We humans like to think that we're pretty tough, but lots of other animals put us to shame. And sometimes, the grittiest creatures on Earth aren't what you'd think.

Take desert kangaroo rats, for example. While you might laugh in the face of danger, they kick it. Literally. I'll let Olivia explain.

OLIVIA: You may not think much of the desert kangaroo rat. This small animal with its large back-legs might seem like a run-of-the-mill rodent at first. But, don't let its modest appearance fool you.

Kangaroo rats are mighty beasts that don't think twice about kicking deadly rattlesnakes in the face. The kangaroo rat lives a simple life. By day, it builds burrows in the sand. And by night, it forages for seeds to store in those burrows. And when it does, it has to avoid predators. Lots of predators.

The rat's flesh is sought by coyotes, foxes, hawks, owls, and snakes, including the sidewinder rattlesnake. You know, the one that does that crazy dance across the sand? Yeah, that guy. 

And they're especially hard to avoid. That's because they're ambush predators that lay in wait at the best seed spots. They have thermal vision, sharp fangs, a potent venom, and can deliver a strike in a tenth of a second.

Luckily, desert kangaroo rats have evolved special countermeasures. This kind of arms race, when two closely interacting species influence how the other evolves is called coevolution.

Take the snake's thermal vision, for example. To get around that, a kangaroo rat can drop its surface body temperature, especially around the feet and ears, and make itself less noticeable. It may also drum one or both of its feet, letting the snake know that its ambush is ruined.

And, if that's not enough to deter the snake, the desert kangaroo rat still has tricks up its sleeve. Or, well, pants? Its most valuable assets are its huge back-legs, which can propel the 10-cm rat up to 3 meters away. Kangaroo rats have disproportionately large muscles, tendons, and bones

which they need to generate and withstand the forces of their jumps. Because unlike their larger namesakes, their big tendons and muscles don't act like springs.

Jumps are made with pure musclepower. That means more stress to the leg bones and muscles, but the rat doesn't have to get to a particular position to leap. And that allows it to react quickly.

An alert kangaroo rat can react in as little as 8 ms, and leap its whole body away in about 50. Roughly one-half the time it takes for the snake to lash out. And it doesn't even need to see the snake coming.

Kangaroo rats have specially modified inner ears, which allow them to hear when a strike happens in complete darkness. They can also use those legs to kick sand at the snake's face, or just kick the snake outright.

A powerful blow can stun the would-be predator, allowing the rat to make its escape. And whether kicking or jumping, the rats have a HUGE advantage. When scientists have watched these battles play out in the wild, only 1 in 23 strikes results in a win for the snake.

Of course, the sidewinders haven't completely lost this evolutionary arms race. Scientists think they can detect the rat's change in termperature, for example. If they realize the element of surprise is gone, they can save their energy for a more unsuspecting meal.

But at least for now, the kangaroo rats seem to have a jump on things.

STEFAN: Well, standing up to a venomous animal is pretty hard core. But grasshopper mice take the idea one step further. Not only are they totally unafraid of scorpions, they use their venom as a painkiller. Here's Olivia again with the details.

OLIVIA: The deserts of the southwestern U.S. are home to an ordinary-looking mouse with a superpower. It's immune to scorpion venom. In fact, grasshopper mice aren't just immune to the nasty stings of the bark scorpions they eat; the rodents have turned the scorpion's best weapon into a potent drug that temporarily dulls their pain, allowing them to eat the toxic critters with impunity.

Bark scorpions are a group of dangerous arachnids. Some species are even toxic enough that they'll

occasionally kill people. But, when they sting mammals like us, it's usually in defense. So, they've evolved a venom that's notoriously agonizing: a feat accomplished by hijacking nerves. It contains toxins that mess with a protein called Nav1.7. That's a sodium ion channel – a tunnel through the membrane which allows sodium molecules to move across – which is used by our pain-sensing nerve cells.

