March 15, 2025 • 43 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 2 of 4, originally published 3/7/2024)
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Speaker 1 (00:06):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lambin. Today we have a vault episode for you.
Of course, this is going to be our second episode
in the four part Life in the Hypogean World series.
So without further ado, let's dive right.
Speaker 2 (00:21):
In Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 1 (00:34):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert.
Speaker 3 (00:37):
Lamb and I am Joe McCormick. And hey, we are
back with part two of our series on cave ecosystems
and Hypogean biology. Cave biology. In the last episode, we
talked about some of the characteristic features of cave ecosystems
and then we ended up discussing how lightless cave environments
(00:58):
shaped the evolution of a creature called Astianax Mexicanus, or
the blind Mexican cavefish, also known as the Mexican tetra,
of which many populations have adapted to life in subterranean
waterways by losing their eyes and the pigments in their flesh.
(01:19):
We ended up kind of doing a deep dive on
the evolutionary logic of this why an animal population that
once had eyes and skin pigment would adapt over many
generations to lose those traits in a cave. So if
you haven't heard part one yet, you should probably go
back and check that one out first. But we are
back again today to talk about some more elements of
(01:39):
cave biology. Specifically, one of the things I wanted to
get into we didn't really have time for last time,
was something else about blind Mexican cavefish, which is, if
they have no sight, what do they do? How do
they navigate their environment and forage for food?
Speaker 1 (01:56):
And this is key because they've got to eat something.
These are extreme environments that are generally regarded as not
being the most bountiful places to scrape out a livelihood
as an organism.
Speaker 3 (02:09):
Right, and there might be some advantages to living in
the cave if you are a fish like this, Like
there might be fewer predators than you would encounter in
the world above, but there are also fewer food resources,
so you know, you have to have to kind of
like shift your specialization.
Speaker 1 (02:27):
Yeah, Like, if I was to decide, again this is
a non evolutionary example, but if I was to decide
I am going to now live in my local Ikea store, well,
you know, I've got to figure out some new things. Right,
It's going to be easy to find a bed when
the lights go out, but I'm going to have to
deal with the night security. My diet is going to
consist entirely of Ikea food. So if you like Lingenberry, yeah, yeah,
(02:51):
make sure you don't have a Lingenberry allergy for sure.
Speaker 3 (02:55):
Yeah. So I wanted to explore this question of how
do blind cavefish since their surroundings, and so I was
looking into this and I discovered that apparently one major
sense mechanism that they rely on is what's known as
the lateral line system. And this is not a sense
that is unique to blind cavefish. The lateral line system
(03:18):
is found in lots of aquatic vertebrates, even those that
can see. Though in blind fish there are usually enhancements
to this system. So it's a sense that they already have,
but it gets stronger in the cave evolved variants. As
a side note, I always really enjoy imagining types of
senses that humans don't have, Like what it would be
(03:41):
like to have a different kind of sense, you know,
like electro reception or magnetic perception or something like that.
And this sense in particular, I'm very excited to try
to imagine because it's very spooky trying to imagine it.
It's something about it feels almost kind of Halloween. This
might make more sense once I explain it. So, the
(04:02):
basic function of the lateral line system is to detect
movements in the water surrounding the animal through high sensitivity
to changes in water pressure. This system it runs along
the length of an animal like a fish, so there'll
be sort of canals of the lateral line sensing organs
(04:24):
around the head of the fish and then running lengthwise
along the body. The system uses sensory organs known as
neuro masts, which contain little hairs suspended within a kind
of jelly filled capsule that bends in response to changes
in water pressure and flow. And then these little hairs
(04:48):
are connected to nerve cells that are strung along the
lateral line system like Christmas tree lights. Sometimes the neuromasts
are exposed to the water on the outside of the skin.
They're just right on the outside of the skin, but
other times they are contained within a kind of duct
or canal that is just under the surface of the skin.
(05:10):
And either way, the purpose of the lateral line system
is to detect vibrations, movements, and objects within the water
by sensing these little changes in water pressure and flow.
So with this system, an animal can get feedback about
its own movement through the water, for one thing, but
(05:31):
it can also sense the presence of currents and nearby
solid masses, either moving or stationary, by the way they
displace water as the fish moves through it. And this
is coming back to what I was saying a minute ago,
something about trying to imagine this is. Oh, it's very
evocative and a little bit scary, almost so imagining living
(05:54):
in a place of dark water. But you can sense
objects around you by the way they move the water,
or by the way they change how the water moves
around you.
Speaker 1 (06:04):
Wow, I mean, we're getting right back into that Gollumn territory,
you know. I'm imagining Gollum sensing that that goblin that
has come down and is washing its hands in the
water and so forth on this lightless underground lake. And
to your point, it is so alien to try and
imagine this. I'm in the water a fair amount. I
swim laps most morning, though swimming this morning, and it's
(06:28):
still you know, it's a very visual exercise. You can
set you can feel things in the water, obviously you can,
and Lord knows, there's a lot to see and feel
in a ymca pool. You know, there are people doing
exercise classes, there are people swimming laps next to you,
sometimes very often in the same lane with you, and
(06:48):
you pick up on those movements, but nowhere near this
level of detail. You know, this is like compared to
what we have. This is like a second site.
Speaker 3 (06:57):
Yeah, I kind of ghostly vision through which you can
sense things around you by the way they move the
water against your body.
Speaker 1 (07:05):
Yeah.
Speaker 3 (07:06):
So anyway, the astianax fish, the Mexican blind cavefish apparently
compensate for their lack of vision by having a more
sensitive lateral line system than their surface dwelling cousins, and
this helps them navigate, forage and survive in their lightless environment.
But this way of living depends not just on heightened
(07:28):
sensitivity to water displacement, but also on changes to behavior.
For example, one thing I was reading about is that
when preparing to mate, apparently male and female blind cavefish
engage in this in these patterns of exaggerated movements of
various body parts like the mouth and the gills, and
(07:51):
these movements are thought to perhaps help the mating partners
find and identify one another without visual cues, does some sense,
like moving around in the water more so that they
can locate one another. Of course, the fish also have
changes to their metabolism to allow them to survive in
a place where food is less abundant than it is
on the surface, so they are thought to have slower
(08:14):
metabolisms to need less food energy. But also I was
reading about an interesting twenty seventeen study published in PLUS
one by some researchers associated with the University of Cincinnati,
and this was on the role of a symmetry in
blind cavefish evolution. We actually did a whole series on
(08:36):
asymmetry and animals a while back in which we talked
about fiddler crabs and such, you know, fiddler crabs having
one claw much much bigger than the other. But we
talked about a lot of examples in the animal world,
and I remember that being a very interesting series.
Speaker 1 (08:49):
Yeah, that was a fun one for sure. You know,
some of the outrageous examples in some of the less
obvious examples of asymmetry.
Speaker 3 (08:58):
Apparently there is a bit of hidden asymmetry in the
blind Mexican cavefish as well. So let's see. This study
was by Amandicaate Powers Aeronym Davis, Shane A. Kaplan and
Joshua be Gross, published in Plus one in twenty seventeen,
and it was called Cranial asymmetry arises later in the
Life History of the blind Mexican cavefish Astianax Mexicanus. And
(09:21):
so we were talking in the previous episode about how
in some of these cavefish populations the fish are not
without eyes from the very beginning. Rather, they do grow
eyes initially in embryonic development, but then the eyes go
through what's called regression where they are absorbed and disappear
as the fish grows, and then the bone around the
(09:43):
eye socket collapses in by the time the fish as
an adult. Young cavefish apparently start life with fairly symmetrical bodies,
and the surface variants in the rivers above where there's
plenty of light also have fairly symmetrical bodies even as adults.
But the young cavefish start out fairly symmetrical, and then
(10:04):
as they mature, a mismatch develops in how the bones
on either side of their skull take shape, and this
asymmetry in their cranial bones seems to match a difference
in behavior between the cave variant of the tetras and
the surface variant still has eyes. The surface variant, when
you place it in a fish tank, will swim in
(10:25):
more random patterns or will just kind of float without
moving in the shaded portions of the tank. The cave variant,
on the other hand, will just keep it'll keep swimming
in circles around the edges of its tank, and this
matches with the fact that has been observed in the wild.
In its natural environment, the cave variant will tend to
(10:48):
follow the rocky walls of the pool where it lives,
swimming counterclockwise around these boundaries in an endless loop. And
one of the studies authors, Amanda Powers, was quoted in
a press release about the research, explaining quote, you could
see how asymmetry might be an advantage in navigation. They
(11:08):
tend to swim in a unidirectional circular motion around their
tanks to explore their surroundings. Having asymmetry in their skull,
we think is attributed to handedness. If their skull is
bent to the left, they could be right handed. They're
feeling the wall to the right with their sensory structures.
