Stuff To Blow Your MindWe are creatures of the epigean world: the world of light on Earth’s surface. But there is another world – a world beneath the surface. In this episode of Stuff to Blow Your Mind, Robert and Joe venture into the world of cave biology or biospeleology.
Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Hey, welcome to Stuff to Blow your Mind.
My name is Robert Lamb and I am Joe McCormick. And hey, here we are together again, Rob. After you you were out for a little bit.
Oh yeah, yeah, we've recorded a few things here or there, so I keep forgetting that, you know, when the actual new Core episodes are that I'm around for. So yeah, I am back. I was just in a little vacation with my family.
Wait a minute, now, you refreshed my brain because I forgot that we did a listener mail episode before this, So listeners, you are reacclimated to Rob.
Now, yeah, I mean listeners I think always have a skewed understanding of when we're out when we're present, because there's you know, even if one of us is out, you may hear us on a vault episode on the weekend. We might have recorded through on something recorded ahead of time on one piece of content, but not on other pieces of content, So you know, you'll just have to take our word for it. Sometimes we are away, sometimes we were present.
We're like a duo of mysterious time traveling cats that appear in your ears.
Yeah all right, Well, you know, I have to say, on this most recent journey with my family, I did not venture into a cave. I don't think I ventured into a cave. Let me refresh. No, no cave venturing.
No, I would think you'd remember.
Come on, I mean, sometimes, you know, these trips, you see a lot of things, and yeah, you know, and you know, cave environments are are always fascinating, but they're not necessarily always going to be like the top of a particular trip. It just varies. But you know, we we frequently discussed biological wonders on the show that leads us to consider extreme environments, and the cave environment is one such environment. I tend to find caves endlessly fascinating, even if it's a lower tier cave, you know, And I've been to some of those where it's like, okay, this is this one was opened by humans a while back. It doesn't have much in the way of a robust ecosystem and so forth, but it's still fascinating. There's still something about the subterranean world.
Have you ever done the cave exploring experience where you go down into a cave and all artificial light sources are turned off and you get to see like the blackest black night you've ever seen. It's a kind of darkness that you cannot imagine otherwise.
Yeah, absolutely done this in a couple of Tennessee caves, I think growing up. One that was like one of these big official caves. I'm blinking on which one it would have been, but went there was like a scout group, and then there was another like local cave that was just on somebody's land and got to go in there and see the main cavern and then have the lights go out. Sometimes they'll do the lights out with a little ghost story or something right, just sort of drive home the creepiness of it. But yeah, you find yourself in just the appsol darkness.
We did this with our tour guide years ago at Oregon Caves National Monument in southwestern Oregon, which is a very cool cave system. And it's hard to explain because again I had not seen anything like this anywhere before, where your eyes are totally wide and you see not even a pinprick of light, just nothing.
Yeah, it's haunting and and it is key to understanding cave environments, as we'll be discussing in this episode. Now, not only are caves home to rather unique organisms in certain caves, as we'll discuss more as we progress, but they've of course always fascinated humans, and our global myths and traditions are full of caverns that house monsters, passages to the underworld, magical treasures, and much more. Also, as we've discussed, these were some of the first enclosed spaces that humans entered and were occupied to some degree in a way, laying the ground for the humble and elaborate shelters that we would construct thereafter. You know, because what is what are some of the rooms in our houses but caves that we have built for ourselves.
Some houses are more cave like than others.
Yeah. Yeah, So we may not think of ourselves as cave people, but we are, in many respects still people of the cave at home, with enclosed spaces, sometimes with windows, sometimes without places in general like this, be it constructed, be it like a large, you know, enclosed concert hall, or an underground facility, or a naturally occurring cave or some sort of a tunnel. I mean, you can't help but dream about those spaces, to find dread and fascination in those spaces. And you know now that I mention it. On my family vacation, I don't think we went We did not go into a cave system, but we did go through tunnels at times, and we were stuck in traffic at a tunnel at one point. And so that alone gives one enough room for pause. It's just as good almost well, I wouldn't say it's not as good, but it does. It summons possibilities, maybe even the possibility of talking about the strange world of cave biology. And that's what we're going to be looking at in this series, right, that's right, talking about the cave environment, talking about some of the organisms and life forms that live there. And to get into this, I want to talk a little bit about just sort of the idea of there being a dual world, something that again is quite reflected in some of our more supernatural understandings of the surface world and the world beneath the earth, you know, the world of the living, the world of the dead, and so forth. I was reading a twenty eleven article in The American Scientist by Abamaro Romero, and he pointed out that cave biologist or biospileiologists divide the world into two. There's the epigean world, or the world upon the earth, the world of light that we predominantly think of as our home, and then there's this other world, this underworld, the Hypogean world beneath the soil.
