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  • [♪ 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 namedstick insectshave 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