I'M LUKE TIPPLE, AND THIS IS YOUR "DAILY BITE."

ON THIS EPISODE, WE'RE TALKING ABOUT BABY SHARKS.

SWIMMING BETWEEN UTERI, FULL OF TEETH,

AND CANNIBALIZING THEIR UNBORN SIBLINGS,

TAKING SIBLING RIVALRY TO A WHOLE NEW LEVEL.

WHEN A MOMMY SHARK AND A DADDY SHARK FALL IN LOVE,

IT'S ABSOLUTELY TERRIFYING.

LET'S BE HONEST -- SHARK SEX ISN'T PRETTY.

IT STARTS WITH A COURTING RITUAL

WHERE THE MALE SHARK LATCHES HIS TEETH

ONTO THE FEMALE AND TWISTS HIS BODY AROUND HERS

IN ORDER TO ALIGN THEMSELVES FOR COPULATION.

THE DAMAGE FROM MATING IS SIGNIFICANT,

AND WE CAN OFTEN SPOT MATURE FEMALE SHARKS

BY THE AMOUNT OF SCARRING ON THEIR BODIES.

NOW IN THE BIG BLUE OCEAN, YOU NEVER KNOW

WHEN YOU'LL RUN INTO A SUITABLE MATE,

SO A FEMALE SHARK MAY ENGAGE WITH SEVERAL MALES

DURING A MATING SEASON, AND THEY CAN STORE THE SPERM

FOR MORE THAN FOUR YEARS BEFORE USING IT FOR FERTILIZATION.

AS A GENETIC DIVERSIFICATION STRATEGY,

THEY MAY EVEN GIVE BIRTH TO SEVERAL MALES YOUNG.

BABY SHARKS COME INTO THE WORLD

IN ONE OF THREE DIFFERENT WAYS --

OVIPAROUS SHARKS ARE EGG-LAYING,

AND THE MOTHERS CAREFULLY HIDE THEIR EGGS FROM PREDATORS

SO THAT THEIR PUPS HAVE THE BEST CHANCE OF SURVIVAL.

VIVIPAROUS SHARKS GIVE BIRTH TO LIVE PUPS.

AND OVOVIVIPAROUS SHARKS, WHICH MAKE UP

THE MAJORITY OF SHARK SPECIES, THEY USE BOTH STRATEGIES.

THE EGGS ARE FORMED AND HATCHED INSIDE THE MOTHER

BEFORE LIVE YOUNG ARE BORN INTO THE SURROUNDING WATER.

HANDS DOWN THE CRAZIEST PART OF THIS STORY, THOUGH,

IS THE BATTLE FOR SURVIVAL GOING ON INSIDE THE WOMB.

SHARK EXPERT VICKY VASQUEZ IS HERE TO DIVE INTO

WHY THIS IS SUCH AN INCREDIBLE PHENOMENON.

SOMETHING THAT IS SUPER MIND-BLOWING ABOUT SHARKS

IS THAT THE FEMALES CAN HAVE TWO UTERUSES,

AND IN SOME OF THE VIVIPAROUS SPECIES,

THOSE BABY SHARKS CAN ACTUALLY SWIM AROUND

AND THEY CAN EVEN GO FROM ONE UTERUS TO THE OTHER.

A RESEARCHER NAMED DR. TOMITA

WAS AT THE OKINAWA AQUARIUM IN JAPAN

WHEN HE SAW THE CRAZIEST THING HAPPEN

WITH THE TAWNY NURSE SHARKS.

THEY WERE ABLE TO COLLECT VIDEO

WHICH SHOWED THEM THE TAWNY BABY SHARKS

BASICALLY SWIMMING FROM ONE UTERUS TO THE OTHER,

AND EVEN TAKING A LITTLE POKE INTO THE OUTSIDE WORLD

TO SAY HELLO.

AND THAT'S REALLY, REALLY COOL,

BECAUSE WE'VE NEVER HAD ANYTHING LIKE THAT BEFORE.

SHARK REPRODUCTION IS SUCH A MYSTERY.