I've never thought of the surface as epigean before, but I love that because I guess with the Greek there Yet, like you say, it means upon the earth, on the earth, so not just like exposed, but it's like like an epiphyte plant that grows on another plant. The epigean world of the surface world isn't like the thing that grows on the surface of this ball of iron.
Yeah, And then everything else, the underworld, the Hypogean world beneath the soil, and the world beneath the soil. No mistake here is vast. So just as the epigean world consists of any part of the biosphere that is even partially or periodically exposed to light, the Hypogean world consists of any underground part of the biosphere. So this includes the living soil in your backyard, you know, networks of mycilium that rooting through it, as well as life between grains of sand buried under the beat underground waters, and indeed cave environments, which to be clear, can also be part of the Epigean world because they of course are gateways and they are going to be transition spaces the mouth of a cave, for example.
Mmmmm yeah.
Now, caves, as Ramera points out in this article, are also This is also a very broad categorization, entailing everything from ice caves to lava tubes. And indeed, caves are merely the subterranean spaces into which humans can venture. There are plenty of subterranean environments and even cavernous spaces in the Earth that are just inaccessible to us because there are no openings to the surface.
That's a really good point. And there's so much that goes on in those those small or tiny spaces that you can't like walk into and observe. You know, you mentioned like the idea of the myceelium that underlies that. You know, you find a mushroom in the forest. It's hard to remember this sometimes, but like the mushroom you find is just like one organ of the fungus organism as a whole. It's the fruiting body that comes up above the surface but underneath there's going to be this whole network that's sort of where the organism really is is under the soil, and you don't even see it. You don't even think to look down there.
Yeah, there's a vital and rich world in there, and it's not a space that human beings can venture into in the way that we can venture into a cave environment, you know. I mean we can study it, we can explore it, we can dig it up, but it's a different sort of world. Now. Romero also mentions the freatic environments of underground lakes and rivers, but the word cave is generally reserved for again, those spaces we can access ourselves, where human explorers and scientists or at least their technological minions, can physically investigate the world below. At least you can get you can get a robot in there, you can stick a probe in somehow invoke your presence there.
A cave is like a space that can be spilunked, and maybe our standards for how small a space can be spilunked are changing.
Right When we had Lee Burger on the show, the author of Cave of Bones, he was talking about exploring various cave environments, a specific cave environment in fact, looking at pre human skeletal remains. And one of the really telling things about his experience is there was this part of the cave that for the longest he could not access. It was like a grueling experience, and you had to send like very young specialized scientists down there to physically occupy the space. And he was able to send them down have them look around, he was able to look at video footage from that space, but eventually he could not resist. You know, he talks about this at length, like he had to go down there. He had to, like he lost weight in order to fit through, narrowly fit through the passageways that were acquired, a grueling experience to actually make it there. So there is something about like the human exploratory experience in all of this, Like even when you have the technology and the organization, there's something about it. You have to be there, you want to be there in the space. Man.
I know I'm not unique in this, but that is one of the things that truly gives me the creeps to like barely trying to wedge your way through passageways inside a cave under the earth that is just ugh.
Yeah, it gave me the all overs here in some of the details in his book, and I think there was also covered in a Netflix special if Memory Is Serving, And yeah, just the idea of like having to squeeze through a space, getting momentarily stuck like the wrench and a chimney, and then having to press on potentially like you know risk you know, dislocating bones and so forth to do it. It's yeah, not for me, but I respect anyone who is capable of it totally. Now, the most common caves are karstick or limestone. I believe we've discussed this before, talking about various geological formations. Limestone, which covers roughly fifteen percent of the world above water, is highly soluble at least fifty percent calcium carbonate according to Romero, so acidic rain water easily modifies it over time, and so the world is littered with such cave systems, some only partially exposed to the surface world but in important ways. Others with key openings that permit light to influence the environments of their interior. So you know, some cave systems, some rather famous ones even do have sunlight spilling into them at least in part, so you end up with this kind of like mixed environment. But the lack of natural light as a whole is extremely important to understanding the individual organisms that make their homes and caves, as well as the overall ecosystems in which they thrive.
