our environments are filled with
information just think of all the stuff
that comes at us each day like light and
sound and even objects
it's almost overwhelming when you think
about it but animals are pretty
resourceful at figuring out how to take
it all in and weed out what's not
important in fact we and other animals
often put our whole bodies into it like
heads knees legs abdomens antennae wings
they're all body parts that animals use
to learn about their environment just
through hearing so let's talk about what
an ear even is how ears evolved how they
work and some of the cool ways animals
have tweaked their ears to work for
their lifestyle
perk up whatever you use for hearing
because i'm going to be sending
vibrations your way for the next 10-ish
minutes i'm raewyn grant and this is
crash course zoology
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like all senses hearing involves
collecting information from the
environment and processing that
information into signals that the brain
can understand
specifically hearing is the ability to
interpret your environment using
vibrations that move through air water
or even solid objects hearing organs
like ears are just one way animals can
interpret the sounds in their
environment
and while we vertebrates trace all our
ears to one ancestor
invertebrates have independently evolved
ears over and over again over 24 times
in just insects alone
so like vision hearing is a spectrum
some animals have very sensitive hearing
and some don't hear at all for those of
us whose hearing organs are ears ears
have two minimum requirements to work a
sensor that turns vibrations into nerve
signals and the brain power to interpret
those signals into different sound
qualities most animals that hear even if
they don't have what we'd call ears
rely on special hair cells which are
named for the little hair-like tufts
that come off of them called stereocilia
sound vibrations cause the stereocilia
to sway which leads to the hair cell
sending signals to nearby neurons that
relay information to the brain our hair
cells are in our internal ear the part
that does the actual hearing that sits
behind a thin membrane or eardrum the
outside part called the pinna works like
a satellite dish to amplify and direct
sound into our internal ear pinna are
very much a mammal thing
some species pinna even move to track
sounds many other vertebrates like most
reptiles and amphibians don't have pinna
their external ears are simple openings
and some animals like fish have no
external ears at all hearing only the
sounds that reach their internal ear
after moving through their body but some
animals have gone a different route
frogs and toads usually have internal
ears but there are over 200 species that
have reduced or even lost many internal
ear parts these mostly earless toads can
still hear some sounds like croak by
feeling vibrations
which might be all they need to chat
with their friends and detect predators
like i said before and like we learned
about vision
hearing and the organs we use to do it
come in a whole spectrum and also like
eyes we can compare hearing organs based
on what they can do
most of our understanding of hearing is
about how big-ish animals handle
far-field sounds
far-field sounds are sounds that have
traveled far enough away from the thing
that makes the noise that they act like
a typical wave very close to the sound
source the sound waves push on the air
or water particles around them so you
get weird auditory effects as the
particles bump into each other
some invertebrates use this kind of
sound called near-field sound to
communicate with each other and detect
predators though the two aren't mutually
exclusive some arthropods can also make
and hear far-field sounds we can also
compare different animals hearing ranges
which is the range of frequencies that
an animal can detect frequencies are the
number of times particles of a material
vibrate in a certain time period when a
sound wave passes through it
one cycle of vibration in one second is
called one hertz us humans have a decent
hearing range of about 20 to 20 000
hertz
we experience frequency as pitch or the
lowness or highness of a sound which we
can also compare but many animals can
hear sounds under 20 hertz which we call
infrasound or above 20 000 hertz which
we call ultrasound like wax moss can
hear 300 000 hertz sounds squeakier than
any bat of course like best eyes or best
brain best ears or hearing isn't really
an award we can give out
animal hearing has evolved to pick up on
the type range and pitch of sounds that
they encounter and need to respond to in
their environment and as we'll see how
animals hear has changed a lot in the
hundreds of millions of years they've
been around
but as for who can hear
well it depends on what you call hearing
and what you call an ear
for us humans hearing usually means
sounds moving in air but there are other
ways vibrations can travel as of 2021 we
know a lot of arthropods keep their ears
to the ground literally by sensing
vibrations as they move through the
earth
and that's pretty much hearing even
though they don't have ears that look
anything like ours other invertebrates
like jellyfish are trickier they
