- You are not looking at a yellow ball. Your brain might think you're
looking at a yellow ball, but look closer. The screen you're watching this
on displays color using only red, green, and blue subpixels. The yellow your brain thinks
it's seeing is actually a mix of red and green light. The camera I'm talking to
right now has a sensor composed of red, green, and
blue-sensitive photosites. Again, no yellow. Of course, I have the
ball here in my hand, so I am looking at a yellow ball, or am I? (light mysterious music) After all my eyes aren't so
different from that camera. The human retina only has
cone cells sensitive to red, green, or blue wavelengths of light. To perceive other colors, we
have to integrate the inputs from those three cone types. When yellow light enters my
eye, it stimulates my red and green-sensitive cone cells, although not as much as pure
red, or green light would. With red and green-sensitive
cones equally excited, my brain tells me I'm looking at yellow. This is how our technology, our cameras, and screens, and projectors
can trick our brains into seeing a whole rainbow of colors using just three wavelengths of light by triggering our three
different types of cone cells in different proportions. So is the ball really yellow? What does yellow even mean? A lot of people would say that color is the wavelengths of light
that an object reflects. In other words, like Aristotle thought, color is a property of the object. But looking at the same ball on a screen, your eyes only sensed red and green light, yet your brain still
perceived it as yellow. So it's also possible,
like Galileo believed, that color isn't a property
of an object at all, but a phenomenon of the mind instead. But whose mind? Because we aren't the only animals that can see the world in color. - We oftentimes don't really think about how other animals see color. So for example, we buy
our dogs bright red, or orange toys that are only
bright red, or orange for us, and not for them, because
they can't see orange, or red as being distinct from green. - [Derek] So maybe we should
start taking in the world from more than just the human perspective. Doing that might just
teach us why color vision evolved in the first place. - We're actually towards the lower end of the spectrum, honestly. We're a step up from our
household pets, maybe, if they're cats, or dogs, but not nearly as good as
many animal groups out there, butterflies, birds, fish,
lizards, jumping spiders. Jumping spiders are harmless
creatures, actually. They don't ever really
get big enough to pose much of a threat to humans. Of course, if you were a small insect, yes, you would absolutely
need to be afraid of a jumping spider. - [Lisa] They'll take
down prey that's sometimes like two, or three times
their own body size. - The Chinese word for jumping
spider translates literally to fly tiger, and that's the
way I like to think about them is the kind of small
cats of the undergrowth. - Jumping spiders are everywhere. They're in your backyard. They're probably in your kitchen. - There are about 6,000 species
of jumping spiders known. Some are sort of furry,
some are sort of shiny, striped, spotted, red, green, blue. Pretty much anything you can imagine. It's like every one is
a little work of art. - As a group, spiders aren't
known for their vision. I mean, most species are nocturnal, and for many, their webs act
as a sort of extra sense organ, so they just don't need to see that well. But jumping spiders, as
active daytime hunters, well, they're different. Not only do they have great eyesight, but different species have
different forms of color vision. Look at those eyes. - Jumping spider eyes are fascinating, and when I say eyes, I mean eight eyes. Jumping spiders split up
things like motion detection and light sensitivity to some eyes, and then color vision and fine
detail vision into others. The pair of eyes that are
perhaps the most fascinating, or most unusual are what
we call the principal eyes, and those are the really big
eyes in the front of the face that make jumping spiders
look a little cute if you're willing to say
a spider looks cute ever, and those are built unlike any other eye in the animal kingdom. - [Derek] It turns out the way jumping spiders perceive
color has everything to do with the anatomy of those principal eyes. - [Nathan] They're really actually built a lot like a Galilean
telescope, or binoculars. That big lens that you see from
the outside of the animal is one of two lenses in these eyes, and in between those two lenses is a long fluid-filled tube. At the end of that fluid-filled
tube is a second lens, and what that lens does is
it magnifies the image that that first lens projects
down that long tube, and in that way, it increases
the ability to see detail by the retina that sits right below it. - [Derek] And when it
comes to seeing detail, it's hard to beat a jumping spider. - For most animals, the bigger the eye, the better it functions. Jumping spiders absolutely
break this rule. The secondary eyes can see the world about as well as the absolute
best insect eyes out there, better than the world's
biggest dragonflies, whose entire head is consumed by an eye. The principal eyes, they can actually see pattern in the world better than a lap dog, a house cat, an elephant, and nearly as good as
the sharp-sighted pigeon, but it's a very narrow slice
of the world that they can see. It's about like your thumb
held at arm's length. - [Derek] And it's only in
that narrow slice of the world that jumping spiders can
see fine detail and color. - So the jumping spider's
secondary eyes give them a full 360-degree view of the world. Now, imagine most of
that's in black and white. When you see something move,
you can swivel to look at it. And now, anything that's