Right because when you think about it, basically everywhere else on Earth. I mean, there are some extreme exceptions, but for most of the world, the food chain is built on access to sunlight. To the sunlight powers the powers the photosynthesis of the autotrophs that are the base of the food chain. Then things start eating them, then other things eat those organisms, and on and on up. If you don't have sunlight to power the autotrophs at the base, what do you do?
I know, yeah, yeah, we'll get back to this in just a minute here, But yeah, humans, of course, we're not the first to venture into the subterranean worlds. Countless other creatures ventured in, resulting in rich a rich, specialized world of troglofauna, and not every creature becomes a permanent denizen of the dark, which, if you watch enough sci fi and horror movies, you would think, oh, yeah, they're just people and creatures fallen in all the time and evolving into chuds. But no, that doesn't happen all the time. But plenty of creatures have specialized evolution narily often evolving defining troglomorphic features that we often associate with these environments, namely blindness and loss of pigmentation with the chutification.
If you will.
Chutification, I guess yeah. You know, you see movies like The Descent and so forth. I love all these movies, you know, more Locks and so forth. But maybe we'll continue to discuss the possibilities there. But briefly, I want to just do a refresher on the various types of troglofauna before we continue. We have troglobites. These are obligate cave dwellers, strictly bound to the under their underground environments, so they are not going to live, they're not going to thrive outside of the darkness of the caves. Then we have troglasines. These are cave guests, creatures that can be found underground sporadically but cannot establish permanent underground populations. Then we have troglophiles. These are creatures that live dominantly above ground, but can also reside in underground habitats. There are two subsets. There's u trogleophiles, surface species that are also able to maintain in a permanent subterranean population, and then there are subtroglophiles creatures that can and do live underground at least temporarily, but are more associated with life above ground. So again you can think about the idea that there's not just surface creatures and underground creatures. You know, there's a lot of back and forth, and there are various phases between what we might think of as a purely surface creature and a purely subterranean creature.
I see none of.
This to be confused with a troglodyte. There are a handful of different uses is for this term. It can refer either to a human cave dweller or and I believe, in one case of an outdated classification of early hominid. But there are also a small genus of small birds related to rims that are called rolodides. These birds are rarely found in caves apparently, but they're so named because they'll venture into various crevices and spaces, including caves, in search of food.
So we have these various kinds of spilunking organisms, those that do still live life on the surface, but have the ability and sometimes the desire to venture into caves or subterranean regions for various reasons. Maybe they want to shelter there for a certain period, maybe they want to go in there to retrieve some kind of resource, who knows. But then there's the other kind, the ones that are fully adapted to life in the caves and it has shaped their evolution, has shaped their morphology. Now their bodies are made for the caves. One of the examples that people listening are probably thinking of are sightless fish, fish that live in cave based in underground waterways and have evolved to completely lose their vision or their eyes. And this has long been a really captivating image within evolution that the idea, you know, people usually think of evolution as a constructive process that sort of powers up an organism. It gives it a new ability or a new adaptation that better suits it to an environment or an environmental niche that it's currently occupying, and so it gets upgraded in some way. These are these are ways that we sometimes discourage people from thinking about evolution. By the way, but it is still in the popular consciousness that that's how it works. This is evolution by what might be thought of as regression, like losing a trait that you previously had. So that raises a lot of questions about how evolution works and why something like that would happen.
You know, it's it's fascinating to think about this about our our are often human focused bias and considering evolution along these lines. Like we think of that classic illustration of the ascent of man, you know, the various Homonid species evolving towards man, and it's easy to look at him and like think, wow, look, with each step he just gets hotter. With each step, he just looks more like us, he becomes more perfect, like clearly modern Homo sapiens are. But even in that, we're seeing the loss of various features. We're seeing like the loss of body hair and so forth.
Yeah, totally, And as you know, big asterisk, if you see an old illustration like that, there's probably a lot of that's inaccurate about it. But some things that broadly would be true is that our ancestors do appear to have lost adaptations that made them really good tree climbers and those adaptations. Those are adaptations that were very useful for our ancestors, and we sort of lost them in favor of other things.
Yeah. And it is also interesting again to think about the world of the dark, the world of the caves, because, like we said, we can have this experience of turning out the lights in the cavern and experiencing the true darkness. But for the most part, ever since human beings were able to capture fire, we have been able to bring light into these spaces. And today when we explore caves, at least on our own terms, we have that light that we bring with us, and we therefore we don't have to we change the environment by bringing light into it the cavern. Changing us over evolutionary time, of course, is not on the table, that's all right.