Oh wow, Yeah, so it seems that the cavefish have
(11:30):
a type of developmental handedness that correlates to a navigation
behavior where they follow the walls of their natural enclosure
in this slow, counterclockwise lapping motion. Though I think it's
still not fully understood how this system emerges, if it
helps them survive how, we don't fully know yet, but
(11:53):
it's interesting to imagine why. Another interesting sort of unrelated
thing that I was just reading in this press release,
also quoting the same author of the paper, I mean powers,
talking about how she and colleagues went into the caves
to observe these fish, and she says in this article quote,
whenever you would touch the surface of the water with
(12:13):
your finger, a swarm of cavefish would come right up
to it. Not many fish would do that. These cavefish
have zero predators, so they're not afraid.
Speaker 1 (12:23):
Oh wow, that you know. That reminds me of other
examples we've talked about in the show, like various birds
of the Galapagos Islands, for example. It reminds me of
some stuff I was just reading the other day about
the dodo bird. You know, where you have these organisms
that adapt over evolutionary time to an environment where there
(12:44):
are no predators or there is just an entirely different
predation situation going on, and yeah, they just lose their fear.
There's no need to engage in that kind of protective
behavior because those things don't exist for them until they
show up on a bub Yeah.
Speaker 3 (13:02):
Yeah, dutch. Okay, So that's lateral line sensations, and then
the asymmetry of the skull, the kind of the circling behavior,
all those differences. But there was one more thing I
wanted to talk about. I mentioned in the previous episode
another sensory enhancement found in some of these cavefish, and
that sensory enhancement was taste. It appeared to be involved
(13:25):
in a pliotropy. Remember that's where a single genetic change
would result in multiple different changes in the phenotype in
the body or the behavior. And in this case, the
idea was that there was a single genetic change that
would result in both regression of the eyes. So this
process we talked about where the eyes are sort of
(13:47):
absorbed and the adult fish doesn't have eyes, and then
along with that development of a greater number of taste buds. Now,
we speculated generally in the last episode that you know,
having enhanced other senses in a cave would probably be
useful in some way. Taste buds are generally useful. They
are our body's chemistry set to know what we're putting
(14:09):
in the digestive tract. But I was wondering if there
was anything else we could know about that, like how
do these extra taste buds work in these fish? What
kind of benefit do they provide? So this is another
thing where the answer is not fully known yet we
know a few kind of interesting morsels. So I came
across a very recent paper addressing this question. It was
(14:31):
by Daniel Burning and Joshua B. Gross published in Frontiers
and Ecology and Evolution twenty twenty three, called the Constructive
Evolution of Taste in astianax Cavefish or Review. And one
of the things I thought was interesting here, Rob, I've
attached a I've attached a diagram for you to look at.
It seems that mostly what we're talking about are what's
(14:53):
called extra oral taste buds, taste buds that are not
inside the mouth like hours, but outside the mouth and
spread out over the jaws in the front of the face.
So in this diagram, it shows blind cavefish from a
couple of different populations compared to the surface variant with
(15:14):
taste buds filled in as red dots on the illustrations.
And while the surface variant looks like it is wearing lipstick,
you know, the red dots, the taste buds are on
the mouth, the cave fish are wearing clown makeup. The
taste buds are all over the face.
Speaker 1 (15:30):
Yeah, it's quite impressive, And again it makes sense for
this kind of an environment, you know, you need to
lean into a different sense situation. It reminds me a
bit of our past discussions about catfish being super tasters,
you know, and really when and again it come back
to that situation where we have our human understanding of
(15:52):
the senses, but we have a human level understanding of
those senses. And so when you're talking about something like
this that is enhanced, it's not taste as we know it,
it's something different.
Speaker 3 (16:06):
Yeah, exactly. So I was wondering, like, what do we
know about what kind of difference this makes? What difference
does it make in the lives of these fish? So
there were a few things mentioned in this review. One
is that multiple studies have found what appears to be
enhanced sensitivity to chemical repellence and taste sources in the
(16:27):
water in the cavefish when compared to the surface fish.
So just one example is a study by Humbach in
nineteen sixty that found in the cave fish compared to
the common minno, that the modality for bitter sensation was
three hundred times more acute roughly, and then the salty
(16:49):
slash acid slash sweet modality difference was about two thousand
to four thousand times more acute in the cavefish, so
much greater, much lower I guess threshold of sensitivity to
these tastes also protus at all. In two thousand and
eight found quote amino acids dissolved in system water were
(17:11):
detected at a much lower concentration in Paschone cavefish. That's
from one particular cave source paschone cavefish, compared to surface fish.
And one possible explanation the authors discuss for this heightened
sensitivity to amino acids could be what they call the
savory taste receptor T one R one. This is the
(17:34):
receptor that binds to glutamate and helps us taste savory
umami flavors. So I wonder if you could really get
these cavefish going bananas over something in the water, a
piece of food or whatever is dispersed in the water
if you hit it with a bit of the good
old MSG.
Speaker 1 (17:53):
I bet, or maybe one of those little fish shaped soy.
Speaker 3 (17:58):
Sauce. Yeah, we get soy sauce, some parmesan cheese, some tomatoes,
some MSG, all the good savory things. However, the authors
do say that several amino acids were used in the
study that found this, so there could be other explanations,
possibly also involving old faction rather than taste. But anyway,
(18:18):
what difference would these these taste bud arrangements and heightened
sensitivity to flavors make in terms of the behavior of
the fish the author's right quote. One recent study argued
that external taste buds are used for preliminary assessment of
food items during random swimming or targeted searches for food.
(18:41):
Extra oral taste buds thus carry in importance for determining
whether to pursue or avoid a food item. So the
way I understand that is it's heightening the fish's foraging
efficiency basically by saying, before you even get something in
the mouth, you're tasting the water around this thing with
(19:03):
the front of your face, and it helps you zero
in more quickly on something that is good to eat
or is not good to eat. Like you're getting you're
getting an idea about whether you're coming close to a
good piece of food or not earlier. So that would
make your foraging more efficient.
Speaker 1 (19:19):
Okay, that makes sense.
Speaker 3 (19:21):
In the end, however, they do say that that this
is just an area that hasn't had enough research yet.
They say that the quote precise function of cavefish extraoral
taste buds remains unclear and largely unexplored, and then they
also say that future studies need to do things like
excluding the role of oldfaction or smell and isolating the
(19:43):
variable of taste. And there are differences that even remain
that they did examine in the paper that we just
don't know how to explain yet, but could possibly be
related to taste. One interesting idea they bring up is
the feeding angle, so they you know, if you look
at the cave variants of these fish versus the surface variants.
(20:06):
One thing you will notice is that they both go
along the bottom. Might there might be like a stony
or sandy or pebbly bottom of the water source where
they are, and they'll go along the bottom with their
mouth down to the bottom, kind of searching for little
bits of food to eat or you know, prey in
the case of you know, carnivory, or just little bits
(20:27):
of dead organic matter, whatever it is they're coming across
to eat. They're scouring the bottom for it. For some reason,
the surface fish have a steeper angle that they forage
at with their body more it's almost more totally vertical,
whereas the cave fish tend to forage at a more
slanted angle that's closer. It's more like fifty five degrees
(20:50):
from the bottom versus more like eighty degrees in the
surface variant. They say this could be related to the
changes in extraoral taste buds. Maybe it's you know, like
they're tasting things differently, thus they have to orient their
bodies differently. Don't know, but interesting question.
Speaker 1 (21:09):
Yeah, yeah, I mean this this is fascinating to think about,
Like it makes one wonder if it has something to
do with their being more predation opportunities for the surface fish,
you know, like you've got to come in at that
steeper angle because there's a greater risk of things, you know,
on the floor of the of the sea or what
(21:32):
have you, that might come after them while they're feeding.
And maybe those are absent in these cave environments. Again,
you know, we're talking about like a lack of just
biodiversity in those environments. You know, there's many fewer predators,
et cetera. But who knows. That's just me spitballing.
Speaker 3 (21:48):
I don't want to act like I'm just look at
a diagram and a paper and answer the question. But
just one thing I wonder about is looking at the
looking again at the diagram of where these extraoral taste
buds are in the surface fish. Remember they're all like
right at the mouth, but in the in the cave variant,
a lot of them tend to be spread out along
the lower jaw. So I wonder if it can come
(22:11):
in at this shallower angle because it's sort of like
feeling and tasting more along the bottom without having to
get the front of its mouth in contact with it.
It can kind of taste with the slope of its
lower jaw.
Speaker 1 (22:24):
Yeah, that would make a lot of sense.
