Survival of the fittest in the sharks womb.
A shark pup's success in life is largely determined by its size at birth and whether the female shark has used a nursery area or a shallow part of the sea with fewer predators than the open sea. Newborn carcharhinid sharks are equipped with fully functional jaws and teeth and have therefore been considered independent of maternal care or support at the point of birth.
Photograph of a Sand Tiger shark by Chris and Monique Fallows. Nature picture library
Shark pups are also very independent, and those that are born live swim away from their mothers as soon as they're born, perhaps to avoid being eaten. Being larger would clearly also have a survival value. One –admittedly hard- way to achieve this is ‘embryonic cannibalism’. This is the name of an unusual mode of reproduction in sharks with many litters, whereby the first embryos in the uterus to reach a certain size consume all of their smaller siblings (called adelphophagy) as well as the unfertilized eggs (called oophagy, oviphagy, or egg-eating) during gestation.
Approximately 14 species of sharks are thought to practice some form of intrauterine cannibalism. The best-known intrauterine cannibal is the sand tiger shark. Although the sand tiger shark has two uteri and produces many eggs, each litter yields just two pups -- one from each uterus. Because of their pre-birth diet, sand tiger pups enter the world bigger than other pups; they measure approximately one meter long. The cannibalistic battle for primacy in utero, with only one surviving represents an evolutionary strategy that allows the largest or strongest male sharks to father the successful baby and thereby outcompete sexual rivals. Paleontological researchers examining incremental growth bands of the tooth of the extinct megatooth shark (Otodus megalodon) recently suggested that the large (2 meters) of the megalodon babies and the rapid growth profiles of their tooth may have also reflected effects of intrauterine cannibalism to reach such enormous proportions, to survive in an ocean infested with much greater predators than today.