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For humans, being upside-down isn’t a comfy way to hang out for very long, but for these six animals, upside-down feels just right!

Hosted by: Michael Aranda

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Sources:
https://www.sciencedirect.com/science/article/abs/pii/S0262407917325137
https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2014.0172
https://www.livescience.com/45147-how-sloths-hang-upside-down.html

https://www.frontiersin.org/articles/10.3389/fevo.2018.00035/full
http://www.soc.hawaii.edu/uhtoday/fall06/research/jellyfish/index.html
https://doi.org/10.3354/meps08623
https://www.biorxiv.org/content/10.1101/784173v2.full.pdf
https://www.hakaimagazine.com/news/the-upside-of-upside-down-jellyfish/

https://www.allaboutbirds.org/guide/Red-breasted_Nuthatch/lifehistory
https://northernwoodlands.org/outside_story/article/nuthatches-the-upside-down-birds
https://www.hitchcockcenter.org/earth-matters/the-sometimes-upside-down-life-of-the-nuthatch/
https://zslpublications.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-7998.2007.00395.x
https://blogs.scientificamerican.com/tetrapod-zoology/exploring-the-vast-nuthatch-empire/

https://kb.osu.edu/handle/1811/5558
https://sites.duke.edu/alexldavis/files/2019/09/Davis2019_Article_LiftAndDragActingOnTheShellOfT.pdf
https://academic.oup.com/cz/article/56/5/550/5559893
http://blogs.ifas.ufl.edu/charlotteco/2015/05/27/whats-up-with-those-baby-horseshoe-crabs-swimming-upside-down/

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001841
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https://www.sciencedaily.com/releases/2008/03/080325203450.htm

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0083476
http://newsroom.unl.edu/releases/2014/01/16/ANDRILL+team+discovers+ice-loving+sea+anemones+in+Antarctic
https://www.jpl.nasa.gov/news/news.php?feature=7543
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5729856/

Images:
https://commons.wikimedia.org/wiki/File:Upside-down_jellyfish_(Cassiopea_xamachana).jpg
https://commons.wikimedia.org/w/index.php?curid=51437096
https://commons.wikimedia.org/wiki/File:Aeroforces.svg
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https://www.inaturalist.org/photos/44287486
https://www.inaturalist.org/photos/2558788
https://www.inaturalist.org/photos/97533235
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https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0083476
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5729856/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5729856/
[♩INTRO].

Some people enjoy hanging upside-down or doing handstands. And while it might be fun for a minute to look at the world from a different perspective, being upside-down isn’t a comfy way to hang out for very long.

There’s something about it that feels… wrong. But with some animals, upside-down feels perfectly right. In fact, this odd orientation gives some of them an evolutionary advantage in their environment. And these animals have some pretty neat adaptations that help them live the upside-down life.

First up, the much-loved three-toed sloth. This sloth is native to rainforests in Central and South America, and you’ll generally find it dangling upside-down from a tree, which is how it spends most of its life. But while this lifestyle might sound kind of lazy, it’s actually pretty strategic.

See, these mammals do everything very slowly. That’s because all they eat are leaves… which are hard to digest and don’t yield much energy. So to make this diet work, sloths have an especially slow metabolism.

That means they don’t have the energy to move around quickly. Unfortunately, that also means they can’t outrun predators. But hanging upside-down in trees helps them stay safely hidden.

It also conveniently keeps them close to their food. And given how little energy they consume, any lifestyle tweak that makes them more energy-efficient is a win. To that end, their long, hook-like claws also connect to a system of internal tendons that pull closed when weighted.

It’s sort of like having a latch that closes when you pull on it. This lets them hang onto branches without using any extra energy. Now, a huge part of sloths’ lives revolves around eating and digesting… which are two things you probably wouldn’t want to do upside-down.

But even though sloths’ abdominal organs and their contents account for up to two-thirds of their body weight, these organs are built for their upside-down life. Sloths have evolved internal anchors made of connective tissue that hold their organs in place. And that keeps the weight from pressing on their lungs.

This lets them breathe freely, even when they hang completely inverted from their hind legs. And that easier breathing is another way they save energy. For some animals, living upside-down isn’t just a way to save energy, it can be a way to get energy.

That brings us to Cassiopea, a genus of upside-down jellyfish that lives in warm coastal regions like the Caribbean and western Atlantic. When you first see this jellyfish, you might think it’s broken or confused. That’s because it spends most of its life slowly pulsating as its bell presses down onto the seafloor.

As strange as it looks, this orientation actually isn’t that much of a departure for a jelly. All jellyfish start out upside-down, tethered to the ground. Then after they break free, most species flip right-side-up.

This one can be found swimming right-side up occasionally, but its natural position is to stay inverted. Not because it’s broken, but to help it get nutrients. First, its arms are home to symbiotic algae, similar to the ones that live in coral.

The algae uses photosynthesis to make food, which it shares with its host. And some researchers think the jelly’s orientation helps the algae get light. On top of that, these jellies live in places that don’t get a lot of water flow, so nutrients tend to settle to the sea floor.

But their upside-down pulsating action draws nutrients up from the sediment and across their many mouths so they can feed. In fact, these nutrients feed the entire ecosystem, including little animals the jellyfish sometimes capture for food. So this odd lifestyle works out for the upside-down jellyfish.

And it has a couple of useful adaptations to help it get by this way. To protect its delicate belly, its body makes compounds that absorb harmful UV light. And while most jellies have a rounded bell, this one has a broad, flat one that’s just right for making contact with the seafloor and stirring up nutrients.

Back on land, a small woodland bird called the nuthatch is known for its habit of foraging upside-down. These birds spend a lot of time on tree trunks and branches, where they’re equally at ease moving up, down, or sideways. But when they look for food, they usually go head-first down tree trunks and underneath branches.