definitely have sensors that could
detect waves but since they live under
water we can't tell if they're
responding to sound waves or
like normal water waves so for a long
time zoologists thought that only
vertebrates could hear airborne sounds
since invertebrates don't have ears like
ours
but that got thrown out with a series of
meticulously designed experiments by
high school teacher charles h turner
let's go to the thought bubble it's
standard these days to account for extra
factors that might influence research
results but in 1907 when turner
published his first hearing study his
experimental design was revolutionary
first he insulated ant nests from ground
vibrations with cotton batting and
carefully controlled the lighting in the
room
then he used a dog whistle organ pipes
and his own voice from different
distances and he repeated the experiment
many times and even used heat filters to
keep the animals from sensing the warmth
of the lights with all those controls
the ants scuttled away when he made
noise which turner interpreted as
evidence that his ants could hear turner
then investigated hearing in moths using
a dog whistle again taking precautions
that the animals weren't just responding
to the sight of the whistle or air
movement
according to his results moths could
hear too he reported in 1914 that the
moths responded to high but not low
frequency sounds and mused that moths
might still be able to hear the lower
sounds but only responded to sounds that
were important
like the high-pitched whistle that
sounded a bit like a bat so he tested
this idea by jostling the mobs when he
made a low frequency sound
soon the mods learned that low pitched
noises were dangerous and fluttered
their wings in response to just the low
frequency sound
so not only did you prove moths could
hear sound even if they didn't respond
to them but also that moths could learn
thanks that bubble
turner was a brilliant and productive
scientist he was the first african
american to earn a phd from the
university of chicago but was excluded
from professorships at white
universities even today many black
entomologists face similar barriers but
turner's work with moz was one of the
earliest examples of conditioning in
insects and foundational to the work of
many later scientists studying insect
learning and behavior in addition to
turner's moths and ants since 1907 we've
learned that lots of insects and at
least some arachnids like spiders can
hear airborne sounds thanks to the work
of many other scientists many insects
even use hairs tuned to particular
frequencies sort of like stereocilia
other insects have a tympano organ in
their legs wings abdomen and other
places that vibrates and ends up sending
signals to the brain invertebrate
hearing evolved dozens of times in lots
of different ways which makes sense
since there are 34-ish phyluv
invertebrates out there but vertebrate
style hearing only evolved once in a
fish ancestor which was passed on to all
tetrapods or animals with four limbs
like mammals birds reptiles and
amphibians we know that because even
though vertebrate ears vary widely they
always use the same hair cell centered
mechanism based on fossil evidence we
think that the earliest vertebrate ears
were entirely internal like fish ears
are today and evolved over 500 million
years ago which is probably why fish
hear low frequency sounds best they pass
through the body more easily than high
frequency sounds then as vertebrates
moved on to land roughly 400 million
years ago things got more complicated
like if you've ever been swimming at a
public pool you might have noticed how
much quieter everything seems underwater
that's because a lot of airborne sounds
bounce off the water surface instead of
traveling through the water the same
thing happened to those early land
vertebrates that only had internal ears
the sounds in the air bounced off their
denser ear tissue and didn't reach the
hair cells so they probably only heard
very loud low frequency sounds that is
until tympanic middle ears or ears with
an eardrum and ossicles evolved which
they did independently at least three
times
these external ears allowed sounds in
the air to come into direct contact with
the inner ear without traveling through
the skull and other tissues this made a
huge difference in how well land animals
could hear ears spread across metazoa
because just like eyes they're super
useful
vertebrates probably first evolved
hearing to gather more information about
their environment and avoid hazards
whereas insects evolved hearing so they
could keep an ear out for vertebrate
predators
except for those noisy cicadas they just
wanted a way to talk to each other some
of our biggest leaps in understanding
how animals interact with sound all
happened very recently and i'm sure
we'll learn even more interesting stuff
soon
next episode we'll get into the really
old and weird ways animals sense their
environment thanks for watching this
episode of crash course zoology which
was produced by complexly in partnership
with pbs and nature it's shot on the
team sandoval peer stage and made with
the help of all of these nice people
if you'd like to help keep crash course
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you