March 8, 2025 • 50 mins
We 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 classic episode of Stuff to Blow Your Mind, Robert and Joe venture into the world of cave biology or biospeleology. (part 1 of 4, originally published 3/5/2024)
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Speaker 1 (00:06):
Hey, welcome to Stuff to Blow your Mind. It is Saturday,
so once more we have a vault episode for you.
This is going to be Life in the Hapogean World,
Part one. This one was originally published three five, twenty
twenty four. This is going to be part one out
of four, so we'll be publishing the other three episodes
in this series on the Saturdays to come.
Speaker 2 (00:29):
Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 1 (00:39):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert.
Speaker 3 (00:42):
Lamb and I am Joe McCormick. And hey here we
are together again, Rob. After you were out for a
little bit.
Speaker 1 (00:48):
Oh yeah, yeah, we've recorded a few thanks here 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.
Speaker 3 (01:01):
Wait a minute, now, you refreshed my brain because I
forgot that we did a listener mail episode before this.
So listeners, are you are reacclimated to Rob? Now?
Speaker 1 (01:10):
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
(01:32):
take our word for it. Sometimes we are away, sometimes
we were present.
Speaker 3 (01:36):
We're like a duo of mysterious time traveling cats that
appear in your ears.
Speaker 1 (01:40):
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 school.
Speaker 3 (01:54):
I would think you'd remember. Come on, I.
Speaker 1 (01:56):
Mean, sometimes you know, these trips, you see a lot
of things, and 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 frequently discussed biological wonders
on the show that leads us to consider extreme environments,
(02:17):
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,
(02:38):
but it's still fascinating. There's still something about the subterranean world.
Speaker 3 (02:42):
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.
Speaker 1 (02:58):
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 blanking 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
(03:20):
little ghost story or something right, just sort of drive
home the creepiness of it. But yeah, you find yourself
in just the absolute darkness.
Speaker 3 (03:28):
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 you just eyes are totally wide and you see
not even a pinprick of light, just nothing.
Speaker 1 (03:48):
Yeah, it's it's haunting and it's 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
(04:09):
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 groundwork for the humble and elaborate
shelters that we would construct thereafter. You know, because what
(04:32):
is what are some of the rooms in our houses
but caves that we have built for ourselves.
Speaker 3 (04:36):
Some houses are more cave like than others.
Speaker 1 (04:39):
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 enclosed concert hall, or an
underground facility, or a naturally occurring cave or some sort
(05:03):
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 in
a tunnel at one point. And so that alone gives
one enough room for pause.
Speaker 4 (05:25):
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.
Speaker 1 (05:39):
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 it, the earth, you know,
(06:01):
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 Autamaro 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
(06:24):
our home. And then there's this other world, this underworld,
the hypogean world beneath the soil.
Speaker 3 (06:30):
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
(06:50):
on the surface of this ball of iron.
Speaker 1 (06:54):
Yeah, and then everything else, the underworld, the hypogean world
beneath the soil, and the world beneath the soil. To
make 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
(07:16):
this includes the living soil in your backyard, networks of
mycilium that rooting through it, as well as life between
grains of sand buried under the beach, underground waters, and indeed,
cave environments, which, to be clear, can also be part
of the epigaean world because they of course are gateways
(07:37):
and there are going to be transition spaces the mouth
of a cave, for example. 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
(07:57):
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.
Speaker 3 (08:09):
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
(08:32):
as a whole. It's the fruiting body that comes up
above the surface. But underneath there's gonna 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.
Speaker 1 (08:46):
Yeah, there's there's an 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
(09:10):
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.
Speaker 3 (09:29):
A cave is like a space that can be spilunked,
and maybe our standards for how small a space can
be spilunked are changing.
Speaker 1 (09:37):
Right when we had Lee Burger on the show, the
author of Cave of Bones, who 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
(09:58):
of the cave that for the longest he could not access.
It was like a grueling experience, and you had to
send a like very young specialized scientists down there to
physically occupy the space. And he was able to, you know,
send them down, have them look around. He was able
to look at video footage from that space. But eventually
(10:19):
he could not resist. And he talks about this at length,
like he had to go down there, 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
and all of this, Like, even when you have the
(10:40):
technology and the organization, there's something about it. You have
to be there. You want to be there in the space.
Speaker 3 (10:46):
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.
Speaker 1 (10:58):
Yeah, it gave maybe all of is 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 dislocating bones and so forth to do it.