But without the ability to modify the features of the environment like that other animals have had to adapt evolutionarily.
Yeah. And in all this we have to again recognize just how vital light is for organisms for the evolution of life on our planet. Both sunlight and moonlight are in play here, as pointed out by Lauren Summer Rooney in the Kingdom of the Blind, published in a twenty eighteen edition of Integrated and Comparative Biology. Light is a fundamental biological cue for almost every animal on Earth, So it's not just about oh can I see the inside of the cave or not? You know, it plays in a navigation foraging, predator avoidance, mate selection, and just like general daily rhythms. So to abandon the world of light is no small thing, right, and yet we do see plenty of organisms that have done just that over time, not only cave organisms but certain creatures of the deep ocean. Though we have to remember that site is still important at varying levels of the water column and in the presence of phosphorescent glows, so there's a fair amount of complexity there. The soil and even interiors of host organisms for various parasites are also dark environments that species have adapted to.
Mm hmm.
Now, as Romero points out, Charles Darwin himself was somewhat stumped by the loss of sight and cavefishes, and apparently ultimately landed on more of a Lamarchian idea that they lost their eyes by simply not using them.
Oh that's kind of interesting. So Lamarckism or Lamarchianism named after a figure named I believe you, Jean Baptist Lamark. It was a sort of alternative theory of inheritance to what would eventually become Mendalian genetics, which in some ways has been partially superseded now. But also but instead of the kind of Mendalian genetic inheritance, Lamark would say that, like the way that an animal uses its body shapes the way its offspring the way their bodies are formed. So like if you are reaching up a lot or something, you might grow your children might have longer arms, or like that a giraffe maybe is reaching its neck up to reach higher leaves, and that means it's children will be born with longer necks and so forth. For the most part, evolution doesn't actually work that way. There are some kind of things of gene expression and regulatory functions epigenetics that some people have argued in minor ways can kind of vindicate Lamarckian thinking, but for the most part, that's not really how inheritance works.
Right right, Yeah, I've been to evolutionary talks, I believe the World Science Festival for some biologists have been like, yeah, yeah, we're kind of we to start talking about epigenetics, sometimes we do get a little Lamarckian in the way we're talking, so it is kind of fascinating.
But the Lamarckian idea would be okay, so fish go down in a cave and then they don't use their eyes because they're living in the dark. Therefore their children or their offspring are born without them or born with eyes that regress. We can talk about that difference in a minute, and in a way you could say that that's loosely kind of right, but it's somewhat different. I think think I would say that the important thing is not that the fish don't use their eyes, it's that they don't need them, so that the eye no longer confers a survival or reproductive advantage.
I think the one of the infectious things about it, at least for non scientists, is that it can feel right in certain ways. Right. It can sort of match up with at least the individual human experience. So we might not think, like, man, I better go to the gym and need to pump iron so that my offspring will will have strong arms. But we might think, oh, well, I've got to keep pumping iron otherwise my arms will get smaller. You know, I've got to do X so that why doesn't happen? And you know, I think we do see this infectious idea in some of our fictions of subterranean monsters, including the Descent of the Hobbit Smeekeel and the Lord of the Rings, who becomes a trogloditic Gollum in the course of a single though of course magically sustained lifetime. He doesn't really become blind or anything, but generally he's depicted as you know, he's changed a lot. He's lost his hair, his pigmentation has changed, his eyes have gotten larger, I believe.
Yeah, though I think the morphotypical changes of Gollum could be largely due to the corruption of the one Ring.
That's the thing. The one ring changes everything. You've got a huge magical factor to consider in that scenario. But sort of just the idea of it is like, yeah, people go into caves, they get weird, they turn into monsters, or if they're not a monster, you know, weight three generations, then just pure monsters.
Caves will do that.
Now, Summer Rooney points out that we're still working out all the details, but we know much more than Darwin did in his time. Of course, the science has come quite a way since then in our understanding of how creatures evolve and what sort of changes are taking place over revolutionary time. The author rights quote eye loss appears to be driven at least in part by direct selective pressure, but both pliotrophy and genetic are also key influencers. Despite substantial recent progress thanks to a combination of developmental and molecular techniques, it is not yet clear how these drivers interact, or crucially, whether their relationships are similar across taxa and habitat types.
