Image source: Virgin Media
Sandtiger shark (Carcharias taurus) pups emerge from the womb as experienced killers. Since they develop teeth and the ability to swim while still in the womb, the strongest embryo will actually kill and eat its siblings (embryophagy) and their yolk sacs. This ensures that the surviving embryo has sole access to the unfertilized eggs that are continuously produced by the mother. Since they have two uteri, these sharks give birth to two live offspring whereas other shark species with less intrauterine killing instincts may deliver a brood of up to a dozen or so. In fact, the sandtiger shark has the lowest reproduction rate among sharks which helps to explain their status as threatened.
Other species of sharks do not have such dramatic sibling rivalries. Instead, they feed on the constant supply of unfertilized eggs produced by the mother (oophagy). This is by far the most common type of intrauterine cannibalism in sharks.