Rose: [SFX: muted growling] Did you hear that? [SFX: more growling] Oh, it's my stomach reminding me of the very important role it plays in my life. But for some animals, evolution wound up sidelining this organ and its digestive juices [SFX: more growls] What? That's right. From puffer fish to platypuses, many species that you'd *assume* were rocking a stomach are, in fact, getting by just fine without one. And scientists are starting to piece together the evolutionary story of why all of these animals made the gutsy move to lose them.

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Stomachs are pretty ancient; the first signs of stomach glands popped up in gnathostomes, also known as the jawed vertebrates, about 450 million years ago. And stomachs have been doing important work ever since; they create a bunch of acid that helps split big proteins up into more digestible chunks plus acid-thriving enzymes that break things down even further. This enabled animals to consume a wider range of proteins in general, opening them up to more diverse food options, like, eventually, a big juicy dinosaur steak or - even more eventually - a technically-still-dinosaur chicken wing. In addition, having a stomach helps your body absorb calcium and phosphate, and helps prevent pathogen hitchhikers from invading via your gut. Basically, having a stomach is a win-win-win.

But there are some gnathostomes that are born without functional stomachs and live their lives with what's called an "agastric gut." This is mostly seen in a bunch of different fish species. In the teleost group, which comprises almost all living agastric fish species, agastric guts have evolved at least 15 separate times. And even some non-teleost fishes, like lungfish and ratfish, have far-gone their stomachs, too. In some cases, like puffer fish, there's still a sack where the stomach *should* be, but that sack doesn't have glands that excrete the acids and enzymes that go hand-in-hand with a gastric stomach. This is also true of the monotremes, platypuses and echidnas. Who would have thought our weird egg-laying, distant, mammalian cousins could get even weirder.

For so many gnathostomes to have successfully ditched their gastric stomachs, there's got to be a lot of reason. But because so many of them have gone agastric, there might be more than one. For example, maintaining a stomach is complicated; your body's got to produce all those powerful acids and enzymes to break down proteins while simultaneously protecting the rest of your body from their corrosive ways. So not having to deal with an entire organ full of digestive juices probably has some kind of energy-saving benefit. But scientists haven't completed any studies that prove that - at least, not yet.

Another reason may be a change in diet. The enzymes you find in a given animal's gut appear to be tailored to the types of protein in its diet. For example, mammals that mostly chow down on plants tend to have higher levels of pepsinogen than those that eat mostly meat. But if a species diet changes enough to continually mess with those gastric functions, there could be an evolutionary pressure to shut it all down.

Maybe a fish starts adding a bunch more calcium to its diet by chowing down on a surplus of shells or coral. Thanks to the extra calcium, this fish is basically taking a bunch of antacids with every meal. So the stomach could be like, "What's the point of me pumping out all these acidic gastric juices if they're just going to get neutralized?" And over multiple generations, evolution responds, "Waste not, want not."

Such a hypothesis might explain why something like the coral-munching parrotfish lacks a gastric stomach, but it doesn't work for all agastric species. Pufferfish may have developed their agastric guts for a completely different reason; they appear to have swapped digestion for defense, using their stomach to fill up on salt water and transform themselves into an adorable, angry nightmare balloon. However, other fish species that bloat their bellies *do* still have gastric functions, so that hypothesis can't apply across the board either, even if it may be the case for puffers.

Not only are scientists still puzzled by exactly why agastric gnathostomes evolved, it's also still unclear how they all get by without their stomachs. Stomachs do a lot of important things, but we still don't know if or how those tasks are being carried out in these stomachless species. At the very least, it also probably varies from species to species. Fish, like the ballan wrasse, appear to have evolved a different way to regulate their appetites, and they might have swapped out individual meals for more continuous feeding schedules. Some animals may have evolved extra teeth to help grind their food up into smaller bits, so their gastrointestinal tract doesn't have to work as hard.

Scientists aren't sure, but that might be why the platypus re-evolved bunch of teeth-like grinding plates in their bills. And other animals may have enhanced the digestive powers outside their stomach, like seahorses that release their acids in their esophagus. Ugh, I'm getting heartburn just thinking about it.

As for our stomach's pathogen-fighting powers, some agastric species may try to avoid the problem entirely by sticking to foodstuffs that are more distantly related to them. Mammals that eat other mammals are often at risk of catching pathogens that just generally infect... well, mammals. But if you're an echidna sticking to an insect-based diet, you don't have to worry as much.

So, despite the very important jobs our stomachs do for us, some animals have found some pretty effective work-arounds. And while there's still a shroud of mystery around the specific ins and outs of why so many gnathostomes have broken the mold and lost their stomachs, one thing is clear: They're doing just fine without them. But in my case, I think I'd be a little lost without the occasional rumble to remind me when I've worked through my lunch hour. [SFX: stomach growl]

Thanks for watching this episode of SciShow. We love talking about the cool and unusual animals that share our planet with us, and if you're looking for even more of that, you should check out our sibling-channel, Bizarre Beasts. Once a month, hosts will introduce you to a new bizarre beast and explore what makes these animals so weird to us; from birds whose babies have claws on their wings to frogs that can't land their own jumps, the show examines the How and the Why of some of the world's most amazingly strange critters.

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