(11:22):
It's yeah, not for me, but I respect anyone who
is capable of it totally. Now, the most common caves
are carstick 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
(11:44):
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 kind of cave systems.
Some rather famous ones even do have sunlight spilling into them,
(12:06):
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.
Speaker 3 (12:24):
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. 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
(12:45):
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?
Speaker 1 (12:52):
I know, yeah, yeah, we'll get back to this in
just a minute here. But the humans, of course, we're
not the first adventure 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
(13:14):
and horror movies, you would think, oh, yeah, they're just
people and creatures fallen in all the time and evolving
in the chuds. But no, that doesn't happen all the time.
But plenty of creatures have specialized evolutionarily, often evolving defining
troglomorphic features that we often associate with these environments, namely
(13:35):
blindness and loss of pigmentation, with.
Speaker 3 (13:38):
The chutification if you will.
Speaker 1 (13:40):
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. You we have troglo bites. These
(14:02):
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.
(14:23):
Then we have troglophiles. These are creatures that live predominantly
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
(14:46):
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.
Speaker 3 (15:03):
I see none of.
Speaker 1 (15:05):
This to be confused with a troglodyte. They're 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 wrens that are called troglodytes. These birds are
(15:28):
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.
Speaker 3 (15:46):
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 very 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
(16:09):
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
(16:30):
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
(16:54):
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
(17:17):
questions about how evolution works and and why something like
that would happen.
Speaker 1 (17:22):
You know, it's it's fascinating to think about this about
our our, our our 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
uh Hamonid species evolving towards man, and we we it's
easy to look at him and like think, wow, look,
with each step he just gets hotter. With each step,
(17:45):
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.
Speaker 3 (17:59):
Yeah, totally, and as 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,
(18:19):
and we sort of lost them in favor of other things.
Speaker 1 (18:22):
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
(18:43):
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
(19:04):
not on the table.
Speaker 3 (19:06):
That's all right, But without the ability to modify the
features of the environment like that other animals have had
to adapt evolutionarily.
Speaker 1 (19:13):
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
(19:36):
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,
(20:00):
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 is a fair amount
of complexity there. The soil and even interiors of host
(20:20):
organisms for various parasites are also dark environments that species
have adapted to 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.
Speaker 3 (20:41):
Oh, that's kind of interesting. So Lamarckism or Lamarchianism named
after a figure named I believe you, Jean Baptiste Lamarck.
It was the sort of alternative theory of inheritance to
what would eventually become Mendalian genetics, which in some ways
has been partially superseded now. But instead of the kind
of Mendalian genetic inheritance, Lamarck would say that, like the
(21:06):
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 its children will be
(21:28):
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.
Speaker 1 (21:48):
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 way we're talking, so
it is it is kind of fascinating.
Speaker 3 (22:03):
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 but it's somewhat different.
(22:26):
I 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.
Speaker 1 (22:38):
I think the one of the infectious things about it,
at least 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 bet I
better go to the gym, and you need a pump
iron so that my offspring will will have strong arms.
(22:58):
But we might think, uh, 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 uh, you know, I think we do see this
infectious idea in some of our fictions of subterranean monsters,
including The Descent of the Hobbit Snekeel and the Lord
of the Rings, who becomes a trogloditic gollum in the
(23:19):
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 pimitation has changed, his eyes have gotten larger,
I believe.
Speaker 3 (23:33):
Yeah, though, I think the morphotypical changes of Gollum could
be largely due to the corruption of the one ring.
Speaker 1 (23:39):
That's the thing. The one ring changes everything you've got.
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.
Speaker 3 (23:59):
Caves will do that.
Speaker 1 (24:01):
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 evolved 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,
(24:24):
but both pliotrophy and genetic drift 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.
Speaker 3 (24:44):
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 cavefish.
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
(25:07):
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
(25:32):
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.
(25:53):
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
(26:14):
of a hidden package deal, a genetic package deal with
an even more advantageous trait balancing it out, and 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
(26:34):
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.
(26:54):
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,
(27:18):
and it happens due to random factors, though sometimes if
a population is small enough. And interesting thing is that
traits resulting 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,
(27:43):
and then all the individuals within the population have the
gene for that trait. Going forward, Yeah.
Speaker 1 (27:48):
Now, now going back to what you mentioned about 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
(28:11):
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
flourish along with whatever other changes they might be bringing.
Speaker 3 (28:31):
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 darkness.
Speaker 1 (28:46):
Yeah, this I can't help be reminded of various gaming
rules and gaming systems and all of this. Like, 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 in a
lightless environment, and you have a weapon that gives you,
say a plus two on attack. But if you're in
(29:08):
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.