Yeah, so this was really interesting, and I ended up going kind of deep on what we do know about eye loss and a related issue of pigmentation loss in cave fish. But I wanted to break down some of the terms that this author uses in that paragraph you just read. So selective pressure, of course would mean an advantage in terms of survival or reproduction, a direct advantage. That's the kind of evolution we think about most often. But there are two other factors that could be influencing the loss of eyes or the loss of pigmentation in fish that the author mentions, and those are plyotrophy and genetic drift. Plyotropy, to sort of oversimplify, means when one gene determines multiple outcomes in the phenotype, and the phenotype is like the body or the behavior of the organism. It's what the genes produce in the animal. So, for example, imagine there is a mutation at one locust, and that one mutation happens to both give you smaller eyes but also longer arms. Or maybe there's a mutation that happened to give you both higher rates of cancer and higher average fertility. And pliotropies like these can They're very interesting because they can sometimes explain the evolution of traits that would seem to be disadvantageous because the seemingly bad trait is actually part of a hidden package deal, a genetic package deal with an even more advantageous trait balancing it out. You can get either both or neither, but not one by itself. So in the case of cavefish, it's possible there is a single mutation that both causes the loss of eyes but also does something else that helps the fish survive and reproduce. And since there's little to no penalty attending the loss of vision in the pitch black cave environment, the helpful half of that pliotropy wins out and the two for one version of the gene is selected for. And then the final factor mentioned here is genetic drift. Genetic drift is change in the frequency of gene variants within a population due to random chance. So genes becoming more or less prevalent within a group of organisms, not for selective reasons, not because they help or hurt the organism, but just randomly, you know. And this happens all the time. Gene variants just become more common less common within a population, and it happens due to random factors. Though sometimes if a population is small enough. An interesting thing is that traits result from random genetic drift can become what's called fixed, meaning that you kind of hit a genetic tipping point and a trait that was randomly fluctuating up and down in the population it becomes prevalent enough that suddenly it's fixed, and then all the individuals within the population have the gene for that trait. Going forward.
Yeah, now, now, going back to what you mentioned about the idea of there possibly being like an advantage and a disadvantage tied up in one change, but The thing is, you find yourself, if you're this fish undergoing this change or some other organism, you find yourself in a lightless environment. And so in this you dip over into the neutral mutation hypothesis, This idea that while naturally occurring mutations for in this case blindness would otherwise spell doom for a fish in a typical environment, in a lighted environment, it doesn't matter in the darkness, and therefore these mutations can flirt along with whatever other changes they might be bringing. That's right.
Then they can flourish for multiple reasons. They could flourish because they're part of a plotropy that has an advantage on the other half, or they can just flourish randomly through genetic drift because there's no penalty to the loss of sight in the in the darkness.
Yeah, this I can't help be reminded of various like gaming rules and gaming systems and all of this. Like it's it's like, imagine that you're playing Dungeons and Dragons and let's say you're venturing into the under dark. You're venturing into a lightless environment, and you have a weapon that gives you, say a plus two on attack. But if you're in the sunlight, it has your hit points or something like something that would make this all right. It has advantages to it, but the disadvantage is far outweigh it in a different environment, but you're not in that environment. Now you're in an environment where the disadvantage does not matter.
Right. Yes, that's analogy, and it's also a good reason to remember to check your build when your environment changes. But yeah, so there is, as we're saying, some possibility for all of these options in explaining the evolution of cavefish blindness. There could be direct selective pressure, there could be indirect selective pressure through pliotropy or the package deal, and there could be random genetic drift. However, it seems that there is some pretty good evidence for the presence of a direct selective pressure for fish to lose their eyes when they live in a lightless environment, and there might be some evidence for plotropy as well. So to get a more concrete idea of what we're talking about, I wanted to focus on a particular species of fish that has been studied a lot, and that species is called Astianax mexicanus, also known as the blind Mexican cave fish, also known as the blind cave tetra. These are small like fish with silver scales. I saw different size estimates for them. Some said they grow to a maximum length of about nine centimeters. Other sources said about twelve centimeters, but they're relatively small in any case. One thing that makes these Astianax fish very interesting is that there exist both surface or epigean and cave or hypogean variants of the same fish species, and they have different body features. And scientists can learn a lot by comparing these extremely closely related populations of animals to one another, cross checking genomic information back and forth, and seeing what that correlates to in terms of their phenotypic traits. So the cave variant of Astianax lives in this big network of underground waterways in the mountainous areas of northeastern Mexico, and they've been in these caves since. One source I was reading said they colonize them probably around the end of the Pleistocene epic, so you know, roughly twelve thousand years ago or so. There are photos you can find online which I'd recommend looking up, which not only show the cave variant of Astianax, but They depict the cave variant and the surface variant side by side to highlight the differences in their bodies. So the cave version is much more pale and translucent. And indeed, in the places where the surface variant has a big, old, bulging fish eye on either side of its skull, the cave variant has only a strange, fatty looking lump underneath its scales, just in front of the red flap of the gill.
