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We humans couldn’t live without our gut microbes, but not all animals rely on microscopic digestive communities like we do. And understanding why these animals ditched their microbial partners can teach us a lot about the costs and benefits of making evolutionary friends.

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Stick bugs:

[♪ INTRO].

We hear all the time about how we couldn't live without our microbiomes, those trillions of symbiotic microorganisms that live on and inside of us. We're especially fond of our gut microbes, for example, because they allow us to digest some of the complex molecules in our foods.

And changing up what's living in our intestines can have a big impact on our health. But not all animals rely on microscopic digestive communities like we do. And understanding why these animals ditched their microbial partners can teach us a lot about the costs and benefits of making evolutionary friends.

Bats are the only group of mammals with powered flight. But that's not their only claim to fame. They're also an outlier amongst mammals for shirking their microbial gut residents.

Scientists used to think that bats would be useful for studying gut microbiomes because they have such diverse diets. There are fruit-eating, insect-eating, and blood-eating bats, so researchers expected each of these different diets to have a characteristic set of gut flora. But that didn't turn out to be the case.

There are some bacteria in bat guts, but not as many as other mammals. And the species of microbes can vary widely from bat to bat, even between closely related species. That led scientists to conclude that they simply don't rely on them the way that we do, which could be because bats fly.

See, all bats have lightning-fast digestion. For example, a study of greater mouse-eared bats found that they start pooping out their meals less than an hour after eating. The researcher actually dyed the bats' food with a magenta substance called fuchsin.

And about half an hour later, their poops turned pink. And bats digest food so quickly for good reason: anything sitting in their stomach would weigh them down as they fly. So, dropping loads more often helps them save energy over a long night of foraging.

And that could also be why they don't have a resident gut microbiome. A large population of microbes might weigh them down. Plus, they don't really need microbial help with digestion, since they've evolved to be more efficient at absorbing nutrients.

Or, the loss of resident microbes may be an indirect consequence of other physiological changes made to accomodate flight, like alterations to the bats' immune systems. Either way, it seems likely to have something to do with flight, since bats aren't the only bony fliers to ditch their gut communities. Birds also seem to have given up their resident gut microbes.

So further study of flying animals, both furry and feathered, will hopefully help pin down exactly how and why they ditched their microbial digestive partners. You might think that caterpillars would need a resident gut microbiome to help them digest all the tough plant material they eat. After all, that's one of the things mammals like us rely heavily on our gut flora for.

Cows and their relatives even have extra stomachs to house special bacteria for the task. But study after study has found that caterpillars don't have microbial helpers. The few bacteria found in their guts are ones that live on the surfaces of leaves, and just happened to have hitched a ride in.

Scientists even double checked to make sure those gut microbes aren't actually important to the bugs. Not only have they compared microbes from different individuals, they've actually dosed caterpillars with antibiotics to see if that messes with their digestion. And it didn't.

If anything, the insects were better off. In fact, caterpillars seem to do what they can to keep microbes from settling into their digestive tracts. Like, you know how we have stomach acid?

Well, they have stomach bases. The pH level in their guts is between 10 and 12, which is comparable to household cleaning products like ammonia and bleach. Both of those are pretty great at killing bacteria, and so are the super-basic caterpillar gut juices.

They probably have such antiseptic insides because microbes aren't always helpful. Many species are pathogenic, and it takes a lot of work to maintain the right internal community. So, for caterpillars, it may be that the risk of infection isn't worth letting microbes stick around.

Plus, their super basic guts help them digest some of the stuff they might otherwise need microbes for. Though, their digestion is still pretty inefficient. They don't get all of the nutrients out of a leaf, only the most accessible ones.

Which is probably why they're notorious for their appetite. For instance, monarch caterpillars can eat about 200 times their birth weight in milkweed in just two weeks. Luckily, there's usually plenty to go around.

One research team even suggested that caterpillars might get a little extra food by digesting the bacteria that make their way into their tummies. Those transient microbes may top off the caterpillar's leafy lunch like microbial croutons. Yum!

There are more than ten thousand species of ants. And while some of them, like fungus-eating and carpenter ants, have evolved relationships with resident microbes over millions of years, others didn't. Studies have found that the abundance and varieties of bacteria in ants' guts can vary widely between closely-related species, and sometimes, even between colonies of the same species, which, like with bats, suggests they're not relying on them.

And the lack of consistent partnering with gut microbes might be why ants are basically everywhere. Some ants do need mini helpers. Up in the canopies of rainforests, for example, plants are pretty much the only reliable food.

So ants that live there may need microbe partners to help them break down complex molecules common in plant material. But those microbes don't work for free. They skim a little off the top of the food that they're helping break down.

