[ ♪ intro ] Sharks are so ferocious that  they will eat each other in the womb!

Or at least that's what some people say. And while it is true that some sharks  cannibalize their siblings before birth, the whole ferocious angle might  just be a product of shark-phobia.

After all, we don’t call  salamanders and flies vicious, and some of them do the same thing! It’s not solely a testament  to natural born aggression. There are a few other explanations  for cannibalism in the womb.

And it can actually benefit a species. Otherwise it wouldn't happen. First though, we have to address sand tiger sharks.

If you’ve ever heard of animals  eating each other in the womb, it was probably referring to these animals. And they have a good reason to do that.  They’re living in an eat-or-be-eaten world. And when you are only as long as the width  of a phone, that’s a tough world to be in.

When these little shark  embryos first hatch in utero, each one is around 60 mm in length. So they spend additional time  in the womb getting bigger before they go out into the  water to live their own lives. But they are not doing that  alone, at least not at first.

A pregnant sand tiger shark can have  several embryos in a single womb. Now, those embryos don’t  all hatch at the same time, so they’re not all the same size. And that is where the harsh  realities of shark life settle in.

Once a hatchling reaches around 100 mm, maybe as tall as a cup of tea, they start eating their siblings. So whichever shark gets big enough to  eat the others first does just that. The biggest hatchling eats  everyone else in the womb, resulting in the birth of only one shark per womb.

But who hatches first is not up to chance. The reason they were the biggest goes  back to when their sperm reached an egg. The fastest sperm reaches the egg first and is  most likely to turn into the first hatchling.

That hatchling will be most  likely to grow the biggest, And eat its siblings, and  survive to pass on their genes. And reducing the number of offspring in  this way turns out to be a good thing for the species, because an egg-bearing sand tiger shark doesn’t have to wait for the best mate to fertilize their eggs. Although sand tiger sharks can have litters  sired by multiple shark daddies at once and there can be offspring from different  sperm in the womb at the same time, the most fit mate’s sperm will likely produce  offspring that kill all the others anyway.

Once they’ve done that, the remaining hatchling gets access to all the nutrients in the womb to grow to around 1,000 mm long. That’s about the height of a three year old human. And there aren’t that many predators  that will eat a baby shark of that size.

So eating their siblings makes them harder prey. But sand tiger sharks are not the only animals that eat each other before birth to grow bigger. Fire salamanders can use this same strategy.

In a fire salamander uterus, there  can be 30 eggs in need of nourishment. They get some nutrients from the yolk  of their egg, because they don't have a placenta like mammals that allows them to get  nutrients directly from the pregnant adult. But their alternative nutrient  source is their siblings.

In the same 3 month gestation period, other species of salamanders that feed just on yolk will be born into  the water and need to metamorphosize. Fire salamanders that eat their  siblings will be born ready for land. Just like sand tiger sharks, the salamanders  that eat their siblings in the womb are more developed and more  fit for life outside the womb.

That’s a huge developmental  advantage awarded to womb cannibals. And while they use cannibalism to  speed up the developmental clock, other animals, like the parasitoid  fly, use it to slow the clock down. The developing parasitoid fly grows inside of cicadas and has much lower survival  rates without their host.

But this fly has about 38 larvae  that all hatch at the same time. So the pregnant fly has a ticking clock that  starts the second those offspring hatch, because they each stay in the womb  until they get their own host. The pregnant fly must find a cicada,  nestle under one of the cicada’s wings and slice its abdomen open to deposit a larva into the warm, nourishing body of its new host.

If the pregnant fly doesn’t find  38 hosts in a matter of weeks, the larvae keep getting bigger inside  the womb and they begin eating each other. These parasitoid flies don’t have the  salamander option to feed off yolk instead. There’s no food in the fly womb, so researchers think that they eat  each other to keep from going hungry.

The good news is that this womb cannibalism  gives the pregnant fly a little more time to find hosts in years when there  aren’t that many cicadas around. So if you’re a parasitoid fly,  eating your siblings in the womb can keep you alive while the  search for hosts continues. If you’re a sand tiger shark, it  keeps your species’ gene pool strong.

And if you’re a fire salamander, it  gets you more prepared for life on land. Because you’re watching this video  right now, you are probably a human, which means you did not participate  in this kind of activity. Humans have a rare thing called  vanishing twin syndrome which is the absorption of one twin by  another and back into the pregnant body.

This is not eating your twin. This happens before you are able  to genuinely consume anything because human fetuses have much less  developed feeding structures than other animals that eat each other in the womb. Which is good news.

And here’s some more good news: you didn’t need to! Human fetuses  are nourished by pregnant adults. They get a steady stream of new food and  don’t need to depend on yolk or cannibalism.

So humans do not devour their siblings in  the womb, but it’s not just sharks doing it either. And the species that exhibit this  behavior have a good reason for doing so. It's not just because they're mean and aggressive and ferocious.

If you enjoyed learning  about these fine young cannibals, you can check out another SciShow video called  Extreme Animal Cannibalism, cause why not? Although you might not want to  do it right before lunch. [ ♪ outro ]