It's the thunderdome, but not as bad as a shark uterus. They generally only give live birth to one shark because that shark has eaten all the others.
2. Sibling cannibalism
Sand tiger sharks have the most ghoulish fix for hungry babies: The fetuses devour each other in-utero until there’s just one pup remaining in each of their twin wombs. This uterine bloodbath is thought to have developed because a litter of sand tiger pups usually has multiple fathers — and each dad is engaged in a proxy war for his genes to take the upper hand. The “winning” fetus gets all the nutrients from its siblings and a roomy womb to develop into an unusually big baby, over 1 meter (3 feet) long at birth. Being born large and well-developed helps these newborn predators survive in a hostile ocean.