Sperm competition in the animal kingdom can be fierce, but some sharks take it to extremes.

Sperm competition in the animal kingdom can be fierce, but some sharks take it to extremes. There may be 12 embryos inside of a pregnant mother, but only one will be born. Not all the pups in the litter will have the same father, and for a male to ensure his offspring will live, the pup will have to cannibalize its siblings.

Source: http://news.discovery.com/animals/sharks/why-shark-exmbryos-eat-each-other-130501.htm