really of curiosity to you, you can add to this world
of black and white vision fine detail and color. But you can only do it moment by moment. So you're really kind of
painting additional details about color and pattern that
you couldn't see otherwise. It's a wild world to try
to put yourself into. (bee buzzing) - As they sweep their
principal eyes across a scene, some species of jumping spiders are adding a lot more color information
to their world than others. Most jumping spiders, including
this one, are dichromats, meaning they have two types
of color-sensitive cone cells in their retinas, just like
dogs and most other mammals. - [Nathan] And by comparing
those two kinds of cells and how they respond to
light in the environment, they get a coarse understanding of color. They can tell the difference between UV, violet, blue, and green. - But some types of jumping
spiders are trichromats with three types of cones, like humans, and other are tetrachromats, like birds. (light music) The weird thing is all these species with expanded color vision aren't necessarily close relatives. - In jumping spiders,
we have huge variation, even from closely related groups, in how well they're able
to see color in the world. Jumping spiders are
reinventing, in some ways, the ability to see color
over and over again in different ways. - That makes jumping
spiders pretty special. I mean, consider primates. Old World monkeys, apes, and humans all have
trichromatic color vision, but we also share a common ancestor. So our color vision
probably evolved only once, and then it stuck around. This is where jumping
spiders really stand out. The ability to see red, for example, has evolved several
times in jumping spiders. Researchers know that, because they've figured out how different groups are related, and by they, I mostly mean jumping spider fanatic Wayne Maddison. - Oh my gosh, Havaika. Fantastic! Beautiful male. Ah, it's been, it's been
30 years since I've seen a live Havaika. - [Megan] He is absolutely
Mr. Jumping Spider. His expertise really is in
jumping spider taxonomy. - I work on the evolutionary
tree of jumping spiders. The evolutionary tree of life is basically the pathway of genetic
descent that links all of us. - [Derek] The position
of different species on this evolutionary tree
can tell us how they ended up with the traits they have. Like if most jumping
spiders don't see red, but two species on two very
different parts of the tree do, chances are that those two species evolved those abilities independently. - In which case, then we can
start to ask questions like what's driving that evolution? Do they have similar ecologies? Do they hunt similar prey? And try to really understand what selective forces are leading to these expanded color vision systems. - It might help them find food, or discriminate tasty prey
from prey that can harm them. - [Nathan] Because, of course, lots of small insects
are brightly colored, and some of them are
using those bright colors to advertise that they're toxic. - [Derek] Another
possibility is that seeing a richer world of color
might help animals, from lizards to spiders,
choose better mates. To test these ideas, the
researchers needed to know which of the 6,000 species
of jumping spiders had expanded color vision and which didn't, and outside of a few well-studied species, no one really knew. So the team set out to collect spiders from every major branch of the
jumping spider family tree. - One of our first things is
just prioritizing where to go and what to look for. And so, it's a lot of
sampling in a lot of places. - Let's see who lives here. - [Derek] It's kind of like "Pokemon GO," except the Pokemon are real, they're smaller than
your pinky fingernail, and they're really good at hiding. - Some jumping spiders have evolved to be really fine-tuned
to a particular situation. So for example, there are termite-eating
specialist jumping spiders that you're only gonna
find around termites. There was this one
species that we only found in piles of bones in South Africa. Who knows what it was doing there, but we quickly learned that that was the only place in the environment we were gonna find it. And so, that's part of the fun of it. I feel like it's a bit of
a treasure hunt, really. (light music) - With no proverbial stone left unturned, the team returns to their
labs with hundreds of spiders representing many different species. They want to figure out which species have expanded color vision, how
each species does it, and why. It's actually a hard question to tell how animals can see color. We can't just connect our
brains to see what they see. So how do we do it? - [Nathan] We begin by using a technique called microspectrophotometry. It's a really long word. What it simply means is a
microscope paired with a device that measures different
wavelengths of light, a spectrophotometer. - [Derek] The researchers
take ultra-thin slices of jumping spider retinas, and then they measure the
wavelengths of light absorbed by individual cone cells. With enough of these measurements, they can tell if a species is a dichromat, trichromat, or tetrachromat, and what wavelengths of light its cells are best at detecting. But that's not the whole story. - Having that knowledge of
what's in the retina tells us what is, or isn't possible
for these animals to see, but it doesn't actually
tell us what they do see, or how they might use that. And so, the gold standard for establishing that an animal can see color
is to do so behaviorally. - In other words, we somehow
need to ask the spiders what they can see and then
understand their answers. Figuring out what's going on inside a spider's mind is difficult. It's no surprise that it takes a group of expert zoologists to do so. But our own minds can