WHAT WE DO KNOW AND WHAT WE HAVE LEARNED

IS BASICALLY FROM DISSECTIONS,

BEING ABLE TO SEE WHAT THE INSIDES LOOK LIKE

AND THEN MAKE THE APPROPRIATE ASSUMPTIONS.

BUT WITH VIDEO, THERE'S NO QUESTION WHAT'S HAPPENING.

THEY HAVE SONOGRAMS SO YOU COULD ACTUALLY SEE

THAT THE EMBRYOS, THESE BABY SHARKS WERE ACTUALLY

SWIMMING FROM ONE UTERUS TO THE OTHER.

IT WAS REALLY ABLE TO SHOW WHAT IT'S LIKE

FOR THESE SHARKS THAT HAVE OOPHAGY,

WHEN THE SHARKS ARE ABLE TO EAT SOMETHING IN THE UTERUS.

A GOOD REASON WHY THEY MIGHT BE ABLE TO SWIM

FROM ONE UTERUS TO THE OTHER

IS PROBABLY BECAUSE THEY GET HUNGRY.

SO THEY MIGHT EAT ALL THE LITTLE GOBSTOPPERS OF SHARKY GOO

FROM ONE UTERUS AND THEN FILL UP

AND THEN GO CHECK OUT AND SEE WHAT'S LEFT IN THE OTHER ONE.

NOW, WHY ARE THEY PEEKING THEIR HEADS OUT?

IF I WAS A HUNGRY BABY SHARK, I ATE EVERYTHING I COULD FIND,

AND NOW I'M TRYING TO, LIKE, PEEK OUT OF MOM

TO SEE IF THERE'S ANY FOOD OUT THERE,

IF IT'S A SAFE TIME TO EXPLORE,

AS ADORABLE AS IT SOUNDS TO BE A BABY SHARK,

THINK ABOUT WHAT WE KNOW ABOUT SHARKS.

A BABY SHARK IN THE WOMB WITH THOSE TEETH,

USING THOSE FINS, SWIMMING AROUND

FROM UTERUS TO THE OTHER, WELL,

IT MIGHT BE WANTING TO MAKE A LITTLE BIT MORE SPACE

WHERE IT IS, MIGHT BE GETTING A LITTLE HUNGRY.

THESE BABY SHARKS CAN ACTUALLY JUMP ON THEIR SIBLINGS.

SO, AS YOU CAN IMAGINE,

THE WOMB BECOMES A LOT MORE COMFORTABLE.

THERE ARE SHARK SPECIES OUT THERE

WITH VERY LOW REPRODUCTIVITY.

SO, IF YOU'RE A SHARK

AND YOU'RE NOT PUTTING OUT A LOT OF BABIES,

YOU PROBABLY WANT TO MAKE SURE ALL OF THOSE BABIES

ARE SUPER, SUPER TOUGH.

SO BASICALLY WHAT'S HAPPENING INSIDE THE WOMB

OF A MOTHER SHARK IS THE SURVIVAL OF THE FITTEST

SO THAT THE STRONGEST BABY SHARK

IS WHAT'S COMING OUT INTO THE WORLD.

TALK ABOUT CHILDHOOD TRAUMA.

SO, FROM DAY ONE, SHARKS ENGAGE IN A BATTLE

FOR SURVIVAL WHERE ONLY THE STRONGEST WILL SURVIVE.

WHETHER THEIR MOTHERS ARE REALLY GOOD

AT FINDING STRONG MATES,

ARE EXPERTS AT HIDING THEIR EGGS,

OR THE SHARKS SURVIVE THE GLADIATOR CANNIBAL PIT

OF THE WOMB, EVOLUTION HAS ENSURED

THAT THE SHARKS THAT MAKE IT TO BIRTH ARE THE STRONGEST

AND FITTEST THEY CAN POSSIBLY BE.

MAYBE THAT'S WHAT MAKES THEM APEX PREDATORS.

STICK WITH US ALL "SHARK WEEK" LONG

AS WE CONTINUE TO BRING YOU THE WILDEST DISCOVERIES IN SHARK.

IN THE NEXT EPISODE OF THE "DAILY BITE,"

"WHEN SHARKS ATTACK."

♪♪