How Ferocious Are Baby Sand Tiger Sharks?

In what might be considered sibling rivalry run amok, scientists say they now understand why sand tiger sharks eat their brothers and sisters while still in the womb. In a 2013 issue of the journal Biology Letters, researchers determined that this cannibalistic in utero behavior is a biological survival of the fittest, in which embryos conceived by different fathers compete to be born.

As many as 12 littermates may start out on the journey, but only one or two embryos will live to become sharks. The embryos that develop the fastest tend to eat the other embryos, a culling process that allows the sand tiger shark to have larger babies than other shark species -- an advantage for newborns in a world full of predators.

It's a shark-eat-shark world:

• Research in the 1980s first revealed shark embryos in the stomachs of other shark embryos, proving that shark fetuses cannibalize each other about five months into their yearlong gestation.
• A shark embryo actually bit a researcher's hand when the researcher reached into the uterus of a female shark during early studies, said marine biologist Demian Chapman, of Stony Brook University of New York.
• The sand tiger shark's cannibalism is called adelphophagy. About 14 species of sharks are thought to practice some form of intrauterine cannibalism.