NARRATOR: The cannibal sharks investigation
heads to Bimini in the Bahamas.
The mangrove swamps here are a precious nursery
for lemon sharks.
Every year, scores of pregnant females
return to these shallow waters where
they were born to give birth.
But in a Machiavellian twist that surprised scientists,
someone was eating the kids.
TRISTAN GUTTRIDGE: Of 100 newborn sharks
born in June of one year, by June the next year,
around 50 are left.
So 50% of the cohort in every year
are eaten or die from starvation.
NARRATOR: This pup is less than two feet long, one
of a litter of up to 17 pups.
Scientists from the Bimini Shark Lab
have been studying the lemons here for more than 20 years.
They found the pups can stay up to eight years
here, learning to hunt and hide among
the twisted mangrove roots.
But the shock discovery was the pups' biggest
threat comes from their own.
TRISTAN GUTTRIDGE: Everything seems to kick off at high tide.
As the water rises, it basically gives
access to shallow water areas for big predators
to come in as well.
That's when these big lemon sharks come in
and start patrolling and cruising the outskirts
of those mangroves.
[music playing]
NARRATOR: The young are on high alert.
They instinctively hug the mangrove roots
which offer some protection.
But if they stray too far, the hunt is on.
TRISTAN GUTTRIDGE: For a baby lemon shark that's in close
proximity to a big adult, if they
haven't seen it before it's seen them, they've got no chance.
They can burst attack very quickly
and ambush really effectively.
I've seen firsthand bite marks.
I've seen adults chasing juveniles.
You know, we know it's happening.
NARRATOR: It's clear evidence shark cannibalism goes on
in the wild.
But why would you eat your own?
TRISTAN GUTTRIDGE: The one great advantage
that an adult lemon shark has is insider knowledge on where
the juveniles are hiding and what
tactics they use to avoid predators,
because it's been one itself.
And that gives it an extra advantage,
and probably why they then pick off the juveniles
of their own species.
What's happening here in Bimini isn't unique.
Throughout the world in other nursery areas,
big sharks are eating little sharks.
It comes down to needing to eat.
Why would they not take advantage of that resource?
NARRATOR: And now, new research suggests this has
been happening for millennia.