It’s not exactly clear what nuthatches gain from this behavior, but scientists think it’s about having a unique perspective. Other birds that compete with them, like woodpeckers and creepers, forage right-side-up. So researchers think that flipping over may help the nuthatch find food that these other birds miss.

And—it may also help them hide food. They often tuck seeds and bugs into small cracks, stashing them for the winter. And these upside-down food stashes may be less visible to other birds.

Now, your average bird wouldn’t be able to just flip upside-down, but the nuthatch is pretty nimble. For a small bird, its feet are unusually large, so they’re strong. At the same time, its claws are narrow, so they can grab tiny footholds.

And they have an unusual way of working their feet that helps them stick to vertical surfaces. With their feet staggered, they use the upper leg to pull in close the tree while pushing outward with the lower leg for stability. …. Then, rather than walking, they use more of a hop to drop down to a new spot.

The research isn’t totally settled on exactly how nuthatches move, but this push-and-pull action seems to help them get around no matter how they’re facing. Moving back to the sea, you can find another kind of upside-down animal right along the shore. Horseshoe crabs, especially young ones, swim upside-down.

And it’s all because of physics! In a 2019 study, researchers were trying to learn how this crab’s weird body shape helps it get around in its environment. So they 3D-printed some horseshoe crabs and analyzed them basically the way you would analyze an airplane.

Except instead of looking at airflow over a wing, they looked at the flow of water over the shell. Overall you’re dealing with the same types of forces, like lift and drag. But whereas an airplane wing is shaped to generate lift, the shape of the horseshoe crab generates negative lift.

In other words, when it’s sitting feet-down, water flowing across the shell holds it firmly on the ground. This is great when horseshoe crabs come to shore to mate. They have to navigate waves, and if the flowing water generated lift on their shells, they’d easily get flipped onto their backs.

Which is a really hard position for them to get out of. But say it wants to get over an obstacle, or get away from its siblings. After all, these are solitary creatures.

Here, swimming is a better strategy than walking, and having a shell that pushes you down is not helpful. But, if the horseshoe crab simply flips onto its back, the fluid dynamics around its shell give it a lift, just like how air lifts an airplane. Thanks to that buoyancy, it takes less energy to swim.

It’s kind of ingenious, really. By flipping back and forth, the horseshoe crab can get either stability or lift all from one shape. One thing that’s not clear is how exactly they know when the time is right to flip over, but scientists think it’s innate.

They learn to swim when they’re still in the egg, as upside-down embryos. And clearly this body plan has worked out for them. Fossil horseshoe crabs with basically the same shape date back hundreds of millions of years.

As it turns out, though, horseshoe crabs aren’t the only creatures that take advantage of physics by hanging out upside-down. Some spiders also spend most of their time upside-down. And in a life spent dangling above the ground, being upside-down turns out to be an energy-efficient way to get around.

Compared to spiders that spend more time on the ground, upside-down spiders have proportionally longer legs. Their shape helps them take advantage of pendulum mechanics. As they scurry along silk lines, gravity helps them propel their weight, kind of like a kid swinging on the monkey bars.

This helps them move faster while using less energy. But that’s not the only advantage of this orientation. For orb weavers, which build those quintessential, spiral-shaped webs, hanging upside-down can also make them better hunters.

When prey hits the web, they need to grab it quickly, before it can escape. And hanging upside-down can help them do that. See, all orb weavers sit in the middle of their webs’ spiral, called the hub.

But most run down their webs faster than they run up. Because gravity. So upside-down orb weavers take advantage of this:.

They build asymmetrical webs, where the part below the hub is bigger than the part above it. This shape makes it more likely that prey will land below the spiders, where they can quickly drop down from their upside-down position and grab it. And by hanging upside-down, they’re perfectly poised to scramble down the web as quickly as possible.

Finally, our last item on the list isn’t just an upside-down animal it’s a whole upside-down ecosystem. Around 2010, scientists testing an underwater robot unexpectedly discovered this unknown ecosystem hanging beneath the Antarctic ice sheet! It was made up of upside-down sea anemones, worms, crustaceans, and even fish that they said swam upside-down.

The anemones sat in tiny burrows in the ice, with little tentacles dangling free below, and the other creatures lived among them. It might seem like an odd choice for a home, but life often happens at interfaces, thanks to all the resources and energy that get exchanged at these boundaries. In this case, the interface is where the liquid ocean water meets the frozen ice sheet.

And since the only solid surface is downward-facing, these creatures just go with it! Most sea anemones burrow into the sea floor, but reversing their orientation lets these anemones occupy a new habitat without changing their lifestyle a whole lot. Unfortunately, since it’s a new discovery, and really hard to access, this upside-down ecosystem hasn’t been studied much.

So there’s lots to learn about exactly how anemones survive in this unusual habitat. They may get food from the open water flowing beneath them. But recent research has also revealed that the undersides of ice sheets can harbor large blooms of microscopic algae.

These algae could serve as the foundation for this and other under-ice food webs. And what makes this discovery extra exciting is that under-ice ecosystems can act as proxies for what life might be like on distant icy ocean worlds like Europa and Enceladus. After all, life can make the most of all sorts of conditions.

We may think there’s a right-side-up way to live. But that’s just because we move around on the ground, supporting our weight on our legs. The way an animal lives mostly has to do with what’s important for its survival in its particular habitat.

And these upside-down animals are a good reminder that now and then it’s worth taking a minute to look at things from a different perspective. Speaking of perspectives…. If you’re interested in learning more about your perspective and the things that can change it, you might like one of the episodes from our spinoff channel, SciShow Psych.

It’s all about how paintings can help you see the world differently. [♩OUTRO].