Yeah, these are haunting images to look at. Again, we can't help but think of like various sneakels and so forth when looking at it.
Yeah, exactly, And I bet these fish are good at riddle games. Now. One thing that I was reading this kind of interesting about these the eyeless variant of the Asti annex fish, is that they undergo what's called eye regression. So I don't know if this is true of all populations, because they're like different cave populations of this fish that have been studied, and they have some differences between them. But I was reading at least in some populations what happens is not that these fish never have eyes, but they begin to grow eyes during embryonic development and then as they develop. They they acquire eyes and then lose the eyes in development, and the eye sockets are paved over by skin and scales, and they there remain these little fatty lumps where the sockets used to be, and the bone kind of collapses around it. So I thought this was kind of interesting. It's not just that they never have eyes. They get eyes and then they lose them, all in the process of a single animal growing up.
Fascinating, you know. I also have to throw in, okay, ASTI annex part of the scientice name for the creature. This is the son of Hector in the Iliad, and I'm guessing here. I couldn't find anything just definitive offhand, but I'm assuming the naming here is because Astianax, the child of Hector, is hidden in the tombs. That is the way that they try to ensure his survival. And so you know, we see some version of that here, a survival in the underworld of the caves as opposed to the tomb.
I did not make that connection. That is interesting. So I mentioned that there was some evidence that actual selection pressure, a direct selection pressure, drove the evolution of eyelessness in these fish, as well as some possible evidence for pliotropy. Let's look at a paper to learn more.
So.
The paper I was reading is called Cavefish and the Basis for Eye Loss by Jaya Krishnan and Nicholas Rohner, published in Philosophical Transactions of the Royal Society b Biological Sciences twenty seventeen, and in the section of their paper discussing the reasons for eye loss, the authors note that it is not unique to the example of these blind Mexican cavefish for an animal to evolve by losing a trait or characteristic that was once positively selected for in that animal's ancestors. Examples include the loss of tails in some primates, including us, the loss of teeth in birds, and the loss of legs in Wales. All of these are examples where an animal lineage acquired an adaptation that was useful long ago, and then some branch of that lineage entered a new ecological niche in which that trait was no longer useful or was actively harmful, and evolved once again to lose the trait that its ancestor bodies had constructed so long ago. And the example of cavefish that lost their eyes was long simply assumed to be a result of genetic drift. Eyelessness appears in the gene pool, and then with no selection pressure favoring eyes to keep the eyelessness suppressed, it would just randomly fluctuate and frequency in the gene pool until at some point it became fixed. However, the authors say that in roughly the last decade before this paper, and this was again twenty seventeen, there had been more attention to the possibility of a positive selection pressure favoring the loss of eyes. There's a reason to lose the eyes, either directly or through pliotropy, So let's talk direct selection first. The main explanation offered for direct selection against eyes is energy conservation, also sometimes expressed as metabolic cost. It costs energy the energy that animals get from food food to have functioning organs, and some types of organs are more energy hungry than others. In fact, the authors demonstrate that neural tissue, which includes brain cells and also eyes and nerves, that type of tissue is one of the most expensive types that an animal can grow and maintain. So any animal's evolution is guided by a balancing of interests. The benefits provided by sensory awareness and the high metabolic cost of having this neural tissue of having these systems. This is true in any animal in any environment, but the balance is especially crucial in harsh environments like caves, where food energy is more scarce than on the surface. It's harder to come by food energy in a cave than it is in the surface waters where the surface variant of the Mexican cavefish lives. And in favor of this explanation the energy cost explanation the author's side of paper from twenty fifteen by Moran and Softly and Warrant published in Science Advances called the Energetic Cost of Vision and Evolution of Eyeless Mexican Cavefish, summarizing the findings of Meranatol, Krishnan and Rohner write quote, this study calculated significant metabolic costs for the optic tectum, and the tectum is the part of the fish's brain that is used to process visual information going on with the quote and the eyes adding up to fifteen percent of the resting metabolism needs in juvenile fish. Such a high cost of vision almost reaching the cost of the human brain at twenty to twenty five percent makes an adaptive loss of eyes owing to energy constraints highly probable. So usually there's a high energy cost you need to eat a lot of food to grow and sustain the sensory organs and visual processing in the brain tissue. But on the surface, evolution is usually willing to pay that cost because it's really beneficial to get visual information about your surroundings that really helps you survive and reproduce. If getting visual information about your surroundings becomes impossible because you live in total darkness, it's time to make budget cuts. Another possible direct selection factor I haven't come across direct evidence for this, but it's just mentioned as a possibility by several sources, and that is that eyes are also a liability in that they are prone to injury and an entry way for infection. So it's kind of like why would you put a windshield on your armored vehicle if you're driving in pitch black with no headlights. It would just be a functionless weak point in the armor of the vehicle that provides no benefit. And it's possible that eyes are like this also. Once there's nothing to see, all they would be is a soft spot to let germs and sharp objects in.
