Today on A Moment of Science, another take on sibling rivalry. Sibling rivalry among sharks, that is. There are nearly four hundred different species of sharks out there and, depending on the species, shark embryos develop in three distinct ways.

Some, like hammerhead and blue sharks, nurture developing embryos through a placenta, much like mammals. Others, like horn sharks, release eggs into the ocean. However, the most common kind of embryonic development in sharks is when the female secretes a thin protective case around the embryo and the embryo remains in one of her two uteri. These sacs have a yolk that provides the embryos with nutrients, but there is no placental connection to the mother. When the embryos are mature enough, they hatch within the uterus, and the female gives birth to live pups.

So where does the sibling rivalry come in? Well, in some species of sharks, the embryos may obtain additional nutrients by swallowing the unfertilized eggs in the mother's uterus, or else--you got it--by eating other developing embryos. This practice is known as intrauterine cannibalism, and was discovered accidentally in 1948 when a scientist was bitten as he probed a very pregnant sand tiger shark's uterus.

Because the world they're born into is hardly any more hospitable than the womb, these shark pups are ready to feed and survive on their own from the moment they come out of the womb. And what better preparation is there for this dog-eat-dog world than surviving a shark-eat-shark womb?