The process is called embryonic cannibalism, or adelphophagy. Once an embryo reaches roughly ten centimetres it develops functional teeth and eats the other embryos in its uterus, after which it feeds on a continuous supply of unfertilised eggs, a strategy known as oophagy. The survivor emerges large enough to be beyond the reach of most predators from birth.
Genetic work has shown a second consequence. A litter often begins with embryos sired by several different males, so the cannibalism resolves a paternity contest inside the mother, and the surviving pup usually has one father.
The arithmetic is brutal for the species. Two pups every second year is close to the lowest reproductive output known in sharks, and it cannot support fishing or netting losses. Sand tigers were shot as dangerous, netted in bathing protection programmes and taken by fisheries; the species is now Critically Endangered despite being unusually placid toward divers.