Speaker 3 (22:25):
But yeah, again don't know interesting questions about Usually when
you when you think about fish and taste together, you're
thinking about what a fish tastes like to you, not
what it's like to taste as a fish.
Speaker 1 (22:36):
Yeah, which again it can put you an entirely different
sense realities, especially considering things like a catfish. But also
you know, getting into taste and smell. When whenever I
read information about, you know, the distances at which fish
in the ocean can detect other things going on, say
blood in the water or some sort of rotting shoe,
(23:01):
I'm always just amazed because again it is an experience
of the ocean that is just unlike anything we can
experience when we venture into it. And the same holds
true of the cave. Like when we enter into the cave,
our experiences are rather different compared to the organisms that
have evolved to thrive there. Now the place I'd like
(23:32):
to turn to next. This gets into something that was
probably one of the key reasons I decided this would
be a good series for us to do, you know,
to return to cave biology. This and the fact that
I was also inspired by some environment related stuff at
the Bishop Museum on a Wahuo recently when I returned
(23:53):
there and got to take in their natural history section
in addition to their cultural and historical sections. But yeah,
this is the revelation that really pushed me over the
edge that for many cave environments, batguano is sunlight. Ooh, batguano,
that's bat poop if you will, if you're not familiar
(24:16):
essentially in these environments, yes, this is the light of
the sun. This plays a vital role in the food
chain because remember light is the first step in the
food chain of the Epigean world. Sunlight reaches the surface
where photo autotrophic organisms create food via sunlight, carbon dioxide
(24:38):
and water. These are autotrophs, and we call this the
trophic level of the food chain.
Speaker 3 (24:44):
Right, So yeah, this came up in the last episode
that almost all of the food chain where we're really
familiar with is happening where the base layer of the
food chain is using energy from the sunlight to make
its food and then we of course you know, other
organisms eat those organisms and on and on it goes.
But if you don't have the sunlight to power the
(25:06):
food synthesis at the base layer of the food chain,
what do you do.
Speaker 1 (25:10):
That's right, yeah, yeah, most of it depends on sunlight.
Plants are autotropes, as are seaweed phytal blanked in some
kinds of bacteria, including bacteria that produce their own food
vio chemosynthesis around volcanic vents. But most of the producers
are producing their own food via the process of photosynthesis
(25:32):
and therefore require the bounty of sunlight. The next trophic
layer of the food chain is, of course the primary
consumers who eat the producers, the herbivores being chief among them.
And then the predators compose the third trophic layer, the
secondary consumers, followed by the tertiary consumers and on up
to the apex predators. And then you have you know,
you also have the other roles in there, the you know,
(25:56):
the scavengers, the decomposers, and so forth. Now, it's pointed
out by sakoy at All in The Life Hidden Inside Caves,
published twenty twenty in the International Journal of Ecology. Cave
ecosystems are intrinsically devoid of primary productivity due to the
absence of light. Again, light may enter the opening of
(26:17):
a cave to different degrees, and other caves may feature
open areas or areas with sort of like naturally occurring skylights.
But past those pools of light there is only darkness,
and that means again no photo autotrophic organisms.
Speaker 3 (26:32):
Right, So this raises a question of how could there
really be anything like an ecosystem inside a cave unless
I don't know, it was just like unless things were
coming and going from the outside to the inside constantly
and eating each other in between. You know, how could
there really be anything sustained within the cave purely in
the dark.
Speaker 1 (26:52):
And that's where bats enter the picture. We've of course
talked about bats before. You have various species of bats
that roost in cave and viral and do so in
vast numbers. So batguano instead offers a major food source,
a kind of alternate sunshine, brown sunshine if you will,
(27:12):
that falls upon the floor of caverns where insectivore or
frugivorous bats roost. So yeah, batsidiat insects bats that eat fruit.
So we're talking about bat species that are generally nocturnal
or crepuscular, that go out and feed and then return
to the seclusion of their caves where they roost on
the ceiling and when they poop, they poop on the
floor below them, and that poop brings in quite a
(27:35):
few nutrients.
Speaker 3 (27:37):
So it's not like the base the photo autotroflayer outside,
because in that case it is that would be organisms
that are fundamentally synthesizing chemical food energy out of what
was originally inorganic sunlight energy. In this case, they are
bringing energy in already in the form of chemical food
(27:59):
energy pooping it out on the floor, but it becomes
like a new base layer of a food chain within
the cave.
Speaker 1 (28:07):
Exactly. Yes, And I will also add it is like
sunlight in that you can replace the word sun or
sunlight with bat guano in any song lyric and it
will work just as well. So please feel free to
try that on your own time.
Speaker 3 (28:21):
I'm walking on guano.
Speaker 1 (28:23):
Yeah, yeah, yeah, it works.
Speaker 3 (28:25):
It works perfectly, don't I feel good?
Speaker 1 (28:29):
So this guano often serves as the primary renewable organic
resource of these caves, and a whole food chain extends
from this, supporting various bacteria, fungi, uh protus, and small arthropods. Now,
according to Sokoi, at all, the exact blend of nutrients
is going to vary depending on broad category and specific
(28:51):
species of bat. But the end result is that the
fields of poop beneath the bats just become teeming with life,
life enough to support an ecosystem of organisms, including those
visiting from the outside, other creatures that spend part of
their time in the caves, and also of course obligate
cave dwellers that are there all the time. Also, this
(29:12):
was interesting, the fermentation of the biomass, along with the
presence of all those warm blooded bat colonies actually heats
up the caves.
Speaker 3 (29:20):
Oh that's interesting. You know, that's a difference that is
acknowledged in some of the literature I've looked at, but
we haven't really talked about much. Which is a difference
between cave environments and the surface is not only the
lack of sunlight, but a much more constant temperature than
you get on the surface. Though I guess this could
be changed if yeah, you're bringing in a lot of biomass,
and that's sort of like warming up the cavern.
Speaker 1 (29:42):
Yeah, like at base level, it's like life in the
wine cellar. But then if you get enough life in
the wine cellar, well things they can have elevated temperatures,
but still going to be pretty dependable. It sounds like, though,
I guess you do have to factor in that there
are sometimes fluctuations in these the occupation by bats and
(30:02):
so forth.
Speaker 3 (30:03):
I do have to apologize I'm only half following the
conversation now because my brain is just running through song
lyrics like I've been waiting so long to be where
I'm going in the guano of your love of.
Speaker 1 (30:16):
Guano on my shoulder shoulders makes me happy. Yeah, yeah,
I'm want to read a quote here from the sequoiad
All paper that gets into some of the details here
of the guano quote. For example, small metazoins such as mites, pseudoscorpions, beetles, thrips, mites,
and flies inhabit the guano of insectivorous bats, whereas the
(30:37):
guano of frugivorous vat bats is frequented by spiders, mites, isopods, millipedes, centipedes, waiters,
bark lice, and insects. Salamander and cavefish populations and invertebrate
communities also rely heavily on nutrients from the bat guano.
They also point out that bat guano constitutes a niche
(30:57):
of several varieties of micro organisms. Includeing fungi, protus, lichen viruses,
and bacteria.
Speaker 3 (31:04):
I wonder if the bat guano is primarily sweet or savory.
Speaker 1 (31:08):
I guess we'd have to ask those blind cavefish from earlier.
Speaker 3 (31:11):
Yeah, though, to be fair, in that quote, it just
said that the cavefish populations rely on nutrients from bat guano.
I don't know if that means they eat it directly
or they eat other things that eat it, or what.
Speaker 1 (31:23):
True true that that is essential and I believe that
something sakoy at All point out is that it's yeah,
there aren't necessarily there are organisms that depend on the
guano that are not directly eating the guano, but they
are able to thrive on the things that do consume
and thrive on the guano directly. Now, it's it's also
worth noting that the presence of the bats themselves also
(31:45):
produces feeding and opportunities for either scavengers or predators. I'm
going to probably have more on this particular tidbit in
the next episode. And yeah, there's another interesting thing that
they point out is all of this can potentially change
over time as well, you know, stable environments but not
necessarily eternal. I was reading a CBC Radio story about
(32:07):
a University of Ottawa study of bat guano in a
specific Jamaican cave or cave system. I believe this cave
is known as Home Away from Home cave. It's very
remote and it has been a subject of some scientific
study and they were looking at it and they were
able to observe a shift over in the past from
(32:27):
insect eating bats to fruit eating bats, though it was
unclear as of twenty twenty one if this was a
change in diet by a specific species or perhaps more
likely the influx of a different bat species that had
a different diet. They also found increased guano levels of cadmium, mercury, lead,
and zinc present during the same time as the Industrial Revolution,
(32:52):
so they were getting into like, you know, we can
see this change, We can see this environmental change brought
on by the Industrial Revolution in the of these bats
in this remote Jamaican cave.