Yeah, kind of like with sci fi visions of spaceships, right, like why have an actual observation lounge or observation bubble or so forth, Why have actual glass there or any kind of like transparent wall when you can if you have like ubiquitous abilities to have video footage and screens around you. Basically the star.
Trek principle exactly, if it's not actually providing you a benefit, then it's just a liability.
Yeah.
Also, the authors of this paper mention some evidence for indirect selection or pliotropy. Specifically, they bring up a signaling pathway identified in the scientific literature as SHH. And do you want to guess what SHH stands for?
Now?
Tell me what stands for sonic hedgehog. That's really what they say, and that's what it is, really, Yes, scientific papers, this is the sonic hedgehog signaling pathway. There's also just an HH or hedgehog pathway, so I don't know attacking on maybe that was discovered first, and then they tacked on sonic to be funny for this other pathway. I don't know for sure, but it is called the sonic hedgehog signaling pathway and the signaling pathway, by the way, is it's like a series of chemical reactions in the body that controls and coordinates the activity of cells in order to do things like grow new tissue during development, or cause an immune system response or something like that. The sonic hedgehog pathway in particular seems important during embryonic development. It sort of guides growth and plays a role in the patterning of tissues that emerge from the growth process. So Krishnan and Rohner point to a study by Yamamoto at all from twenty nine, not twenty nineteen, from two thousand and nine, sorry, in Developmental biology, which found that by altering expression of the sonic hedgehog signaling pathway in embryos of the surface dwelling relative of the blind Mexican cavefish, you would actually get multiple changes together. This is our package deal. You would get regression of the eyes like we talked about, so the eyes would sort of like be absorbed and you would no longer have functioning eyes as an adult fish. But you would also get enhancement of taste, buds and jaws. So that's your package deal. And in the dark you can imagine why that would be a good package deal. The author's write quote SAHH has wide range effects on the development of various organs, and it has been identified as a strong candidate for eye regression. Its elevated expression in the oral pharyngial region in cavefish taste buds, and the fact that eye size and number of taste buds are correlated in hybrids makes SAHH a promising candidate for functional studies. So that's another factor possibly at work here. So to review, you got eyes and visual tissue in the brain require a lot of food to sustain, and food is especially scarce in the cave. Also, the eyes are not really useful in the cave, so you know, the trade off there seems pretty obvious. There's less evidence for this, but it has been suggested that eyes are also a liability in terms of infection and injury, and it seems there are certain genetic and developmental changes that offer a package deal where you get eye regression, your eyes are absorbed, you have no eyes as an adult fish, but more taste buds and in the dark. That's a good deal to take because remember taste buds. We think of them mainly in terms of like providing pleasure, but taste buds give you important survival information. They are your your body's chemistry set, along with your ural factory, you know, smell and taste together help you sort of test incoming materials for safety and nutritional value. And that is actually a very valuable thing in the wild, especially if you can't look at stuff you're going to eat.
Yeah. Yeah, we have to be reminded of that because we are such site dependent organists that we easily think of like the world we can see and then the other senses that provides sort of backup information, right, or at least it's easy to think about that in terms of a lot of stimuli. Obviously it's different once you get into, say, you know, the actual experiences of consuming food. But even that, the visual factor is a huge part of it totally. When you pay for a nice meal, when you pay for a nice cocktail or zero proof cocktail, whatever floats your boat, you know, I mean presentation, visual presentation is a huge part of what you're signing up for.