It has a gate, of sorts, which, when open, lets a flood of sodium ions through to generate an electrical pulse. And that pulse is essentially the start of the "ouch" signal that's sent to our brains. Normally, Nav1.7 senses small changes in ion concentrations that happen in response to injury, but bark scorpion venom has components which force its gate open in the absence of these, triggering pain. Really, really bad pain.

Most mammals also have Nav1.7 channels in their pain-sensing neurons. So, scorpion venom hurts like heck in humans, most rodents, and lots of other furry critters. A pretty effective way to say "stay back." But grasshopper mice, which eat these toxic scorpions, have found a unique way to get around this.

You might think they've changed their Nav1.7 proteins to be resistant to the venom like scorpion-eating bats have. But...nope! Instead, they have an altered version of a different sodium channel protein – Nav1.8. It's found in the same nervs as Nav1.7 and one of its jobs is to help transmit pain signals to the brain.

Basically, it's one of the runners in a neuronal relay and can take the baton from Nav1.7. In other mammals, Nav1.8 doesn't react to scorpion venom. But in these mice, it does. The venom toxins bind to it and shut it down, dulling the mice's pain. So, the venom still turns on the pain pathway by opening Nav1.7; that signal is just immediately stopped from going anywhere.

If that sounds incredible, well, the researchers that discovered it were pretty surprised, too. But, they confirmed the pain-killing effects in the lab. Injecting the mice with scorpion venom allowed them to shrug off injections of pain-causing substances like formaldehyde,

which otherwise made the mice quite unhappy. And when they looked at these mice's genes, they found the difference comes from swithcing just two amino acids out of the hundreds that make up the Nav1.8 protein. That's all it took to transform the scorpion's agony-inducing venom into a painkiller instead. So, don't let mice's meek reputation fool you. Turns out some of them are way more hardcore than you think.

STEFAN: Well, I'm starting to think we don't give rodents enough credit. Now for our next secretly hardcore creature: Trinidadian guppies. They're cute and tiny, but don't let those looks fool you! They don't cower when danger looms; they dare bigger fish to attack. Here's Michael to explain why.

MICHAEL: Convincing a predator to attack might seem counterproductive if you don't wanna be eaten. And getting them to attack your head might seem like the absolute worst idea ever. But that's exactly what these little guppies do. They taunt larger fish and get them to attack at their eyes. And that's actually safer for them. Let me explain.

Trinidadian guppies are eaten by bigger fish called cichlids, which are ambush predators. When a cichlid is hungry, it hides and waits for a smaller fish to swim within reach. And then, BOOM! It's like a yummy fish dinner was delivered straight to its door. But these guppies have figured out how to make the cichlids attack on their terms, allowing them to lure the predators into missing.

First, they draw the cichlids' attention using conspicuous coloration: highly noticeable colors that get the hunter looking where the guppy wants it to attack. But here's the thing: if the guppies were always conspicuously colored, they'd know where a cichlid would attack, but not when. Which is why they can change their color.

And specifically, they can change the color of their eyes. Normally, their irises are silver, but they can turn them black. See, the color comes cells that have sacs of pigment inside. Melanin, the black pigment that guppies use to change their eye color, is found in a special type of cell known as a melanophore.

And when melanin is at the center of the melanophore, light can pass through most of the cell. But if it's spread out, it creates a layer that absorbs

light and makes the cell look black.

So when a guppy spots a predator, hormonal and nervous responses signal their melanophores to rearrange. Proteins pull melanin from the center of each cell out across the entire thing and the whole eye goes black in about three seconds.

Those black eyes are much more noticeable, so when a guppy changes its eye color, it's basically daring the cichlid to aim for its head. Which, I know, seems like the last place you'd wanna be bitten, but if the attack is aimed there, instead of its middle, where the cichlid would usually attack, the guppy can bend around its center to dodge the predator. So basically, they use their eyes kind of like a matador cape: luring danger into lunging at the wrong angle and allowing the guppy to twist away at the last second and escape.

