[♩INTRO]
It's not easy living in a cave.
There isn't a ton of space, the pickings are slim,
and then there's the whole living-in-total-darkness bit.
So it's not surprising that the species that live in caves, or troglobites,
have made some adjustments.
Many are eyeless, for example, or have become basically colorless.
But cave-dwelling species haven't just lost stuff.
The animals on this list have all gained amazing traits
because they live their lives in the dark abyss.
And those features are helping scientists understand evolution and adaptation better.
Mexican cave fish are maybe best known for their ghostly appearance,
but don't judge them purely on their looks.
While they may have lost their eyes,
they're still incredibly good at finding food
thanks to taste buds in some pretty weird places,
like their lips, gill arches,
and on both the underside and the topside of their heads.
This ensures they don't miss a tasty treat when rummaging around
in the sediment in search of worms, snails, or insects.
In total, they have 3 to 6 times the number of taste buds
that their surface-dwelling counterparts have
and that gives them a big advantage when it comes to finding food.
In one study, equal numbers of both types of fish were housed together
in a warm, dark experimental tank,
and then presented with bits of food one at a time.
The cave fish got a hold of 80 percent of the food offered,
which the researchers thought was due to both their heightened sense of taste
and their strategy of searching the bottom for food.
It's especially impressive that they can out-sniff their kin
given that their brains are 30% smaller, though not all parts have shrunk.
To help deal with all the taste information coming in,
the Mexican cave fish actually has an enlarged forebrain,
including an especially big teleost gustatory center
the bundle of neurons that deals with taste.
So pretty much their whole head is involved in tasting somehow
like a giant tongue.
Whip spiders are not the kind of arachnid you want to come acrossin the dark.
At least, not if you're a small cave-dweller.
They have long, really long, very, too long spindly legs
and two overgrown forearms that look like
some kind of terrifying hybrid between a venus fly trap and pincers.
And they have a brutish personality to go with that frightening appearance.
Encounters between two whip spiders generally end
in one of them cannibalizing the other
except if they're both troglobites.
It turns out cave varieties are just big softies by comparison.
In a 2015 paper published in the Journal of Arachnology,
biologists put pairs of whip spiders together in a gladiator style arena,
and watched what happened.
It must have been a Friday...
It took cave-dwelling ones longer to get to the point of
waving those terrifying forearms at their opponent, and when they did,
their displays were shorter.
This might be because cave life is so cramped that they bump
into each other more often than on the surface.
And because food is more limited in caves,
they don't have the energy to waste on squabbling.
The whip spider's gentle temperament is an example
of behavioral evolution, because adapting to an extreme environment
doesn't always mean changing physically.
It's probably no surprise to you that life in a cave is pretty dark.
While many animals have lost the ability to see, some have gone
the other direction and developed ways to brighten things up a bit.
Glow worms, which aren't worms at all, but the maggots of a fungus gnat,
use their bright butts to hunt for food in caves and other dark habitats
in New Zealand, like the famously beautiful Waitomo caves.
They fashion fishing lines out of a silk-like material dotted with sticky globs
of saliva and urea regurgitated from their mouths which are then
dangled them from the cave ceiling.
But rather than simply waiting for something to happen across their snares,
they lure unsuspecting insects towards them by glowing.
This light or bioluminescence is the byproduct of a chemical reaction
involving oxygen and a compound called luciferin.
It's the same way fireflies light up,
but the ability evolved separately in them and the New Zealand glow worm.
And in the glow worm, the process of making light happens in
excretory organs in their rear ends called malpighian tubules,
whereas fireflies have specialized light-producing organs in their abdomen.
All this might seem like a lot of effort for a meal,
but eating is literally the maggots' only job for 10 to 11 months.
Then, they pupate and emerge as adults.
The gnats only have two to four days to reproduce because
they don't have mouths, so they quickly die of starvation…
if they're not ensnared by one of their cousins first.
When you're living life in a cave, hearing can be hard.
With all those echoey walls, it's difficult to localize sound.
