially multiple males. The embryos develop in two separate uteruses. As the largest embryo in each uterus develops teeth (at about 5 months), it begins eating its siblings.
The "winner" in each uterus then continues eating unfertilized eggs the mother produces, a behavior called oophagy. By the time the two surviving pups are born, they're already 3+ feet long and experienced predators.
This brutal system ensures only the strongest offspring survive. It may also serve to eliminate offspring from rival males — genetic studies show the surviving pups are often full siblings rather than half-siblings, suggesting babies fathered by different males get eaten.
Other sharks have different strategies: some lay eggs, some give live birth to many small pups, and some (like great whites) also practice oophagy but without sibling cannibalism. Shark reproduction is remarkably diverse.