And the international team of researchers who wrote about this amazing tactic in a 2020 Current Biology paper found that larger guppies are even better at this, as their center pivot point is even further from their eye. So they think this adaptation may have allowed this species to evolve a bigger size overall. So next time you're faced with a stressful situation, consider tackling it like a guppy: head-on.

STEFAN: Now those fishies aren't the only fearless little animals out there. Fuzzy velvet ants look like they'd be fun to cuddle. I mean, as much as any ant could. But most creatures are smart enough not to mess with them.

And for several good reasons. I'll pass it back to Michael for the lowdown.

MICHAEL: No one wants to be eaten. That's why the animal world is full of animal adaptations to stop predators like hard shells, bright markings, and painful stings. Still, most creatures regularly end up on someone's menu, except for velvet ants. These little insects combine a ton of defenses into one very unappetizing package.

And they're so terrifying that pretty much nothing eats them. Velvet ants aren't technically ants, they're wasps. The males can fly by the females don't have wings.

What they do have is a massive stinger. In fact, relative to their size, they've got the longest stinger of any wasp. 

And some seriously painful venom, too. Still, they don't rely on it alone to deter predators. They also have a super-tough exoskeleton. It takes about five times the force to crush a velvet ant as it does to squish a bald faced hornet, for example. Plus, they're round and slippery, making them hard to bite down on, something researchers realized first-hand while trying to crush these things. The critters kept slipping out of their grasp. Of course, the velvet ant doesn't want it to come to that, so they employ a ray of warning signals. Their velvet sections are brightly-colored as a bold, visual aposematic signal. They also literally sound the alarm by rubbing a section of their abdomen to make a loud squeaking noise. And they can release smelly alarm pheromones! These by themselves can deter some predators, and the combinations of defenses and warnings is super effective! It seems to train other animals not to mess with them too. Like, in one experiment, lizards that had a run-in with a velvet ant still avoided them over a year later. In another, birds were wary of eating one of their favorite tasty treats simply because researchers had painted them to look like velvet ants. Predators don't even need to have encountered a specific species of them Many velvet ants mimic each other's coloring in what scientists call Müllerian mimicry. So if a predator encounters one, they're unlikely to mess with any of the others. In fact, scientists have set up showdowns between velvet ants and many potential predators, just to see what bites. They tried spiders, lizards, actual ants, birds, moles, gerbils, shrews-- almost all of them avoided the velvet ants entirely. And the few that did catch them mostly spat them up. As far as anyone can tell, toads are the only animals that occasionally eat velvet ants. But they don't have an easy time of it. They swallow their food whole, and velvet ants can survive for over twenty minutes inside a toad's stomach. So even toads end up spitting them out more than not. Velvet ants' extreme investment in defense likely has something to do with their lifestyle. Females can't fly away from predators, plus they spend lots of time on the ground looking for places to lay their eggs. See, these ants are parasites. To reproduce, they find nests of other bees and wasps and lay their eggs inside.

When the large hatch, they feed on the other species helpless young. Oh, also those nests are often well defended by other stinging insects. So I guess if you're regularly that rude to your neighbors, you'd better be as tough as a velvet ant. 

STEFAN: So to sum up velvet ants, small but mighty. but next, let's shine a light on an even smaller hardcore creature, rotifers. And it can be a pretty bright light because these microscopic critters can withstand way more radiation than you or I can. Which apparently has something to do with their aquatic lifestyle. Hank, lay it all out for us. 

HANK: Bdelloid rotifers are microscopic animals that look like cigars with a brush on the end. They are not very impressive looking, but they are some of the toughest creatures in the animal world. Their superpower?

If their DNA is shredded to pieces, whether because of lack of water or a blast of radiation, they can put it back together. They might even use the situation to their advantage and acquire new genes in the process. It's all in the name of survival in aquatic habitats that can dry out and leave the rotifers without a place to live.