Speaker 3 (33:03):
Heavy metal guano.
Speaker 2 (33:05):
Yeah.
Speaker 1 (33:06):
Now, of course we have various other creatures besides the bat,
but also from the era of bats to consider that
are now extinct that would have entered into caves and
and would have defecated. We have like the cave bear,
that is extinct, the extinct cave hyena copy lights from
(33:26):
both of these species. Fossilized fecal matter has been discovered
in caves. Yeah, now another tidbit, and this is something
you could definitely go in deeper on. We could come
back to another episode in the future at some point.
Sequoiat All point out that the other thing about bat
guano is that human beings figured out that, hey, this
(33:48):
stuff has value. So cave environments have also long been
exploited by human beings due to its economic value as
a fertilizer and also, for at least for a while there,
it was harvested to produce gunpowder. And all of this
can impact these cave environments. Here's another interesting thing. Bats,
(34:10):
the bats that enter into these caves, these caves, they
are not set in stone. They are changing. We just
discuss how these caves form over geologic time in the
first episode. But the bats by roosting in the caves
physically change them roosting on the ceiling with their little claws,
(34:30):
so that physically changes the cave. And then also there's
chemical augmentation of the caves via their urination. Because yeah,
they're going both number one and number two in that
cave system and it does have an impact.
Speaker 3 (34:43):
Oh do you know if the urination does it primarily
like build structures or dissolve parts of the cave.
Speaker 1 (34:50):
I believe it is more dissolving. Yeah, okay, And again
we're dealing often with limestone cave systems and so forth.
So that's my understanding here, because I know what you're thinking.
We end up with actual like.
Speaker 3 (35:05):
Bat piece stalagmites.
Speaker 1 (35:06):
Yeah, bat piece stalagmites. I do not have an answer
to that, but I think it is more of a
dissolving of the cave system that is in play here.
Speaker 3 (35:15):
Wait a minute, hold on, I just googled bat piece
stalagmites with large numbers of bats, thick and hard stalactites
and stalagmites of crystallized bat you'rein occasionally form.
Speaker 1 (35:25):
Well, there you go. Okay, so that is also possible.
It's just a wonderful world down there. It's a whole
new world.
Speaker 3 (35:33):
Sorry, I should say, since I read that directly, that
was from something called the Internet Center for Wildlife Damage Management.
I don't know what that is, but that's what they claim.
Speaker 1 (35:41):
Yeah, there was another art. I didn't get into this
article a lot. Maybe I could come back more in
the next episode. But there was a twenty twenty one
article in The New York Times titled how bats and
their poop erase ancient cave art, and that one got
into this issue of it because again they're changing the caves,
and sometimes that can change things that prehistaric humans did.
Speaker 3 (36:02):
Wow.
Speaker 1 (36:03):
They point out that large quantities of the urine and
the guano it ferments, it can saturate the air with
quote aerosolized particles of phosphoric acid. So it's a rich
world down there. Goodbye horsetoodle. Now an interesting and largely
unanswered question for me, they would get some answers on it.
(36:28):
Pertains to the question what might cave environments have been
like before the evolution of cave roosting bats, because again,
bats are mammals, and mammals have not in their highly
successful mammals, they've been around for a while, but they
haven't always been around. So what would have potentially pooped
up these caves and sustained these ecosystems before bats.
Speaker 3 (36:50):
That's a good point. So like the base layer of
the food, the food web within the cave is emerging
from back guano, and there were at a time, no
back to bring the guano in. Could there be an
ecosystem in the cave at all?
Speaker 1 (37:04):
Exactly? Yeah, And I think by and large the answer
is yes, there would have been things that have done that.
And we know that in part because even today bats
are not the only trogs to poop in a cave.
There are considerable factors, but crickets are often brought up
as another major cave pooper that allows the sun to
(37:25):
shine in these lightless places. So it would seem that
prebat organisms would have taken advantage of the cave niche
to do, you know, to whatever degree they could. One
of the things about investigations into cave fossils is that
fossils that you find in deep caves aren't necessarily telling
you about subterranean life. Tracks in the floor or even
(37:48):
ceilings of deep caves may actually be insightful about life
along shorelines of the surface. You know. That's the degree
of time that we're dealing with, and considering the likes of,
for instance, a five hundred meter deep that's a third
of a mile deep cavern in France. I was reading
about this is and I may be butchering French here,
(38:09):
likely am castle book cave. Here, for instance, you'll find
dino footprints that were first etched in mud or sand
on a beach one hundred million years ago, and they've
just been gradually forced underground over time.
Speaker 3 (38:24):
So it's not like they were formed in the cave.
These are rocks that were formed on the surface and
then they later emerged in a cave.
Speaker 1 (38:32):
Yeah, and kind of similar to like, oh, you can't
find a trillobte in a mountain and be like these
were the mountain trilobites. It's not really what's happening. But
of course this doesn't mean we don't have evidence of
dinosaur age cave dwellers. As reported by Nature in twenty twenty,
cockroaches preserved in amber from ninety nine million years ago
(38:52):
are likely the oldest evidence we have of organisms evolved
for life in a cave environment.
Speaker 3 (39:00):
Million year old cave cockroaches. What were they like?
Speaker 1 (39:03):
Oh, they were they were kind of scary. I included
an image here of one for you, Joe. This is
one of the species that they were looking at, and
you'll see that it has front appendages, these kind of
like raptorial four legs, much like a praying mantis, which
means they were likely a highly predatory species in their
(39:23):
cave environments. MM. So that's that's one of the features
that they found on these these amber preserved specimens. There's
lost coloration, there's reduced wing and eye size, elongated antennae,
and and reduced leg spines for passive defense. So two
species were discovered in amber in this cave from me
(39:46):
and mar, one of which apparently had these raptorial four legs,
meaning it was it was likely primarily a predator. And
according to a paper in Cosmos magazine by James Urckhart,
one of the one of the interesting things about this
find is that the specimens in question would seem to
have mid Cretaceous origins, but all cave animals living today
(40:09):
have a late Sinozoic origin. Now, why these older cave
organisms died out is apparently a mystery, because it would
seem like an ideal place to shelter and survive surface
world extinction events, but obviously that's not the case. The
article also stresses that it's not impossible that these cockroaches
may have survived even into modern times in one form
(40:32):
or another, because our understanding of living insects is of
course incomplete, so it's not impossible even though we're gazing
backwards through time with this particular specimen, its genetic legacy
could still be alive today. The article here is quite good.
It also stresses some of the larger challenges of understanding
(40:52):
such ancient cave environments in figuring out what they consisted of.
Other Mesozoic cave organisms, it certainly existed, but either we
get into the same problems we encounter with the inherent
incompleteness of a fossils in general on the fossil record,
because either a cave system collapsed long ago or key
(41:15):
to this example, didn't allow fossil preservation. This case, amber
containing cave organisms is rather unique because a tree is
not going to grow in said cave. They say this
is kind of like a one in a million fine
because it probably depended on resin from a tree growing
(41:36):
directly in the mouth of a cave, like in exactly
the right place, exactly the wrong right time, in order
to capture this creature that otherwise would not be venturing
out of the caves to climb around on trees.
Speaker 3 (41:48):
Yeah, that's interesting, and I was wondering I was going
to ask about exactly that like, how does a cave
cockroach get trapped in amber? So I guess the answer
is probably very rarely happened. We're incredibly lucky to have
this unique find.
Speaker 1 (42:01):
Yeah, and it just raises the question, like what else
would have been down there that we didn't crawl into
the amber. You know, there's again just so many mysteries
within within the fossil record, and back to the earlier point,
so many mysteries remaining and just sort of the in
the the existing organic world. You know, we're still making
(42:22):
discoveries regarding insects, cave environments, of cave ecosystems. It's I mean,
it's really exciting.
Speaker 3 (42:31):
No doubt, And you know what, I think we're going
to have to end today's episode there, but we've got
more cave stuff to talk about. So we're going to
be back with part three definitely.
Speaker 1 (42:40):
That's right. We're gonna get into some cave creditors and
who knows what else. All right, stuff to blow your mind.
As you know. Core episodes are on Tuesdays and Thursday.
We're primarily a science and culture podcast, and then on
Mondays we do listener mail. On Wednesdays we do a
short form episode, and then on Fridays we set us
(43:01):
on most serious concerns to just talk about a weird
film on Weird House Cinema. We bust out a rerun
on the weekends.
Speaker 3 (43:07):
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 (43:29):
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're listening to your favorite shows.
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lambin. Today we have a vault episode for you.