Speaker 3 (29:25):
Right. Yes, that's a good 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 and explaining the
evolution of kvefish blindness. There could be direct selective pressure,
(29:45):
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 pliotropy as well.
(30:06):
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 minnow like fish with silver scales. I
(30:30):
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
(30:50):
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
(31:12):
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 colonized 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
(31:33):
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
(31:56):
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.
Speaker 1 (32:07):
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.
Speaker 3 (32:16):
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
Astianax fish is that they undergo what's called eye regression.
So I don't know if this is true of all populations,
(32:36):
because they are 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 developed the they acquire eyes and then lose
(32:58):
the eyes in development, and the eye sockets are paved
over by skin and scales, and 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.
Speaker 1 (33:20):
Fascinating, you know. I also have to throw in, okay,
Astianax part of the scientific 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
(33:41):
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.
Speaker 3 (33:56):
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
(34:16):
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
(34:37):
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 law of tails in some primates,
(35:02):
including us, the loss of teeth in birds and the
loss of legs in whales. 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
(35:24):
to lose the trait that its ancestors' 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 in frequency in
(35:48):
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
(36:10):
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 to have functioning organs, and some types of
(36:31):
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
(36:54):
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
(37:15):
where the surface variant of the Mexican cavefish lives. And
in favor of this explanation the energy cost explanation, the
author's cite of paper from twenty fifteen by Moran, Softly
and Warrant published in Science Advances called the Energetic Cost
of Vision and Evolution of Eyeless Mexican cavefish. Summarizing the
(37:36):
findings of Moran at all Krishna and Rohner Wright 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
(38:00):
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
(38:24):
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
(38:46):
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 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
(39:11):
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.
Speaker 1 (39:23):
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
(39:48):
around you. Basically the Star Trek principle exactly.
Speaker 3 (39:52):
If it's not actually providing you a benefit, then it's
just a liability.
Speaker 1 (39:56):
Yeah.
Speaker 3 (39:57):
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,
(40:19):
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
(40:40):
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, it seems important during embryonic development.
(41:00):
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 from not twenty nineteen, from
two thousand and nine sorry in Developmental Biology, which found
that by altering expression of the sonic hedgehog signaling pathway
(41:23):
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
(41:47):
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. It's elevated expression in the oral
pharyngeal region in cavefish taste buds, and the fact that
(42:08):
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 in 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
(42:29):
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
(42:52):
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. Can you know,
smell and taste together help you sort of test incoming
(43:13):
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 gonna eat.
Speaker 1 (43:22):
Yeah. Yeah, we have to be reminded of that because
we are such site dependent organisms that we easily think
of like the world we can see and then the
other senses that provide 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.
(43:43):
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.
Speaker 3 (43:58):
It absolutely is. Even I would say this, even if
you think you quote no better. Do you know what
I mean about that?
Speaker 1 (44:05):
Row like that?
Speaker 3 (44:06):
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.
Speaker 1 (44:18):
Yeah, on some level, you were still anticipating the flavor,
anticipating the taste experience based on and to a large
degree on visual data totally.
Speaker 3 (44:28):
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
(44:49):
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 Jay 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
(45:12):
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 milanophores and color molecules in the fish's body,
(45:32):
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 results in these different cavefish populations.
(45:54):
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
(46:14):
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
(46:36):
it doesn't matter. On the surface, pigmentation can also play
a role in camouflage, conceealing 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. Even
(47:00):
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
(47:24):
protests at All from two thousand and seven indicating it's
more likely a result of drift, though some amount of
indirect selection through pleiotropy is possible.
Speaker 1 (47:35):
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
(47:59):
to discuss I 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.
Speaker 3 (48:10):
I can't wait.
Speaker 1 (48:12):
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 hedgehog. We'd also love to hear from you
on that point as well. But yeah, yeah, particular cave
(48:33):
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 of memory serves was discovered late
enough that they were able to preserve the cave environment
(48:54):
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
(49:16):
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
(49:38):
to be reminded of this. We have 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
(49:58):
stars where if you listen to the podcas cast downloaded
and subscribe, that also.
Speaker 3 (50:02):
Helps us out huge thanks as always to our excellent
audio producer JJ Posway. If you would like to get
in touch with us with feedback on this episode or
any other, to suggest a topic for the future, or
just to say hello, you can email us at contact
at stuff to Blow your Mind dot com.