You might be thinking, wait this sounds familiar because tardigrades famously survive under similar conditions. We talked about them way back in sci show's fifth video ever. But stay with me because rotifers are arguably even more hardcore, and they do it in their own way.

There are over 400 species of bdelloid, and many of them can tolerate desiccation or drying out. They live in droplets of mosses or in temporary ponds which can dry out and force the rotifers to enter a sort of suspended animation known as anhydrobiosis. Their bodies contract into a dried-out form that's less than ten percent water which is too dry for biochemical reactions to happen.

It's hitting a complete pause button on being alive. There are serious challenges to surviving this dried-out state.

One of them is the fact that it's extremely bad for DNA.

At this level of desiccation, DNA molecules just snap. Both strands of the backbone holding them together give way in what's called a double-strand break.

And DNA doesn't work if there is a break in the middle of a sequence the cell needs to use, even worse, most organisms aren't very efficient at repairing double-strand breaks. The usual repair process might stick the wrong ends together or introduce new base pairs, either of which would cause potentially dangerous mutations. Bdelloids are known for their resistance to radiation, which also causes double-strand breaks.

A 2008 study by researchers in Massachusetts found that bdelloids could withstand a dose of radiation that caused about 500 breaks per copy of their genome and only suffered and drop in their ability to reproduce of about 20%. But their resistance to radiation is probably a coincidence, an unintended but super awesome side effect of the fact that both radiation and drying out cause the same DNA snapping problem. The weirdest thing of all is that bdelloids don't seem to fix their DNA until they rehydrate. It's not clear how they do this, but it seems like maybe the proteins that fix their DNA pull through the desiccation process even when the DNA is damaged.

That was what researchers found in a 2012 study in the journal PNAS, bdelloids experienced DNA damage from radiation at about the same level as other similar animals like nematodes, but the damage to their proteins was kept relatively in check, so maybe when the rotifers are exposed to water again, the proteins fix the DNA, and the rotifers swim away. But stitching up their shredded DNA isn't the only thing rotifers can do. They can also borrow genes from others organisms, including bacteria, plants, and fungi.

Acquiring genes in the absence of sexual reproduction, which by the way, bdelloids don't seem to go for, is called horizontal gene transfer. Bacteria do it all the time, but it's incredibly uncommon in animals. 

And yet, as many as 8-9% of bdelloid genes may come from other organisms.

At least one study has found that desiccating species of bdelloids acquire more foreign DNA than their cousins who don't dry out, which leads to the hypothesis that they actually do it while stitching their DNA back together.

Got some free DNA ends here? Got a spare gene? Stick it in there, maybe it will even work. We don't know for sure this is even what's going on, especially because even bdelloids that don't dry out still have some foreign genes. But this chaotic borrowing leads to perhaps the example of how rotifers are so good at not dying.

A 2015 study in the journal Plos One showed how horizontal gene transfer could help bdelloids survive desiccation, as well as shed new light on an old mystery.  Many desiccation-resistant organisms including tardigrades fill their bodies with something to take the place of water when they dry out. For a lot of these organisms that something is a sugar called trehalose. Trehalose hasn't been found in bdelloids, but when researchers took a closer look at one species's genome, they found that it had some genes for both making and breaking down trehalose.

One seemed to have been acquired from plants, and one from bacteria. Both appeared to be expressed in the bdelloid's genome, so it was using the genes to make something, although we don't know whether the products were biochemically active. The breakdown gene was expressed more than the building one, leading researchers to speculate that trehalose could be produced in bdelloids, then broken down too quickly to be detected.

Whether this is an actual mechanism for bdelloids to survive desiccation is far from clear, still, it's pretty incredible for an organism to potentially piece together an entire biochemical pathway from bits it finds lying around. Which makes rotifers some of the most incredible survivors of the animal world, like some bizarre zombie Macgyver they're able to piece themselves together all the way back from the practically dead.

We've still got a lot to learn about how they do it, but they sure are talented at protecting themselves. 

