How Animals Hear: Crash Course Zoology #7 - Free Educational videos for Students in K-12

How Animals Hear: Crash Course Zoology #7 - By CrashCourse

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00:0-1	our environments are filled with information . Just think of
00:03	all the stuff that comes at us each day ,
00:06	like light and sound and even objects . It's almost
00:10	overwhelming when you think about it . But animals are
00:13	pretty resourceful figuring out how to take it all in
00:16	and weed out what's not important . In fact ,
00:19	we and other animals often put our whole bodies into
00:23	it like heads , knees , legs , abdomens and
00:27	10 I wings . They're all body parts of animals
00:31	used to learn about their environment just through hearing .
00:34	So let's talk about what an ear even is ,
00:37	how it has evolved , how they work . And
00:40	some of the cool ways animals have tweaked their ears
00:43	to work for their lifestyle . Perk up whatever you
00:46	use for hearing because I'm going to be sending vibrations
00:50	your way for the next 10 ish minutes . I'm
00:53	Ray Wynne Grant and this is crash course zoology ,
01:06	like all senses , hearing involves collecting information from the
01:10	environment and processing that information into signals that the brain
01:14	can understand specifically Hearing is the ability to interpret your
01:19	environment using vibrations that move through air , water or
01:23	even solid objects . Hearing organs like ears are just
01:27	one way animals can interpret the sounds in their environment
01:31	. And while we vertebrates trace all our ears to
01:34	one ancestor , invertebrates have independently evolved ears over and
01:39	over again over 24 times in just insects alone .
01:43	So like vision , hearing is a spectrum . Some
01:47	animals have very sensitive hearing and some don't here at
01:50	all . For those of us who is hearing organs
01:53	are ears . Ears have to minimum requirements to work
01:57	a sensor that turns vibrations into nerve signals and the
02:01	brain power to interpret those signals into different sound qualities
02:05	. Most animals that here , even if they don't
02:08	have what we call ears , rely on special hair
02:12	cells which are named for the little hair like Tufts
02:14	that come off of them called stereo cilia . Sound
02:18	vibrations cause the stereo cilia to sway , which leads
02:22	to the hair cell sending signals to nearby neurons that
02:25	relay information to the brain . Our hair cells are
02:29	in our internal ear , the part that does the
02:32	actual hearing that sits behind a thin membrane or eardrum
02:36	. The outside part called the pinna works like a
02:38	satellite dish to amplify and direct sound into our internal
02:42	ear . Pinna are very much a mammal thing .
02:46	Some species pinna even move to track sounds . Many
02:50	other vertebrates , like most reptiles and amphibians don't have
02:53	pinna . Their external ears are simple openings and some
02:58	animals like fish , have no external ears at all
03:01	, hearing only the sounds that reach their internal ear
03:04	after moving through their body . But some animals have
03:07	gone a different route . Frogs and toads usually have
03:10	internal ears , but there are over 200 species that
03:14	have reduced or even lost many internal ear parts .
03:18	These mostly fearless toads , can still hear some sounds
03:22	like a crock by feeling vibrations which might be all
03:27	they need to chat with their friends and detect predators
03:30	. Like I said before . Unlike we learned about
03:32	vision hearing and the organs we used to do it
03:35	come in a whole spectrum and also like eyes .
03:39	We can compare hearing organs based on what they can
03:42	do . Most of our understanding of hearing is about
03:45	how big ish animals handle . Far field sounds .
03:49	Far field sounds are sounds that have traveled far enough
03:52	away from the thing that makes the noise that they
03:55	act like a typical wave very close to the sound
03:58	source . The sound waves push on the air or
04:00	water particles around them , so you get weird auditory
04:04	effects as the particles bump into each other . Some
04:07	invertebrates use this kind of sound called near field sound
04:10	to communicate with each other and detect predators . Though
04:13	the two aren't mutually exclusive , some arthropods can also
04:17	make . And here far field sounds . We can
04:20	also compare different animals . Hearing ranges which is the
04:24	range of frequencies that an animal can detect frequencies are
04:28	the number of times particles of a material vibrate in
04:31	a certain time period . When a sound wave passes
04:34	through it , one cycle of vibration in one second
04:37	is called , one hurts us . Humans have a
04:40	decent hearing range of about 20 to 20,000 hertz .
04:45	We experience frequency as pitch or the loneliness or highness
04:50	of a sound which we can also compare . But
04:52	many animals can hear sounds under 20 hertz which we
04:55	call infrasound or above 20,000 hertz . Which we call
05:00	ultrasound . Like wax mouse can hear 300,000 hertz sounds
05:05	squeaky er than any bat of course like best eyes
05:09	or best brain . Best ears or hearing isn't really
05:13	an award . We can give out . Animal hearing
05:15	has evolved to pick up on the type range and
05:18	pitch of sounds that they encounter and need to respond
05:21	to in their environment . And as we'll see how
05:24	animals here has changed a lot in the hundreds of
05:28	millions of years they've been around but as for who
05:32	can hear . Well it depends on what you call
05:36	hearing and what you call an ear for us .
05:39	Humans hearing usually means sounds moving in air but there
05:43	are other ways vibrations can travel . As of 2021
05:47	we know a lot of arthropods keep their ears to
05:50	the ground literally by sensing vibrations as they move through
05:54	the earth and that's pretty much hearing even though they
05:57	don't have ears that look anything like ours . Other
06:01	invertebrates like jellyfish are trickier . They definitely have sensors