Speaker 2 (50:25):
Stuff to Blow Your Mind is production of iHeartRadio. For
more podcasts from My Heart Radio, visit the iHeartRadio app,
Apple Podcasts, or wherever you listen to your favorite shows.
Hey, welcome to Stuff to Blow your Mind. It is Saturday,
so once more we have a vault episode for you.
This is going to be Life in the Hapogean World,
Part one. This one was originally published three five, twenty
twenty four. This is going to be part one out
of four, so we'll be publishing the other three episodes
in this series on the Saturdays to come.
Speaker 2 (00:29):
Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 1 (00:39):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert.
Speaker 3 (00:42):
Lamb and I am Joe McCormick. And hey here we
are together again, Rob. After you were out for a
little bit.
Speaker 1 (00:48):
Oh yeah, yeah, we've recorded a few thanks here 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.
Speaker 3 (01:01):
Wait a minute, now, you refreshed my brain because I
forgot that we did a listener mail episode before this.
So listeners, are you are reacclimated to Rob? Now?
Speaker 1 (01:10):
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
(01:32):
take our word for it. Sometimes we are away, sometimes
we were present.
Speaker 3 (01:36):
We're like a duo of mysterious time traveling cats that
appear in your ears.
Speaker 1 (01:40):
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 school.
Speaker 3 (01:54):
I would think you'd remember. Come on, I.
Speaker 1 (01:56):
Mean, sometimes you know, these trips, you see a lot
of things, and 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 frequently discussed biological wonders
on the show that leads us to consider extreme environments,
(02:17):
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,
(02:38):
but it's still fascinating. There's still something about the subterranean world.
Speaker 3 (02:42):
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.
Speaker 1 (02:58):
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 blanking 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
(03:20):
little ghost story or something right, just sort of drive
home the creepiness of it. But yeah, you find yourself
in just the absolute darkness.
Speaker 3 (03:28):
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 you just eyes are totally wide and you see
not even a pinprick of light, just nothing.
Speaker 1 (03:48):
Yeah, it's it's haunting and it's 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
(04:09):
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 groundwork for the humble and elaborate
shelters that we would construct thereafter. You know, because what
(04:32):
is what are some of the rooms in our houses
but caves that we have built for ourselves.
Speaker 3 (04:36):
Some houses are more cave like than others.
Speaker 1 (04:39):
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 enclosed concert hall, or an
underground facility, or a naturally occurring cave or some sort
(05:03):
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 in
a tunnel at one point. And so that alone gives
one enough room for pause.
Speaker 4 (05:25):
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.
Speaker 1 (05:39):
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 it, the earth, you know,
(06:01):
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 Autamaro 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
(06:24):
our home. And then there's this other world, this underworld,
the hypogean world beneath the soil.
Speaker 3 (06:30):
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
(06:50):
on the surface of this ball of iron.
Speaker 1 (06:54):
Yeah, and then everything else, the underworld, the hypogean world
beneath the soil, and the world beneath the soil. To
make 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
(07:16):
this includes the living soil in your backyard, networks of
mycilium that rooting through it, as well as life between
grains of sand buried under the beach, underground waters, and indeed,
cave environments, which, to be clear, can also be part
of the epigaean world because they of course are gateways
(07:37):
and there are going to be transition spaces the mouth
of a cave, for example. 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
(07:57):
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.
Speaker 3 (08:09):
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
(08:32):
as a whole. It's the fruiting body that comes up
above the surface. But underneath there's gonna 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.
Speaker 1 (08:46):
Yeah, there's there's an 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
(09:10):
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.
Speaker 3 (09:29):
A cave is like a space that can be spilunked,
and maybe our standards for how small a space can
be spilunked are changing.
Speaker 1 (09:37):
Right when we had Lee Burger on the show, the
author of Cave of Bones, who 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
(09:58):
of the cave that for the longest he could not access.
It was like a grueling experience, and you had to
send a like very young specialized scientists down there to
physically occupy the space. And he was able to, you know,
send them down, have them look around. He was able
to look at video footage from that space. But eventually
(10:19):
he could not resist. And he talks about this at length,
like he had to go down there, 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
and all of this, Like, even when you have the
(10:40):
technology and the organization, there's something about it. You have
to be there. You want to be there in the space.
Speaker 3 (10:46):
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.
Speaker 1 (10:58):
Yeah, it gave maybe all of is 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 dislocating bones and so forth to do it.
(11:22):
It's yeah, not for me, but I respect anyone who
is capable of it totally. Now, the most common caves
are carstick 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
(11:44):
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 kind of cave systems.
