When Shark Fetuses Attack

  • By Anna Rothschild
  • Posted 05.22.15
  • NOVA

Sand tiger shark fetuses eat each other in the womb, in a behavior called "embryonic cannibalism." Learn more in this episode of Gross Science.

Running Time: 02:23


When Shark Fetuses Attack

Posted: May 21, 2015

As humans, we tend to think of the womb as a nice, warm, safe place. But if you were a sand tiger shark, the womb might seem like the most dangerous place on the planet.

I’m Anna Rothschild, and this is Gross Science.

Sand tiger sharks are large sharks that live in warm and temperate waters throughout most of the world. Females have two wombs—two “uteri.” And when a female’s pregnant, she can have about six or seven babies in each womb, each one in its own egg case.

Eventually, one of the babies will hatch from its egg. This hatchling might be little, but it already has a set of razor sharp teeth. In fact, one scientist claims he was bitten by a fetus while he was dissecting a dead female!

There’s a reason for these sharp teeth though. You see, soon after emerging, the hatchling will start devouring its growing brothers and sisters until it eats up every last one in the womb, a process called “embryonic cannibalism.” In the end the mother gives birth to just two babies—the cannibal pup from each uterus.

This might just sound like sibling rivalry gone wild, but the hatchling actually gets a huge benefit from this blood bath. By feeding on the flesh of its siblings, the hatchling grows quite large. In fact, by the time it’s born, it can be over three feet long! And this gives it an advantage in the ocean—very few predators would dare hunt a three-foot-long killer with razor sharp teeth.

There might be another reason for this strange process, as well. It turns out that female sand tiger sharks mate with multiple males, and it’s possible that they have little choice in the matter, though they might get to choose who they mate with first. Embryonic cannibalism may be a way for them to take back control. You see, females can carry the babies of multiple males at the same time. So, a female might mate with her favorite male first, to give that male’s babies a head start to becoming the cannibal hatchling.




Host, Writer, Animator, Editor
Anna Rothschild
DP, Sound, Intern Extraordinaire
Kristen Clark
Many thanks to Dr. Demian Chapman and Dr. Kevin Feldheim.
Slow Bass
Music Provided by APM


Hatchling and Embryo
©Debra Abercrombie
Sand Tiger Sharks Mating
©Grant Kennedy
Carcharias taurus Newport
Wikimedia Commons/Jeff Kubina
“We Can Do It!”
J. Howard Miller/Public Domain


(used with permission from author)
Squeak Pack/squeak_10
Running water bubbles-02
Zombie pack vol. 1/Zombie eating flesh
120 horror strings
Produced by WGBH for PBS Digital Studios


(main image: Shark)
©WGBH Educational Foundation 2015


Want more info?

Chapman, D., Wintner, S., Abercrombie, D., Ache, J., Bernard, A., Shivji, M., & Feldheim, K. (2013) The behavioural and genetic mating system of the sand tiger shark, Charcharias Taurus, an intrauterine cannibal. Biology Letters, 9: 20130003.

Related Links