STEFAN: And last but certainly not least, we have one more microscopic animal to talk about. I mean, you didn't really think we'd get through a whole episode about hearty critters without mentioning tardigrades, did you? Of course not. Hank, take it away. 

HANK: Do me a favor, and picture in your mind the toughest animal on earth, whatever you think it is. And now imagine what that animal would do in the most inhospitable environment you could imagine. So, for example, if you thought the grizzly bear on top of Mount Everest, being attacked by a swarm of silverback gorillas, you'd be wrong. That is neither the toughest animal nor is it the most inhospitable environment, but I thank you for the visual image, that was a good one.

So you want to know what the toughest animal on earth is, well voila, there you have it. My friends, it is the tardigrade, also called a water bear, or a moss piglet because they're plum, waddly, and they like to suck on moss and you may have noticed they're actually kind of cute. They're what scientists call extremophiles, which means they don't give a crap about where they live.

The tardigrade secret is that when the environment gets too tough, they just shrivel up and die for a while with the option of reviving when conditions improve. And that is the weird thing about tardigrades, they are so extravagantly tough, like for no real reason, they're just supposed to waddle around on moss and suck up water, that's their job, and yet, in their dormant state, they can withstand temperatures close to absolute zero and up to 300 degrees Fahrenheit. They can survive being exposed to 1,000 times the radiation that would kill an elephant.

They can withstand pressures up to 6 times what you could find in the deepest oceans on earth. What-what is the point of that? there's no place that that would be useful

on earth.

And you had better believe that we've been sending these little waddlers into outer space because what is the most inhospitable environment. Yes, it is space. In fact, scientists think tardigrades may be the key to understanding how life began on earth.

Back in 2007, NASA put a bunch of tardigrades on the space shuttle then they opened up an airlock door and left them outside in the vacuum of space for 10 days, being exposed to crazy amounts of UV radiation. Then they brought them back to earth, and when they got there, the tardigrades were like, what's up?  They were happy and healthy and some of them laid tardigrade eggs and had little tardigrade babies that were completely normal. And we keep doing it.

Earlier this year, on the very last mission of the space shuttle Endeavor, we sent some tardigrades up, and the European space agency sent some tardigrades into space as part of a mission called "Tardigrades in Space". Which isn't clever till you realized that they shortened it to Tardis. So the question is, why do we keep shoving these adorable little beasts into the vacuum of space, it doesn't seem like a very nice thing to do.

Well one, because we want to understand how tardigrades work. Just scientifically how they can possibly survive these intense horrible inhospitable environments. And two, because we're interested one proving the panspermia hypothesis.

That is right, panspermia, a word that I  am not going to make a joke about. So imagine for a moment that meteorite slamming into our planet and this meteorite is so large that it actually ejects pieces of the earth into outer space. Now imagine on those pieces of earth that got ejected into outer space, there are tardigrades.

If that little organism could survive the vacuum of space long enough to then fall down onto another planet, it could seed that planet with life. If life can be transmitted in that way, then it becomes much more likely that life is a very very common thing in our universe. Panspermia hypothesis has been around for a long time, but thanks to

tardigrades, starting to look a lot more credible. So we can already thank these little beasts for being a great proof of concept for us, but of course, they will never know that we are so in their dept, they'll just keep walking around on moss, sucking water off and occasionally visiting other planets. 

STEFAN: Ah those cute little moss piglets. What a wonderful one to end on. Thank you for watching this Sci Show compilation, if you enjoyed learning about these amazing abilities, I bet you'd love our monthly After Hours podcast. In it, Hank and our content editor Christy explore the wildest features of life on earth, stuff that we can't really talk about here because it's not so family-friendly. We make the podcasts as a thank you to our amazing Patreon community because we couldn't do any of this without them. So if you're one of those awesome people, thank you. And if you're not a patron, you can learn how to support the team here at sci show and even listen to a sample of After Hours by heading over to patreon.com/scishow.