It absolutely is. Even I would say this, even if you think you quote know better, do you know what I mean about that rot like that? Some people can think like, oh, you know, I'm just in it for the tasty food. I don't really care if it looks good. But actually you do care in subconscious ways that you're not admitting to yourself.
Yeah, on some level, you were still anticipating the flavor, anticipating the taste experience based on to a large degree, on visual data.
Totally. So one more thing I want to say about these the blind Mexican cavefish, because I was looking into this cavefish species and it seems that eye loss or I regression is strongly associated with a parallel morphotype, which is pigmentation loss. So a source I was using on pigmentation loss in cavefish is a chapter in a book. The chapter is called Evolutionary Genetics of Pigmentation Loss in Blind Mexican Cavefish by Joshua B. Gross and Clifford J. Tabin. It is from the book In Search of the Causes of Evolution, From Field Observations to Mechanisms, Princeton University Press, twenty eleven, edited by Peter and Rosemary Grant. So most of this chapter is focused on the specific genetic mutations and regulatory changes that result in the reduction in or total loss of pigment in blind Mexican cavefish, the reduction in the melanophores and color color molecules in the fish's body, and they find that in different populations they're actually you get these similar, similar adaptations of the loss of pigment in the body, but they have different underlying genetic or regulatory mechanisms, so it's evolved in different ways to get sort of the same result in these different cave fish populations. But also this chapter gets a little bit into what the environmental selection pressure is related to, or the lack of selection pressures related to pigmentation might be. So, for example, on the surface, biological pigments like melanin help protect the cells of animals from ultraviolet radiation in the sunlight. In the cave, there's no light, so this radiation shielding effect is useless on the surface. Pigments are also arranged in patterns on the body as ornamentation, which plays a role in sexual selection. So a fish with certain types of pigmentation patterns might be seen as a more desirable mate, but in the cave, your mate can't see you, so it doesn't matter. On the surface, pigmentation can also play a role in camouflage concealing your body within the environment, but in the cave. Once again, this doesn't make any difference. So much like with eyes, pigmentation would be a phenotypic trait that confers huge advantages on the surface where there's light, and it suddenly confers little to no advantage underground. So even if it's just subject to genetic drift, pigmentation, like eyes, could fluctuate out of the population. But are there any reasons to think it is actively selected against? It seems like there is less evidence for this than there is in the case of eyes. There's probably good reason to think that eyes are being selected against in the cave, less so for pigmentation. The authors here cite research by protests at all from two thousand and seven than indicating it's more likely a result of drift, though some amount of indirect selection through pleiotropy is possible.
Okay, all right, we have more complex understanding, perhaps than of what might be going on here with these changes. All Right, We're going to go ahead and close out this episode of Stuff to Blow your mind, but we'll be back with part two in this series on Thursday, and don't worry, we will get into the guano. We will get into that back guano because it is vitally important to this discussion. And also I think we're going to discuss guano in a way that may turn the concept on its head for you. I know when I was reading about it in the way that we're going to discuss it, it made me think about it in a new light.
I can't wait.
In the meantime, we'd love to hear from you if you have thoughts about particular caves that you've been to, cave environments that you're familiar with, If you have thoughts on scientists naming things after everything from the Iliad to Sonic the Hedge. We'd also love to hear from you on that point as well. But yeah, yeah, particular cave environments that you've really enjoyed. Like I say, I always enjoy checking out caves. I believe Colossal Cave in Tucson, Arizona is one that I visited many years back, and I really want to go back there in the future because this is one if memory serves was discovered late enough that they were able to preserve the cave environment to a large degree by use of essentially like an air lock system. So that they're not just opening up this environment in a way that destabilizes what has evolved beneath the surface. But anyway, more on this sort of thing in the next episode. In the meantime, we'll remind you once more that Stuff to Blow your Mind is primarily a science podcast Science and culture podcasts, with core episodes on Tuesdays and Thursdays. Mondays we do listener mail. Wednesdays we do a short form episode, and on Fridays we set aside most serious concerns to just talk about a weird film on Weird House Cinema. Check us out on social media. If you haven't, We're on What're We're on the Instagram. You can find us on the other major social platforms. I think we're on TikTok. I had to be reminded of this. We have some some wonderful folks that are helping us handle our social these days, and I think we're on TikTok. We look for us there. I guess I think we're there. But yeah, if you use social media and you have the power to follow us in any of these places, yeah, give us a follow. We appreciate it. If you can give the show some stars where if you listen to the podcast downloaded and subscribe, that also.
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