06:05	that could detect waves . But since I live underwater
06:09	, we can't tell if they're responding to sound waves
06:12	or like normal water waves . So for a long
06:16	time sociologists thought that only vertebrates could hear airborne sounds
06:21	, since invertebrates don't have ears like ours , But
06:24	that got thrown out with a series of meticulously designed
06:28	experiments by high school teacher Charles . H . Turner
06:31	. Let's go to the thought bubble . It's standard
06:34	these days to account for extra factors that might influence
06:37	research results . But in 1907 , when Turner published
06:41	his first hearing study , his experimental design was revolutionary
06:45	. First he insulated ant nests from ground vibrations with
06:50	cotton batting and carefully controlled the lighting in the room
06:54	. Then he used a dog whistle , organ pipes
06:57	and his own voice from different distances . And he
07:00	repeated the experiment many times , and even used heat
07:04	filters to keep the animals from sensing the warmth of
07:07	the lights . With all those controls , the ants
07:10	scuttled away when he made noise , which Turner interpreted
07:13	as evidence that his aunt's could here . Turner then
07:17	investigated hearing in moths using a dog whistle again ,
07:20	Taking precautions at the animals weren't just responding to the
07:23	site of the whistle or air movement . According to
07:26	his results . Moths could here , too , he
07:29	reported in 1914 that the moths responded to high but
07:33	not low frequency sounds , and mused that moths might
07:38	still be able to hear the lower sounds , but
07:41	only responded to sounds that were important , like the
07:44	high pitched whistle that sounded a bit like a bat
07:47	. So he tested this idea by jostling the mobs
07:50	when he made a low frequency sound . Soon the
07:54	moths learned that low pitched noises were dangerous and fluttered
07:57	their wings in response to just the low frequency sound
08:01	. So not only did he prove moths could hear
08:03	sound even if they didn't respond to them , but
08:06	also that moths could learn thanks . Thought bubble Turner
08:11	was a brilliant and productive scientist . He was the
08:14	first african american to earn a PhD from the University
08:17	of Chicago , but was excluded from professorships at white
08:21	universities . Even today . Many black entomologists faced similar
08:25	barriers , But Turner's work with Moss was one of
08:28	the earliest examples of conditioning and insects and foundational to
08:33	the work of many later scientists studying insect learning and
08:36	behavior . In addition to Turner's moths and ants since
08:40	1907 , we've learned that lots of insects and at
08:43	least some Iraq needs , like spiders can hear airborne
08:47	sounds . Thanks to the work of many other scientists
08:50	, Many insects even use hairs tuned to particular frequencies
08:55	, sort of like stereo cilia . Other insects have
08:58	a TIM panel organ in their legs , wings ,
09:00	abdomen and other places that vibrates and ends up sending
09:05	signals to the brain . Invertebrate hearing evolved dozens of
09:09	times in lots of different ways , which makes sense
09:13	since there are 34 ish file of invertebrates out there
09:16	. But vertebrate style hearing only evolved once in a
09:20	fish ancestor , which was passed on to all tetrapods
09:25	or animals with four limbs , like mammals , birds
09:28	, reptiles and amphibians . We know that because even
09:31	though vertebrate years vary widely , they always use the
09:35	same hair , cell centered mechanism based on fossil evidence
09:39	. We think that the earliest vertebrate years were entirely
09:43	internal like fish years are today and evolved over 500
09:48	million years ago , which is probably why fish hear
09:51	low frequency sounds best . They passed through the body
09:55	more easily than high frequency sounds . Then , as
09:58	vertebrates moved onto land roughly 400 million years ago ,
10:02	things got more complicated . Like if you've ever been
10:06	swimming at a public pool , you might have noticed
10:08	how much quieter everything seems underwater . That's because a
10:12	lot of airborne sounds bounce off the water surface instead
10:16	of traveling through the water . This same thing happened
10:19	to those early land vertebrates but only had internal years
10:24	. The sounds in the air bounced off their denser
10:26	ear tissue and didn't reach the hair cells , so
10:29	they probably only heard very loud , low frequency sounds
10:33	that is until tim panic , middle ears or ears
10:37	with an ear drum and obstacles evolved which they did
10:40	independently . At least three times these external ears allowed
10:46	sounds in the air to come into direct contact with
10:49	the inner ear without traveling through the skull and other
10:52	tissues . This made a huge difference in how well
10:56	land animals could hear ears spread across meadows . Oh
10:59	, a because just like eyes , they're super useful
11:03	vertebrates probably first evolved hearing to gather more information about
11:08	their environment and avoid hazards . Whereas insects evolved hearing
11:12	so they could keep an ear out for vertebrate predators
11:15	. Except for those noisy cicadas , They just wanted
11:18	a way to talk to each other . Some of
11:19	our biggest leaps and understanding how animals interact with sound
11:23	. All happened very recently and I'm sure we'll learn
11:27	even more interesting stuff soon . Next episode will get
11:31	into the really old and weird ways animals sense their
11:34	environment . Thanks for watching this episode of crash course
11:38	ideology , which was produced by complexity in partnership with
11:41	PBS and Nature . It's shot on the team Sandoval
11:44	Pierre stage and made with the help of all of
11:46	these nice people . If you'd like to help keep
11:49	crash course free for everyone forever , you can join
11:51	our community on patreon . Yeah .

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