Of course, this is going to be our second episode
in the four part Life in the Hypogean World series.
So without further ado, let's dive right.
Speaker 2 (00:21):
In Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 1 (00:34):
Hey, welcome to Stuff to Blow your Mind. My name
is Robert.
Speaker 3 (00:37):
Lamb and I am Joe McCormick. And hey, we are
back with part two of our series on cave ecosystems
and Hypogean biology. Cave biology. In the last episode, we
talked about some of the characteristic features of cave ecosystems
and then we ended up discussing how lightless cave environments
(00:58):
shaped the evolution of a creature called Astianax Mexicanus, or
the blind Mexican cavefish, also known as the Mexican tetra,
of which many populations have adapted to life in subterranean
waterways by losing their eyes and the pigments in their flesh.
(01:19):
We ended up kind of doing a deep dive on
the evolutionary logic of this why an animal population that
once had eyes and skin pigment would adapt over many
generations to lose those traits in a cave. So if
you haven't heard part one yet, you should probably go
back and check that one out first. But we are
back again today to talk about some more elements of
(01:39):
cave biology. Specifically, one of the things I wanted to
get into we didn't really have time for last time,
was something else about blind Mexican cavefish, which is, if
they have no sight, what do they do? How do
they navigate their environment and forage for food?
Speaker 1 (01:56):
And this is key because they've got to eat something.
These are extreme environments that are generally regarded as not
being the most bountiful places to scrape out a livelihood
as an organism.
Speaker 3 (02:09):
Right, and there might be some advantages to living in
the cave if you are a fish like this, Like
there might be fewer predators than you would encounter in
the world above, but there are also fewer food resources,
so you know, you have to have to kind of
like shift your specialization.
Speaker 1 (02:27):
Yeah, Like, if I was to decide, again this is
a non evolutionary example, but if I was to decide
I am going to now live in my local Ikea store, well,
you know, I've got to figure out some new things. Right,
It's going to be easy to find a bed when
the lights go out, but I'm going to have to
deal with the night security. My diet is going to
consist entirely of Ikea food. So if you like Lingenberry, yeah, yeah,
(02:51):
make sure you don't have a Lingenberry allergy for sure.
Speaker 3 (02:55):
Yeah. So I wanted to explore this question of how
do blind cavefish since their surroundings, and so I was
looking into this and I discovered that apparently one major
sense mechanism that they rely on is what's known as
the lateral line system. And this is not a sense
that is unique to blind cavefish. The lateral line system
(03:18):
is found in lots of aquatic vertebrates, even those that
can see. Though in blind fish there are usually enhancements
to this system. So it's a sense that they already have,
but it gets stronger in the cave evolved variants. As
a side note, I always really enjoy imagining types of
senses that humans don't have, Like what it would be
(03:41):
like to have a different kind of sense, you know,
like electro reception or magnetic perception or something like that.
And this sense in particular, I'm very excited to try
to imagine because it's very spooky trying to imagine it.
It's something about it feels almost kind of Halloween. This
might make more sense once I explain it. So, the
(04:02):
basic function of the lateral line system is to detect
movements in the water surrounding the animal through high sensitivity
to changes in water pressure. This system it runs along
the length of an animal like a fish, so there'll
be sort of canals of the lateral line sensing organs
(04:24):
around the head of the fish and then running lengthwise
along the body. The system uses sensory organs known as
neuro masts, which contain little hairs suspended within a kind
of jelly filled capsule that bends in response to changes
in water pressure and flow. And then these little hairs
(04:48):
are connected to nerve cells that are strung along the
lateral line system like Christmas tree lights. Sometimes the neuromasts
are exposed to the water on the outside of the skin.
They're just right on the outside of the skin, but
other times they are contained within a kind of duct
or canal that is just under the surface of the skin.
(05:10):
And either way, the purpose of the lateral line system
is to detect vibrations, movements, and objects within the water
by sensing these little changes in water pressure and flow.
So with this system, an animal can get feedback about
its own movement through the water, for one thing, but
(05:31):
it can also sense the presence of currents and nearby
solid masses, either moving or stationary, by the way they
displace water as the fish moves through it. And this
is coming back to what I was saying a minute ago,
something about trying to imagine this is. Oh, it's very
evocative and a little bit scary, almost so imagining living
(05:54):
in a place of dark water. But you can sense
objects around you by the way they move the water,
or by the way they change how the water moves
around you.
Speaker 1 (06:04):
Wow, I mean, we're getting right back into that Gollumn territory,
you know. I'm imagining Gollum sensing that that goblin that
has come down and is washing its hands in the
water and so forth on this lightless underground lake. And
to your point, it is so alien to try and
imagine this. I'm in the water a fair amount. I
swim laps most morning, though swimming this morning, and it's
(06:28):
still you know, it's a very visual exercise. You can
set you can feel things in the water, obviously you can,
and Lord knows, there's a lot to see and feel
in a ymca pool. You know, there are people doing
exercise classes, there are people swimming laps next to you,
sometimes very often in the same lane with you, and
(06:48):
you pick up on those movements, but nowhere near this
level of detail. You know, this is like compared to
what we have. This is like a second site.
Speaker 3 (06:57):
Yeah, I kind of ghostly vision through which you can
sense things around you by the way they move the
water against your body.
Speaker 1 (07:05):
Yeah.
Speaker 3 (07:06):
So anyway, the astianax fish, the Mexican blind cavefish apparently
compensate for their lack of vision by having a more
sensitive lateral line system than their surface dwelling cousins, and
this helps them navigate, forage and survive in their lightless environment.
But this way of living depends not just on heightened
(07:28):
sensitivity to water displacement, but also on changes to behavior.
For example, one thing I was reading about is that
when preparing to mate, apparently male and female blind cavefish
engage in this in these patterns of exaggerated movements of
various body parts like the mouth and the gills, and
(07:51):
these movements are thought to perhaps help the mating partners
find and identify one another without visual cues, does some sense,
like moving around in the water more so that they
can locate one another. Of course, the fish also have
changes to their metabolism to allow them to survive in
a place where food is less abundant than it is
on the surface, so they are thought to have slower
(08:14):
metabolisms to need less food energy. But also I was
reading about an interesting twenty seventeen study published in PLUS
one by some researchers associated with the University of Cincinnati,
and this was on the role of a symmetry in
blind cavefish evolution. We actually did a whole series on
(08:36):
asymmetry and animals a while back in which we talked
about fiddler crabs and such, you know, fiddler crabs having
one claw much much bigger than the other. But we
talked about a lot of examples in the animal world,
and I remember that being a very interesting series.
Speaker 1 (08:49):
Yeah, that was a fun one for sure. You know,
some of the outrageous examples in some of the less
obvious examples of asymmetry.
Speaker 3 (08:58):
Apparently there is a bit of hidden asymmetry in the
blind Mexican cavefish as well. So let's see. This study
was by Amandicaate Powers Aeronym Davis, Shane A. Kaplan and
Joshua be Gross, published in Plus one in twenty seventeen,
and it was called Cranial asymmetry arises later in the
Life History of the blind Mexican cavefish Astianax Mexicanus. And
(09:21):
so we were talking in the previous episode about how
in some of these cavefish populations the fish are not
without eyes from the very beginning. Rather, they do grow
eyes initially in embryonic development, but then the eyes go
through what's called regression where they are absorbed and disappear
as the fish grows, and then the bone around the
(09:43):
eye socket collapses in by the time the fish as
an adult. Young cavefish apparently start life with fairly symmetrical bodies,
and the surface variants in the rivers above where there's
plenty of light also have fairly symmetrical bodies even as adults.
But the young cavefish start out fairly symmetrical, and then
(10:04):
as they mature, a mismatch develops in how the bones
on either side of their skull take shape, and this
asymmetry in their cranial bones seems to match a difference
in behavior between the cave variant of the tetras and
the surface variant still has eyes. The surface variant, when
you place it in a fish tank, will swim in
(10:25):
more random patterns or will just kind of float without
moving in the shaded portions of the tank. The cave variant,
on the other hand, will just keep it'll keep swimming
in circles around the edges of its tank, and this
matches with the fact that has been observed in the wild.
In its natural environment, the cave variant will tend to
(10:48):
follow the rocky walls of the pool where it lives,
swimming counterclockwise around these boundaries in an endless loop. And
one of the studies authors, Amanda Powers, was quoted in
a press release about the research, explaining quote, you could
see how asymmetry might be an advantage in navigation. They
(11:08):
tend to swim in a unidirectional circular motion around their
tanks to explore their surroundings. Having asymmetry in their skull,
we think is attributed to handedness. If their skull is
bent to the left, they could be right handed. They're
feeling the wall to the right with their sensory structures.