Some rather famous ones even do have sunlight spilling into them,
(12:06):
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.
Speaker 3 (12:24):
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. 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
(12:45):
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?
Speaker 1 (12:52):
I know, yeah, yeah, we'll get back to this in
just a minute here. But the humans, of course, we're
not the first adventure 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
(13:14):
and horror movies, you would think, oh, yeah, they're just
people and creatures fallen in all the time and evolving
in the chuds. But no, that doesn't happen all the time.
But plenty of creatures have specialized evolutionarily, often evolving defining
troglomorphic features that we often associate with these environments, namely
(13:35):
blindness and loss of pigmentation, with.
Speaker 3 (13:38):
The chutification if you will.
Speaker 1 (13:40):
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. You we have troglo bites. These
(14:02):
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.
(14:23):
Then we have troglophiles. These are creatures that live predominantly
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
(14:46):
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.
Speaker 3 (15:03):
I see none of.
Speaker 1 (15:05):
This to be confused with a troglodyte. They're 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 wrens that are called troglodytes. These birds are
(15:28):
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.
Speaker 3 (15:46):
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 very 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
(16:09):
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
(16:30):
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
(16:54):
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
(17:17):
questions about how evolution works and and why something like
that would happen.
Speaker 1 (17:22):
You know, it's it's fascinating to think about this about
our our, our our 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
uh Hamonid species evolving towards man, and we we it's
easy to look at him and like think, wow, look,
with each step he just gets hotter. With each step,
(17:45):
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.
Speaker 3 (17:59):
Yeah, totally, and as 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,
(18:19):
and we sort of lost them in favor of other things.
Speaker 1 (18:22):
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
(18:43):
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
(19:04):
not on the table.
Speaker 3 (19:06):
That's all right, But without the ability to modify the
features of the environment like that other animals have had
to adapt evolutionarily.
Speaker 1 (19:13):
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
(19:36):
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,
(20:00):
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 is a fair amount
of complexity there. The soil and even interiors of host
(20:20):
organisms for various parasites are also dark environments that species
have adapted to 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.
Speaker 3 (20:41):
Oh, that's kind of interesting. So Lamarckism or Lamarchianism named
after a figure named I believe you, Jean Baptiste Lamarck.
It was the sort of alternative theory of inheritance to
what would eventually become Mendalian genetics, which in some ways
has been partially superseded now. But instead of the kind
of Mendalian genetic inheritance, Lamarck would say that, like the
(21:06):
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 its children will be
(21:28):
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.
Speaker 1 (21:48):
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 way we're talking, so
it is it is kind of fascinating.
Speaker 3 (22:03):
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 but it's somewhat different.
(22:26):
I 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.
Speaker 1 (22:38):
I think the one of the infectious things about it,
at least 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 bet I
better go to the gym, and you need a pump
iron so that my offspring will will have strong arms.
(22:58):
But we might think, uh, 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 uh, you know, I think we do see this
infectious idea in some of our fictions of subterranean monsters,
including The Descent of the Hobbit Snekeel and the Lord
of the Rings, who becomes a trogloditic gollum in the
(23:19):
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 pimitation has changed, his eyes have gotten larger,
I believe.
Speaker 3 (23:33):
Yeah, though, I think the morphotypical changes of Gollum could
be largely due to the corruption of the one ring.
Speaker 1 (23:39):
That's the thing. The one ring changes everything you've got.
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.
Speaker 3 (23:59):
Caves will do that.
Speaker 1 (24:01):
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 evolved 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,
(24:24):
but both pliotrophy and genetic drift 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.
Speaker 3 (24:44):
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 cavefish.
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
(25:07):
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
(25:32):
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.
(25:53):
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
(26:14):
of a hidden package deal, a genetic package deal with
an even more advantageous trait balancing it out, and 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
(26:34):
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.
(26:54):
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,
(27:18):
and it happens due to random factors, though sometimes if
a population is small enough. And interesting thing is that
traits resulting 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,
(27:43):
and then all the individuals within the population have the
gene for that trait. Going forward, Yeah.
Speaker 1 (27:48):
Now, now going back to what you mentioned about 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
(28:11):
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
flourish along with whatever other changes they might be bringing.
Speaker 3 (28:31):
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 darkness.
Speaker 1 (28:46):
Yeah, this I can't help be reminded of various gaming
rules and gaming systems and all of this. Like, 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 in a
lightless environment, and you have a weapon that gives you,
say a plus two on attack. But if you're in
(29:08):
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.
