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 actually
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.
Ew.