Nutrient embezzlement, if you will. And down on the forest floor, there's an abundance of easier to digest, non-vegetarian food options. So for ants living there, the cost of keeping their microbial moochers may have outweighed the benefits.

That's not the only potential explanation, of course. One research team noted that it's possible that, for some species, a lack of gut bacteria is a side-effect of their overall fastidiousness. Many ants keep their nests immaculate, even going so far as to produce antibiotics to actively stop microbial growth.

So the idea is that, if those compounds end up in their guts, intentionally, or by accident, they could kill the bacteria there, too. And it may be that to take over new lands and habitats, ants had to ditch their microbes. Bacteria are often more sensitive than their hosts, and because of that, they can hold species back.

Take carpenter ants, for example. The ants can withstand higher temperatures than the bacteria they rely on to process certain nutrients. So if you put them in hotter conditions, they struggle, in part because their microbes do.

Ants not relying on microbiomes wouldn't have that kind of limitation, so they may have been free to conquer more extreme habitats. This hasn't been explored directly in ants, but there is evidence for the idea in other animals. For example, a 2015 study demonstrated that brine shrimp struggle in lower salinities because their gut microbes do.

If those microbes are removed, they do just fine in much fresher waters, as long as you feed them a diet of what they don't need the microbes' help to break down. Ultimately, with so many potential reasons to cut bait, it seems likely that ants' fluidity when it came to microbial partnerships helped them become some of the most common and diverse organisms on Earth. Aptly named “stick insects” have clever camouflage which allows them to hang out in foliage where they consume a dense diet of leaves.

Like other plant eaters, they can make the most of their meals by breaking down complex plant molecules like cellulose, the tough, fibery substance that makes the walls of plant cells sturdy. And while many plant-eating animals call in microbial help, the stick insect does not. It has a long, skinny gut with no room set aside for resident microbes.

Like in caterpillars, microbes found in stick insect guts are probably passing through after catching a ride on the lunch-time leaf. And it seems like the insects don't need any microbes to stick around. They've already taken what they'd need from the microbes: their genes.

In their genomes, stick insects have the code needed to create molecular tools called enzymes which break down the big, tough-to-digest molecules found in plants. And they've probably had those genes for tens of millions of years. According to research from 2016, at least 60 million years ago, some ancestor to modern stick and leaf bugs acquired the code through horizontal gene transfer.

That's when a chunk of one organism's DNA makes its way into the nucleus of another species and integrates into its genetic code. Lots of animals, including humans, have bacterial genes scattered throughout their DNA. But stick insects seem to have received a bunch of different pieces of DNA code that they use every day.

Then, over time, they created duplicate versions of these stolen microbial genes, as well as some of their own. With more code, they could make slightly different versions of key enzymes to more efficiently digest the nutrients in their greens. Which is good, because, again, that's all they eat.

They're actually the only order of insects that is 100% vegetarian. But even though the stick insects got vital genes from microbes, they're not full of gratitude. They're full of antibiotics.

When they detect an infectious microbe, their immune systems jump into kill mode. And they have an impressive repertoire of microbe-killing molecules with which they can dispatch potential invaders. So there you have it: four animals that don't seem to rely on gut bacteria.

But there are probably more; maybe even a lot more. The results from studies on bats, caterpillars, ants, and stick insects all remind us to be cautious about assuming that microbes in or on something are welcome there. But also, we may someday discover that some of these animals do actually have microbial buddies in their digestive systems.

In science, you have to be careful when claiming negatives. Just because you haven't found something yet doesn't mean it's not there. For things like stick insects, we can say that there's no evidence so far that they rely on gut microbes.

And the sum of evidence, especially for caterpillars, is pretty convincing. But more sensitive technology might someday change the story. That's what happened with oysters.

In the 1980s, researchers used microscopes and culture plates to try to grow bacteria from samples from the surfaces of Pacific oysters. And at the time, nothing showed up, so the authors suggested the species simply might not have symbiotic microbes. But in the decades since, genetic sequencing has revealed they do have microbial residents.

And research from 2019 on a related species found bacteria living all over oysters; in their gills, glands, mucus, and guts. Really, we need to be careful about making any assumptions about what's living on and in different species, positive or negative. And the more we study the microbiomes of different species, the more we'll learn about the costs and benefits of living in a germ-riddled world.

Thanks for watching this episode of SciShow! And a special thank you to our patrons. We simply wouldn't be able to produce all these educational science videos we put out without our wonderful community of patrons, so, thanks!

If you want to learn about how to join them in supporting SciShow, you can head over to And if you want to learn more about microbes and what they can do for us and other species, maybe check out our episode on microbes that could save the environment! [♪ OUTRO].