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Although cannibalism is considered to be the result of serious mental illness and damage in humans, for sharks, it is just another method of natural selection in which the strongest ones survive. According to Demian Chapman and colleagues from Stony Brook University in New York, sharks begin their voracious behaviors in the wombs of their mothers. Chapman observed the babies of sand tiger sharks and discovered that once these babies develop eyes and a set of sharp teeth, they begin eating off their fellow brothers and sisters in the womb, giving themselves the best chances of survival after birth. Despite being just 10 centimeters long, these baby sharks are vicious creatures to be reckoned with.

When a female shark is impregnated, any fertilized eggs can end up in either one of the two uteri in the shark's body. Since female sharks will continue to develop eggs, a large number of embryos can develop within the body. Despite this large number of eggs, only a select few sharks emerge at the very end of the gestation period. Chapman stated that the first embryo to develop will most likely be the one to survive the dangers of the womb as it eats up all of its siblings and unfertilized eggs. Not only does this process of survival of the fittest ensure that the baby shark will have less competition once it enters the world, it also allows for nourishment. Since the diet is composed mostly of the yolk of unfertilized eggs and the flesh of its siblings, the first hatchling tends to grow at a rapid pace, giving it the upper hand in the womb and eventually outside of it as well.

Chapman studied 15 pregnant sand tiger sharks that had died due to nets in the South Africa region. The research team retrieved tissue samples from both the mothers and embryos and discovered that female sharks tend to mate with two or more sharks. The wombs revealed that when there were only hatchlings left, the baby sharks tended to be half-siblings as opposed to full ones. This finding suggests that for sharks, sexual reproduction is not certain for male sharks. Impregnating a female shark does not automatically mean that the male shark's genes will get passed on. Furthermore, researchers have found that female sharks can also store sperm and decide on which ones they want to fertilize their eggs with.

Reproduction for male sharks can be one hard thing to accomplish.