Evolution

For most animals the fight for survival starts on arrival into the world. But for sand tiger sharks, it begins in the uterus.

Though several eggs are fertilised following mating, only two pups will be born - one developing in each uterus. The majority of these eggs are fated to become dinner. From the moment these sharks develop their teeth they begin to eat their brothers and sisters, as well as

any unfertilised eggs. This process is called adelphophagy ("to eat one's brother). As a result of this nutrient-rich diet, sand tiger pups are born larger than other shark pups, generally measuring a metre long on birth.

Adelphophagy is not the only form of intrauterine cannibalism seen in sharks. A far more common type of in-uterus cannibalism is oophagy, when growing embryos eat unfertilised eggs in their mother. Around 14 species of sharks are known to practice some form on intrauterine cannibalism. It sure is a tough world when you have to fight to even make it to birth!

Photo credit: David Doubilet.

http://animals.howstuffworks.com/fish/shark-pup2.htm

http://www.flmnh.ufl.edu/fish/Gallery/Descript/Sandtiger/Sandtiger.html

http://www.elasmo-research.org/education/topics/lh_intrauterine_cannibalism.htm

http://australianmuseum.net.au/Oophagous-sharks