Oh wow, Yeah, so it seems that the cavefish have
(11:30):
a type of developmental handedness that correlates to a navigation
behavior where they follow the walls of their natural enclosure
in this slow, counterclockwise lapping motion. Though I think it's
still not fully understood how this system emerges, if it
helps them survive how, we don't fully know yet, but
(11:53):
it's interesting to imagine why. Another interesting sort of unrelated
thing that I was just reading in this press release,
also quoting the same author of the paper, I mean powers,
talking about how she and colleagues went into the caves
to observe these fish, and she says in this article quote,
whenever you would touch the surface of the water with
(12:13):
your finger, a swarm of cavefish would come right up
to it. Not many fish would do that. These cavefish
have zero predators, so they're not afraid.
Speaker 1 (12:23):
Oh wow, that you know. That reminds me of other
examples we've talked about in the show, like various birds
of the Galapagos Islands, for example. It reminds me of
some stuff I was just reading the other day about
the dodo bird. You know, where you have these organisms
that adapt over evolutionary time to an environment where there
(12:44):
are no predators or there is just an entirely different
predation situation going on, and yeah, they just lose their fear.
There's no need to engage in that kind of protective
behavior because those things don't exist for them until they
show up on a bub Yeah.
Speaker 3 (13:02):
Yeah, dutch. Okay, So that's lateral line sensations, and then
the asymmetry of the skull, the kind of the circling behavior,
all those differences. But there was one more thing I
wanted to talk about. I mentioned in the previous episode
another sensory enhancement found in some of these cavefish, and
that sensory enhancement was taste. It appeared to be involved
(13:25):
in a pliotropy. Remember that's where a single genetic change
would result in multiple different changes in the phenotype in
the body or the behavior. And in this case, the
idea was that there was a single genetic change that
would result in both regression of the eyes. So this
process we talked about where the eyes are sort of
(13:47):
absorbed and the adult fish doesn't have eyes, and then
along with that development of a greater number of taste buds. Now,
we speculated generally in the last episode that you know,
having enhanced other senses in a cave would probably be
useful in some way. Taste buds are generally useful. They
are our body's chemistry set to know what we're putting
(14:09):
in the digestive tract. But I was wondering if there
was anything else we could know about that, like how
do these extra taste buds work in these fish? What
kind of benefit do they provide? So this is another
thing where the answer is not fully known yet we
know a few kind of interesting morsels. So I came
across a very recent paper addressing this question. It was
(14:31):
by Daniel Burning and Joshua B. Gross published in Frontiers
and Ecology and Evolution twenty twenty three, called the Constructive
Evolution of Taste in astianax Cavefish or Review. And one
of the things I thought was interesting here, Rob, I've
attached a I've attached a diagram for you to look at.
It seems that mostly what we're talking about are what's
(14:53):
called extra oral taste buds, taste buds that are not
inside the mouth like hours, but outside the mouth and
spread out over the jaws in the front of the face.
So in this diagram, it shows blind cavefish from a
couple of different populations compared to the surface variant with
(15:14):
taste buds filled in as red dots on the illustrations.
And while the surface variant looks like it is wearing lipstick,
you know, the red dots, the taste buds are on
the mouth, the cave fish are wearing clown makeup. The
taste buds are all over the face.
Speaker 1 (15:30):
Yeah, it's quite impressive, And again it makes sense for
this kind of an environment, you know, you need to
lean into a different sense situation. It reminds me a
bit of our past discussions about catfish being super tasters,
you know, and really when and again it come back
to that situation where we have our human understanding of
(15:52):
the senses, but we have a human level understanding of
those senses. And so when you're talking about something like
this that is enhanced, it's not taste as we know it,
it's something different.
Speaker 3 (16:06):
Yeah, exactly. So I was wondering, like, what do we
know about what kind of difference this makes? What difference
does it make in the lives of these fish? So
there were a few things mentioned in this review. One
is that multiple studies have found what appears to be
enhanced sensitivity to chemical repellence and taste sources in the
(16:27):
water in the cavefish when compared to the surface fish.
So just one example is a study by Humbach in
nineteen sixty that found in the cave fish compared to
the common minno, that the modality for bitter sensation was
three hundred times more acute roughly, and then the salty
(16:49):
slash acid slash sweet modality difference was about two thousand
to four thousand times more acute in the cavefish, so
much greater, much lower I guess threshold of sensitivity to
these tastes also protus at all. In two thousand and
eight found quote amino acids dissolved in system water were
(17:11):
detected at a much lower concentration in Paschone cavefish. That's
from one particular cave source paschone cavefish, compared to surface fish.
And one possible explanation the authors discuss for this heightened
sensitivity to amino acids could be what they call the
savory taste receptor T one R one. This is the
(17:34):
receptor that binds to glutamate and helps us taste savory
umami flavors. So I wonder if you could really get
these cavefish going bananas over something in the water, a
piece of food or whatever is dispersed in the water
if you hit it with a bit of the good
old MSG.
Speaker 1 (17:53):
I bet, or maybe one of those little fish shaped soy.
Speaker 3 (17:58):
Sauce. Yeah, we get soy sauce, some parmesan cheese, some tomatoes,
some MSG, all the good savory things. However, the authors
do say that several amino acids were used in the
study that found this, so there could be other explanations,
possibly also involving old faction rather than taste. But anyway,
(18:18):
what difference would these these taste bud arrangements and heightened
sensitivity to flavors make in terms of the behavior of
the fish the author's right quote. One recent study argued
that external taste buds are used for preliminary assessment of
food items during random swimming or targeted searches for food.
(18:41):
Extra oral taste buds thus carry in importance for determining
whether to pursue or avoid a food item. So the
way I understand that is it's heightening the fish's foraging
efficiency basically by saying, before you even get something in
the mouth, you're tasting the water around this thing with
(19:03):
the front of your face, and it helps you zero
in more quickly on something that is good to eat
or is not good to eat. Like you're getting you're
getting an idea about whether you're coming close to a
good piece of food or not earlier. So that would
make your foraging more efficient.
Speaker 1 (19:19):
Okay, that makes sense.
Speaker 3 (19:21):
In the end, however, they do say that that this
is just an area that hasn't had enough research yet.
They say that the quote precise function of cavefish extraoral
taste buds remains unclear and largely unexplored, and then they
also say that future studies need to do things like
excluding the role of oldfaction or smell and isolating the
(19:43):
variable of taste. And there are differences that even remain
that they did examine in the paper that we just
don't know how to explain yet, but could possibly be
related to taste. One interesting idea they bring up is
the feeding angle, so they you know, if you look
at the cave variants of these fish versus the surface variants.
(20:06):
One thing you will notice is that they both go
along the bottom. Might there might be like a stony
or sandy or pebbly bottom of the water source where
they are, and they'll go along the bottom with their
mouth down to the bottom, kind of searching for little
bits of food to eat or you know, prey in
the case of you know, carnivory, or just little bits
(20:27):
of dead organic matter, whatever it is they're coming across
to eat. They're scouring the bottom for it. For some reason,
the surface fish have a steeper angle that they forage
at with their body more it's almost more totally vertical,
whereas the cave fish tend to forage at a more
slanted angle that's closer. It's more like fifty five degrees
(20:50):
from the bottom versus more like eighty degrees in the
surface variant. They say this could be related to the
changes in extraoral taste buds. Maybe it's you know, like
they're tasting things differently, thus they have to orient their
bodies differently. Don't know, but interesting question.
Speaker 1 (21:09):
Yeah, yeah, I mean this this is fascinating to think about,
Like it makes one wonder if it has something to
do with their being more predation opportunities for the surface fish,
you know, like you've got to come in at that
steeper angle because there's a greater risk of things, you know,
on the floor of the of the sea or what
(21:32):
have you, that might come after them while they're feeding.
And maybe those are absent in these cave environments. Again,
you know, we're talking about like a lack of just
biodiversity in those environments. You know, there's many fewer predators,
et cetera. But who knows. That's just me spitballing.
Speaker 3 (21:48):
I don't want to act like I'm just look at
a diagram and a paper and answer the question. But
just one thing I wonder about is looking at the
looking again at the diagram of where these extraoral taste
buds are in the surface fish. Remember they're all like
right at the mouth, but in the in the cave variant,
a lot of them tend to be spread out along
the lower jaw. So I wonder if it can come
(22:11):
in at this shallower angle because it's sort of like
feeling and tasting more along the bottom without having to
get the front of its mouth in contact with it.
It can kind of taste with the slope of its
lower jaw.
Speaker 1 (22:24):
Yeah, that would make a lot of sense.
