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Check out Brilliant.org/SciShow to learn more. [♪ INTRO]. You think your stomach growls?

Listen to this. [rasping noise]. Those rasps are coming from  the stomach of a ghost crab. But this sound doesn’t mean “I’m  hungry.” It means “Back off.” And it gets even weirder.

Because the crab  is growling…with the teeth in its stomach. Which are a thing that it has. And it’s a window into evolution in action — it’s what happens when an old  structure gets used for a new purpose.

Most crustaceans have a structure  called a gastric mill in their stomachs. It acts like a mortar and pestle, with  teeth that scrape together to grind up food and make it small enough for digestive  enzymes to break it down the rest of the way. But in a 2019 study, researchers  discovered that the mill could be used for another purpose — to make sound.

They observed that ghost crabs would  make a rasping sound when provoked. They also knew that ghost crabs  can produce a warning sound by rubbing their claws together. But in this case, the crabs were  growling with their claws outstretched — telling the researchers the sound must  have been coming from somewhere else.

So they used a technique called  laser Doppler vibrometry, which helps identify sources of vibration,  to see where the sound was coming from. And the signal was strongest  from the crabs’ stomachs. So the researchers took a closer  look by feeding the crabs some dye and scanned them with x-rays.

The x-ray showed the crabs  grinding their stomach teeth in time with the rasping sound they were making. In trying to figure out where this new  stomach-growling behavior came from, the researchers also learned  more about how it was used. For example, some crabs  growled faster or more often, which the researchers think might  indicate how worked up they are.

And bigger crabs growled more often,  perhaps to tell possible threats, “Hey, I’m huge so don’t mess with me!” But if the crabs already had  functional claw-based warning sounds, why evolve a separate one? If you’re a crab, growling with  your stomach frees up those claws so they can be used as extra defense. And using an existing structure for a new purpose is more likely to happen than  evolving something totally new.

Because evolution works with what it’s got.  In fact, existing structures get co-opted to do new things all the  time — including make sounds. For example, fish called grunts  gnash their teeth together to make a characteristic  grunting sound when distressed. But this is the first known example of  stomach teeth used for communication.

And the authors of this study  think it might not be the only one, since grinding and churning stomachs  exist in a number of animals. So watch out for those talkative tummies. They may be saying more than  just asking for a snack!

Our stomachs tell us when they need  food, but what about our brains? If you want to feed your brain,  Brilliant is here for you. Their bite-sized Daily Challenges will  take care of that mental tummy rumble.

But if you’re hungry for even more, each one ties into a full  course available on Brilliant. Brilliant is a place where you can learn  about all things math, science, engineering, and computer science. If you’re interested,  you can go to Brilliant.org/SciShow for the chance to save 20% on  an annual premium subscription. [♪ OUTRO].