[♩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]