Speaker 3 (29:25):
Right. Yes, that's a good 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 and explaining the
evolution of kvefish blindness. There could be direct selective pressure,
(29:45):
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 pliotropy as well.
(30:06):
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 minnow like fish with silver scales. I
(30:30):
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
(30:50):
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
(31:12):
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 colonized 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
(31:33):
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
(31:56):
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.
Speaker 1 (32:07):
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.
Speaker 3 (32:16):
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
Astianax fish is that they undergo what's called eye regression.
So I don't know if this is true of all populations,
(32:36):
because they are 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 developed the they acquire eyes and then lose
(32:58):
the eyes in development, and the eye sockets are paved
over by skin and scales, and 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.
Speaker 1 (33:20):
Fascinating, you know. I also have to throw in, okay,
Astianax part of the scientific 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
(33:41):
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.
Speaker 3 (33:56):
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
(34:16):
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
(34:37):
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 law of tails in some primates,
(35:02):
including us, the loss of teeth in birds and the
loss of legs in whales. 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
(35:24):
to lose the trait that its ancestors' 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 in frequency in
(35:48):
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
(36:10):
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 to have functioning organs, and some types of
(36:31):
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
(36:54):
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
(37:15):
where the surface variant of the Mexican cavefish lives. And
in favor of this explanation the energy cost explanation, the
author's cite of paper from twenty fifteen by Moran, Softly
and Warrant published in Science Advances called the Energetic Cost
of Vision and Evolution of Eyeless Mexican cavefish. Summarizing the
(37:36):
findings of Moran at all Krishna and Rohner Wright 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
(38:00):
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
(38:24):
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
(38:46):
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 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
(39:11):
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.
Speaker 1 (39:23):
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
(39:48):
around you. Basically the Star Trek principle exactly.
Speaker 3 (39:52):
If it's not actually providing you a benefit, then it's
just a liability.
Speaker 1 (39:56):
Yeah.
Speaker 3 (39:57):
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,
(40:19):
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
(40:40):
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, it seems important during embryonic development.
(41:00):
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 from not twenty nineteen, from
two thousand and nine sorry in Developmental Biology, which found
that by altering expression of the sonic hedgehog signaling pathway
(41:23):
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
(41:47):
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. It's elevated expression in the oral
pharyngeal region in cavefish taste buds, and the fact that
(42:08):
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 in 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
(42:29):
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
(42:52):
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. Can you know,
smell and taste together help you sort of test incoming
(43:13):
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 gonna eat.
Speaker 1 (43:22):
Yeah. Yeah, we have to be reminded of that because
we are such site dependent organisms that we easily think
of like the world we can see and then the
other senses that provide 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.
(43:43):
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.
Speaker 3 (43:58):
It absolutely is. Even I would say this, even if
you think you quote no better. Do you know what
I mean about that?
Speaker 1 (44:05):
Row like that?
Speaker 3 (44:06):
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.
Speaker 1 (44:18):
Yeah, on some level, you were still anticipating the flavor,
anticipating the taste experience based on and to a large
degree on visual data totally.
Speaker 3 (44:28):
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
(44:49):
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 Jay 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
(45:12):
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 milanophores and color molecules in the fish's body,
(45:32):
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 results in these different cavefish populations.
(45:54):
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
(46:14):
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
(46:36):
it doesn't matter. On the surface, pigmentation can also play
a role in camouflage, conceealing 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. Even
(47:00):
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
(47:24):
protests at All from two thousand and seven indicating it's
more likely a result of drift, though some amount of
indirect selection through pleiotropy is possible.
Speaker 1 (47:35):
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
(47:59):
to discuss I 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.
Speaker 3 (48:10):
I can't wait.
Speaker 1 (48:12):
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 hedgehog. We'd also love to hear from you
on that point as well. But yeah, yeah, particular cave
(48:33):
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 of memory serves was discovered late
enough that they were able to preserve the cave environment
(48:54):
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
(49:16):
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
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major social platforms. I think we're on TikTok. I had
(49:38):
to be reminded of this. We have 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
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We appreciate it. If you can give the show some
(49:58):
stars where if you listen to the podcas cast downloaded
and subscribe, that also.
Speaker 3 (50:02):
Helps us out huge thanks as always to our excellent
audio producer JJ Posway. If you would like to get
in touch with us with feedback on this episode or
any other, to suggest a topic for the future, or
just to say hello, you can email us at contact
at stuff to Blow your Mind dot com.
Speaker 2 (50:25):
Stuff to Blow Your Mind is production of iHeartRadio. For
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