Speaker 3 (22:25):
But yeah, again don't know interesting questions about Usually when
you when you think about fish and taste together, you're
thinking about what a fish tastes like to you, not
what it's like to taste as a fish.
Speaker 1 (22:36):
Yeah, which again it can put you an entirely different
sense realities, especially considering things like a catfish. But also
you know, getting into taste and smell. When whenever I
read information about, you know, the distances at which fish
in the ocean can detect other things going on, say
blood in the water or some sort of rotting shoe,
(23:01):
I'm always just amazed because again it is an experience
of the ocean that is just unlike anything we can
experience when we venture into it. And the same holds
true of the cave. Like when we enter into the cave,
our experiences are rather different compared to the organisms that
have evolved to thrive there. Now the place I'd like
(23:32):
to turn to next. This gets into something that was
probably one of the key reasons I decided this would
be a good series for us to do, you know,
to return to cave biology. This and the fact that
I was also inspired by some environment related stuff at
the Bishop Museum on a Wahuo recently when I returned
(23:53):
there and got to take in their natural history section
in addition to their cultural and historical sections. But yeah,
this is the revelation that really pushed me over the
edge that for many cave environments, batguano is sunlight. Ooh, batguano,
that's bat poop if you will, if you're not familiar
(24:16):
essentially in these environments, yes, this is the light of
the sun. This plays a vital role in the food
chain because remember light is the first step in the
food chain of the Epigean world. Sunlight reaches the surface
where photo autotrophic organisms create food via sunlight, carbon dioxide
(24:38):
and water. These are autotrophs, and we call this the
trophic level of the food chain.
Speaker 3 (24:44):
Right, So yeah, this came up in the last episode
that almost all of the food chain where we're really
familiar with is happening where the base layer of the
food chain is using energy from the sunlight to make
its food and then we of course you know, other
organisms eat those organisms and on and on it goes.
But if you don't have the sunlight to power the
(25:06):
food synthesis at the base layer of the food chain,
what do you do.
Speaker 1 (25:10):
That's right, yeah, yeah, most of it depends on sunlight.
Plants are autotropes, as are seaweed phytal blanked in some
kinds of bacteria, including bacteria that produce their own food
vio chemosynthesis around volcanic vents. But most of the producers
are producing their own food via the process of photosynthesis
(25:32):
and therefore require the bounty of sunlight. The next trophic
layer of the food chain is, of course the primary
consumers who eat the producers, the herbivores being chief among them.
And then the predators compose the third trophic layer, the
secondary consumers, followed by the tertiary consumers and on up
to the apex predators. And then you have you know,
you also have the other roles in there, the you know,
(25:56):
the scavengers, the decomposers, and so forth. Now, it's pointed
out by sakoy at All in The Life Hidden Inside Caves,
published twenty twenty in the International Journal of Ecology. Cave
ecosystems are intrinsically devoid of primary productivity due to the
absence of light. Again, light may enter the opening of
(26:17):
a cave to different degrees, and other caves may feature
open areas or areas with sort of like naturally occurring skylights.
But past those pools of light there is only darkness,
and that means again no photo autotrophic organisms.
Speaker 3 (26:32):
Right, So this raises a question of how could there
really be anything like an ecosystem inside a cave unless
I don't know, it was just like unless things were
coming and going from the outside to the inside constantly
and eating each other in between. You know, how could
there really be anything sustained within the cave purely in
the dark.
Speaker 1 (26:52):
And that's where bats enter the picture. We've of course
talked about bats before. You have various species of bats
that roost in cave and viral and do so in
vast numbers. So batguano instead offers a major food source,
a kind of alternate sunshine, brown sunshine if you will,
(27:12):
that falls upon the floor of caverns where insectivore or
frugivorous bats roost. So yeah, batsidiat insects bats that eat fruit.
So we're talking about bat species that are generally nocturnal
or crepuscular, that go out and feed and then return
to the seclusion of their caves where they roost on
the ceiling and when they poop, they poop on the
floor below them, and that poop brings in quite a
(27:35):
few nutrients.
Speaker 3 (27:37):
So it's not like the base the photo autotroflayer outside,
because in that case it is that would be organisms
that are fundamentally synthesizing chemical food energy out of what
was originally inorganic sunlight energy. In this case, they are
bringing energy in already in the form of chemical food
(27:59):
energy pooping it out on the floor, but it becomes
like a new base layer of a food chain within
the cave.
Speaker 1 (28:07):
Exactly. Yes, And I will also add it is like
sunlight in that you can replace the word sun or
sunlight with bat guano in any song lyric and it
will work just as well. So please feel free to
try that on your own time.
Speaker 3 (28:21):
I'm walking on guano.
Speaker 1 (28:23):
Yeah, yeah, yeah, it works.
Speaker 3 (28:25):
It works perfectly, don't I feel good?
Speaker 1 (28:29):
So this guano often serves as the primary renewable organic
resource of these caves, and a whole food chain extends
from this, supporting various bacteria, fungi, uh protus, and small arthropods. Now,
according to Sokoi, at all, the exact blend of nutrients
is going to vary depending on broad category and specific
(28:51):
species of bat. But the end result is that the
fields of poop beneath the bats just become teeming with life,
life enough to support an ecosystem of organisms, including those
visiting from the outside, other creatures that spend part of
their time in the caves, and also of course obligate
cave dwellers that are there all the time. Also, this
(29:12):
was interesting, the fermentation of the biomass, along with the
presence of all those warm blooded bat colonies actually heats
up the caves.
Speaker 3 (29:20):
Oh that's interesting. You know, that's a difference that is
acknowledged in some of the literature I've looked at, but
we haven't really talked about much. Which is a difference
between cave environments and the surface is not only the
lack of sunlight, but a much more constant temperature than
you get on the surface. Though I guess this could
be changed if yeah, you're bringing in a lot of biomass,
and that's sort of like warming up the cavern.
Speaker 1 (29:42):
Yeah, like at base level, it's like life in the
wine cellar. But then if you get enough life in
the wine cellar, well things they can have elevated temperatures,
but still going to be pretty dependable. It sounds like, though,
I guess you do have to factor in that there
are sometimes fluctuations in these the occupation by bats and
(30:02):
so forth.
Speaker 3 (30:03):
I do have to apologize I'm only half following the
conversation now because my brain is just running through song
lyrics like I've been waiting so long to be where
I'm going in the guano of your love of.
Speaker 1 (30:16):
Guano on my shoulder shoulders makes me happy. Yeah, yeah,
I'm want to read a quote here from the sequoiad
All paper that gets into some of the details here
of the guano quote. For example, small metazoins such as mites, pseudoscorpions, beetles, thrips, mites,
and flies inhabit the guano of insectivorous bats, whereas the
(30:37):
guano of frugivorous vat bats is frequented by spiders, mites, isopods, millipedes, centipedes, waiters,
bark lice, and insects. Salamander and cavefish populations and invertebrate
communities also rely heavily on nutrients from the bat guano.
They also point out that bat guano constitutes a niche
(30:57):
of several varieties of micro organisms. Includeing fungi, protus, lichen viruses,
and bacteria.
Speaker 3 (31:04):
I wonder if the bat guano is primarily sweet or savory.
Speaker 1 (31:08):
I guess we'd have to ask those blind cavefish from earlier.
Speaker 3 (31:11):
Yeah, though, to be fair, in that quote, it just
said that the cavefish populations rely on nutrients from bat guano.
I don't know if that means they eat it directly
or they eat other things that eat it, or what.
Speaker 1 (31:23):
True true that that is essential and I believe that
something sakoy at All point out is that it's yeah,
there aren't necessarily there are organisms that depend on the
guano that are not directly eating the guano, but they
are able to thrive on the things that do consume
and thrive on the guano directly. Now, it's it's also
worth noting that the presence of the bats themselves also
(31:45):
produces feeding and opportunities for either scavengers or predators. I'm
going to probably have more on this particular tidbit in
the next episode. And yeah, there's another interesting thing that
they point out is all of this can potentially change
over time as well, you know, stable environments but not
necessarily eternal. I was reading a CBC Radio story about
(32:07):
a University of Ottawa study of bat guano in a
specific Jamaican cave or cave system. I believe this cave
is known as Home Away from Home cave. It's very
remote and it has been a subject of some scientific
study and they were looking at it and they were
able to observe a shift over in the past from
(32:27):
insect eating bats to fruit eating bats, though it was
unclear as of twenty twenty one if this was a
change in diet by a specific species or perhaps more
likely the influx of a different bat species that had
a different diet. They also found increased guano levels of cadmium, mercury, lead,
and zinc present during the same time as the Industrial Revolution,
(32:52):
so they were getting into like, you know, we can
see this change, We can see this environmental change brought
on by the Industrial Revolution in the of these bats
in this remote Jamaican cave.