So instead of listening for sound,
several species of cave crickets have found a way to feel sound instead.
Insects usually hear airborne sounds using a structure
called the tympanal organ, which looks kind of like an exposed drum
on the side or leg of an insect's body.
But cave crickets don't have them.
Instead, they rely on something called the subgenual organ
which senses vibrations from the ground.
This organ is found on a section of their legs called the tibia
because it's in a similar position as the mammalian shinbone,
which makes sense because their legs are the parts in contact with the ground.
There are other insects with this vibration-sensing organ,
but theirs pick up on higher frequency sounds because those travel better in air,
whereas cave cricket subgenual organs
are tuned to the low frequencies of ground vibrations.
And in addition to finding food,
the crickets use their leg-hearing to locate mates.
Males will vibrate their abdomens to send signals through the ground
which are, hopefully, felt by a female nearby.
Now that's blind dating.
Swimming around in caves in southern Europe
is the world's only exclusively cave-dwelling salamander: the olm.
Like many troglobites, its sense of sight is pretty much non-existent
because its eyes never fully develop and end up covered in skin.
But it more than makes up for it with its amazing sense of smell.
Scientists have shown the salamanders can quickly locate living
and dead food items in total darkness,
and they can even find meals in the presence of a fast-moving current.
It's not that they have a particularly special "nose" either
the olm's olfactory epithelium,
the tissue that picks up smell signals and sends them to the brain,
looks pretty much the same as any other vertebrates.
But that layer of cells is up to 5 times thicker than in other salamanders,
though it's not clear how that translates to better sniffing.
And like with the crickets, olms don't just use smells to find food.
The animals are often found cuddling up in little rock crevices, and even opt
for snuggling if given the option between an empty or an occupied nook.
And they find and select between cuddle buddies
using their super sense of smell.
Most of the time, they don't even mind cozying
up to a member of the same sex.
But olms can live for nearly a century, take 12-15 years to mature,
and females only lay eggs once every 12.5 years on average.
So when breeding is on the table, males take over crevices to use as love
nests, and the smelly signals they send out can help other males avoid a fight.
After all, you don't want your friends getting in the way when you're
trying to woo a mate that's in the mood less than once a decade.
For the longest time scientists thought the tiny organ on each of the
cave beetle's antennae, called Hamann's organ, helped them smell.
But thanks to some pretty clever and kind of mean experiments in the 1970s,
they discovered it actually detects moisture levels in the air
like built in humidity sensors.
To figure this out, researchers cut the beetle's antennae to various lengths
and put them in a choice chamber with two different levels of humidity,
knowing the bugs prefer moist environments.
The ones with their Hamman's organs cut off weren't able
to find the more humid chamber.
Scientists aren't exactly sure how Hamann's organs work,
but they think water droplets in the air may cause sensory cells
or hygroreceptors to swell or stretch, which then send signals to the brain.
The brain combines those signals with temperature information
from thermoreceptors, and the result is a humidity sense.
To some extent, we can sense humidity too, but the beetles
are way better at it thanks to their special organs.
But why they need to be so sensitive to moisture
is still a bit of a scientific mystery.
One hypothesis is that the moisture content of their food matters a lot.
Many insects get water they need from their meals,
but because food is scarce where the cave beetle lives,
it needs to make sure what it's eating is nice and juicy, not dry like a cracker.
Or, it could be because female cave beetles only lay a few large eggs that take
a long time to develop, so laying them somewhere moist prevents
them from drying out before the little ones hatch.
Either way, finding moisture is probably pretty important to them,
so they've become outstanding humidity sensors.
Sure, living in a cave is lonely, dark, and challenging for all kinds of reasons.
But it isn't that bad.
You might wind up looking a little pasty, but as the creatures on this list show,
you could also develop some pretty awesome super powers.
Thanks for watching this episode of SciShow!
And if you liked learning about these troglobites' super powers,
you might like our news episode where we explain how
Mexican cave fish have turned being diabetic into a good thing.
[♩OUTRO]
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