Speaker 3 (33:03):
Heavy metal guano.
Speaker 2 (33:05):
Yeah.
Speaker 1 (33:06):
Now, of course we have various other creatures besides the bat,
but also from the era of bats to consider that
are now extinct that would have entered into caves and
and would have defecated. We have like the cave bear,
that is extinct, the extinct cave hyena copy lights from
(33:26):
both of these species. Fossilized fecal matter has been discovered
in caves. Yeah, now another tidbit, and this is something
you could definitely go in deeper on. We could come
back to another episode in the future at some point.
Sequoiat All point out that the other thing about bat
guano is that human beings figured out that, hey, this
(33:48):
stuff has value. So cave environments have also long been
exploited by human beings due to its economic value as
a fertilizer and also, for at least for a while there,
it was harvested to produce gunpowder. And all of this
can impact these cave environments. Here's another interesting thing. Bats,
(34:10):
the bats that enter into these caves, these caves, they
are not set in stone. They are changing. We just
discuss how these caves form over geologic time in the
first episode. But the bats by roosting in the caves
physically change them roosting on the ceiling with their little claws,
(34:30):
so that physically changes the cave. And then also there's
chemical augmentation of the caves via their urination. Because yeah,
they're going both number one and number two in that
cave system and it does have an impact.
Speaker 3 (34:43):
Oh do you know if the urination does it primarily
like build structures or dissolve parts of the cave.
Speaker 1 (34:50):
I believe it is more dissolving. Yeah, okay, And again
we're dealing often with limestone cave systems and so forth.
So that's my understanding here, because I know what you're thinking.
We end up with actual like.
Speaker 3 (35:05):
Bat piece stalagmites.
Speaker 1 (35:06):
Yeah, bat piece stalagmites. I do not have an answer
to that, but I think it is more of a
dissolving of the cave system that is in play here.
Speaker 3 (35:15):
Wait a minute, hold on, I just googled bat piece
stalagmites with large numbers of bats, thick and hard stalactites
and stalagmites of crystallized bat you'rein occasionally form.
Speaker 1 (35:25):
Well, there you go. Okay, so that is also possible.
It's just a wonderful world down there. It's a whole
new world.
Speaker 3 (35:33):
Sorry, I should say, since I read that directly, that
was from something called the Internet Center for Wildlife Damage Management.
I don't know what that is, but that's what they claim.
Speaker 1 (35:41):
Yeah, there was another art. I didn't get into this
article a lot. Maybe I could come back more in
the next episode. But there was a twenty twenty one
article in The New York Times titled how bats and
their poop erase ancient cave art, and that one got
into this issue of it because again they're changing the caves,
and sometimes that can change things that prehistaric humans did.
Speaker 3 (36:02):
Wow.
Speaker 1 (36:03):
They point out that large quantities of the urine and
the guano it ferments, it can saturate the air with
quote aerosolized particles of phosphoric acid. So it's a rich
world down there. Goodbye horsetoodle. Now an interesting and largely
unanswered question for me, they would get some answers on it.
(36:28):
Pertains to the question what might cave environments have been
like before the evolution of cave roosting bats, because again,
bats are mammals, and mammals have not in their highly
successful mammals, they've been around for a while, but they
haven't always been around. So what would have potentially pooped
up these caves and sustained these ecosystems before bats.
Speaker 3 (36:50):
That's a good point. So like the base layer of
the food, the food web within the cave is emerging
from back guano, and there were at a time, no
back to bring the guano in. Could there be an
ecosystem in the cave at all?
Speaker 1 (37:04):
Exactly? Yeah, And I think by and large the answer
is yes, there would have been things that have done that.
And we know that in part because even today bats
are not the only trogs to poop in a cave.
There are considerable factors, but crickets are often brought up
as another major cave pooper that allows the sun to
(37:25):
shine in these lightless places. So it would seem that
prebat organisms would have taken advantage of the cave niche
to do, you know, to whatever degree they could. One
of the things about investigations into cave fossils is that
fossils that you find in deep caves aren't necessarily telling
you about subterranean life. Tracks in the floor or even
(37:48):
ceilings of deep caves may actually be insightful about life
along shorelines of the surface. You know. That's the degree
of time that we're dealing with, and considering the likes of,
for instance, a five hundred meter deep that's a third
of a mile deep cavern in France. I was reading
about this is and I may be butchering French here,
(38:09):
likely am castle book cave. Here, for instance, you'll find
dino footprints that were first etched in mud or sand
on a beach one hundred million years ago, and they've
just been gradually forced underground over time.
Speaker 3 (38:24):
So it's not like they were formed in the cave.
These are rocks that were formed on the surface and
then they later emerged in a cave.
Speaker 1 (38:32):
Yeah, and kind of similar to like, oh, you can't
find a trillobte in a mountain and be like these
were the mountain trilobites. It's not really what's happening. But
of course this doesn't mean we don't have evidence of
dinosaur age cave dwellers. As reported by Nature in twenty twenty,
cockroaches preserved in amber from ninety nine million years ago
(38:52):
are likely the oldest evidence we have of organisms evolved
for life in a cave environment.
Speaker 3 (39:00):
Million year old cave cockroaches. What were they like?
Speaker 1 (39:03):
Oh, they were they were kind of scary. I included
an image here of one for you, Joe. This is
one of the species that they were looking at, and
you'll see that it has front appendages, these kind of
like raptorial four legs, much like a praying mantis, which
means they were likely a highly predatory species in their
(39:23):
cave environments. MM. So that's that's one of the features
that they found on these these amber preserved specimens. There's
lost coloration, there's reduced wing and eye size, elongated antennae,
and and reduced leg spines for passive defense. So two
species were discovered in amber in this cave from me
(39:46):
and mar, one of which apparently had these raptorial four legs,
meaning it was it was likely primarily a predator. And
according to a paper in Cosmos magazine by James Urckhart,
one of the one of the interesting things about this
find is that the specimens in question would seem to
have mid Cretaceous origins, but all cave animals living today
(40:09):
have a late Sinozoic origin. Now, why these older cave
organisms died out is apparently a mystery, because it would
seem like an ideal place to shelter and survive surface
world extinction events, but obviously that's not the case. The
article also stresses that it's not impossible that these cockroaches
may have survived even into modern times in one form
(40:32):
or another, because our understanding of living insects is of
course incomplete, so it's not impossible even though we're gazing
backwards through time with this particular specimen, its genetic legacy
could still be alive today. The article here is quite good.
It also stresses some of the larger challenges of understanding
(40:52):
such ancient cave environments in figuring out what they consisted of.
Other Mesozoic cave organisms, it certainly existed, but either we
get into the same problems we encounter with the inherent
incompleteness of a fossils in general on the fossil record,
because either a cave system collapsed long ago or key
(41:15):
to this example, didn't allow fossil preservation. This case, amber
containing cave organisms is rather unique because a tree is
not going to grow in said cave. They say this
is kind of like a one in a million fine
because it probably depended on resin from a tree growing
(41:36):
directly in the mouth of a cave, like in exactly
the right place, exactly the wrong right time, in order
to capture this creature that otherwise would not be venturing
out of the caves to climb around on trees.
Speaker 3 (41:48):
Yeah, that's interesting, and I was wondering I was going
to ask about exactly that like, how does a cave
cockroach get trapped in amber? So I guess the answer
is probably very rarely happened. We're incredibly lucky to have
this unique find.
Speaker 1 (42:01):
Yeah, and it just raises the question, like what else
would have been down there that we didn't crawl into
the amber. You know, there's again just so many mysteries
within within the fossil record, and back to the earlier point,
so many mysteries remaining and just sort of the in
the the existing organic world. You know, we're still making
(42:22):
discoveries regarding insects, cave environments, of cave ecosystems. It's I mean,
it's really exciting.
Speaker 3 (42:31):
No doubt, And you know what, I think we're going
to have to end today's episode there, but we've got
more cave stuff to talk about. So we're going to
be back with part three definitely.
Speaker 1 (42:40):
That's right. We're gonna get into some cave creditors and
who knows what else. All right, stuff to blow your mind.
As you know. Core episodes are on Tuesdays and Thursday.
We're primarily a science and culture podcast, and then on
Mondays we do listener mail. On Wednesdays we do a
short form episode, and then on Fridays we set us
(43:01):
on most serious concerns to just talk about a weird
film on Weird House Cinema. We bust out a rerun
on the weekends.
Speaker 3 (43:07):
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 (43:29):
Stuff to Blow Your Mind is production of iHeartRadio. For
more podcasts from my heart Radio, visit the iHeartRadio app,
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