be just as complicated, yet we rarely talk with
mental health experts to help us interpret our
thoughts and emotions. Whether you're feeling depressed, anxious, or are just stressed and
want someone to talk to, a therapist lets you see things from a different perspective, and that's where today's
sponsor, BetterHelp, comes in. I know finding a therapist
you're comfortable around can be intimidating, especially
if you're only limited to the options in your city, but BetterHelp lets you get around this, because it's an online platform, and they make it really easy to connect with a professional therapist, who can help you work through
whatever you're facing. It's easy to sign up. There's a link in the description, betterhelp.com/veritasium, and after answering a few questions, BetterHelp will match you with one of their more
than 30,000 therapists. Each therapist is licensed,
has a master's, or a PhD, and has spent over three years and over 1,000 hours with people. And if you don't click
with your first therapist, you can simply switch
to a new one for free without things getting awkward. If you feel like you'd benefit
from talking to someone, getting advice, feedback, and help, then visit betterhelp.com/veritasium
to get started. Clicking that link both
helps support this channel, but it also gets you 10%
off your first month. So I wanna thank BetterHelp
for sponsoring this video, and now, back to jumping
spiders and how they see color. - These animals are particularly motivated to investigate things that move, and these responses
can be guided by color. - [Derek] The theory is simple. You show the spider a
screen with a moving shape that differs from the background in color, but not in brightness, and you see if the spider tries to follow. - [Nathan] The problem with
letting the jumping spider actually turn and respond is
that they'll absolutely do so, but it changes some of what they see. So what we want is to
really have some control over what the spiders can
see at any given moment. - So the researchers hold them in place with tiny magnets attached to their heads. - [Nathan] What we do is we
give them a ball to stand on. They actually hold it with their feet, and we can monitor how that
ball moves around in their feet to know where they would want to go. - If the spider turns
the ball to the left, it's probably trying to look to the right to follow the moving shape, and that's evidence the
spider can discriminate between the colors of the
shape and the background. (playful music) Once the team knows which
species can see which colors, the next question is how do they do it? What's different in the DNA
of these spiders that can see and discriminate more colors? If you ask Megan Porter, a
lot of it comes down to genes that encode proteins called opsins. - The way that animals
achieve color vision is to have different copies
of this opsin gene, and those variations then
are what produce proteins that are sensitive to
different colors of light. The first technique
that we generally go to with a new species is called
transcriptome sequencing, and this is where we
can take the entire head of a jumping spider, and
we can get the sequences for every single gene that
is expressed in that tissue. - [Derek] This method gives
the researchers an inventory of all the genes being expressed, in other words, all the genes
that are copied out of the DNA and sent to make a protein. Then the team can figure out where each of these genes is expressed, in which eyes, and in
which parts of the eye. - And we do that using a fancy technique called
immunohistochemistry. - [Derek] The researchers basically create glowing molecular tags
specific to each protein they're interested in. - [Megan] And then looking
for which parts glow in the right color, we can figure out where each opsin is being expressed, where the protein is located. - The team is especially
interested in genes that are expressed in the
retinas of the principal eyes. These are the genes most
likely to be related to changes in color vision. Already, this process of asking which species have expanded color vision and how they accomplish it has led to some surprising discoveries. The researchers already
knew that the ability to see and discriminate more colors
had evolved more than once among jumping spiders. But they hadn't realized just how widespread this ability would be. After measuring 45 species
across the evolutionary tree, the team has already found as many as 12 independent
changes in color vision. In evolutionary terms, jumping
spiders seem to be evolving new expanded forms of
color vision all the time, and different species have acquired their new visual capabilities
in very different ways. Take, for example, the ability to see red. Most jumping spiders
only have green-sensitive and UV-sensitive photo
pigments in their retinas. But some species became sensitive to red when their green-sensitive opsin gene was accidentally duplicated in the genome, and the new copy started to evolve, shifting its sensitivity
to longer wavelengths. - So in our eyes, that's
exactly what happened. The opsin gene for our
green-sensitive visual pigment was duplicated, and the
second version evolved to be more red-sensitive, and we see this happen over
and over and over again in jumping spiders. - But other jumping spiders see red in a totally different way. Rather than evolving new photo pigments, they added an internal filter to some of their
green-sensitive cone cells, which cuts out green light, and forces those cells to respond only to longer wavelengths, like red. - They can basically
create two kinds of cells from the same type of photoreceptor simply by using a filter
in front of some of them and not in front of others. - All this evolutionary innovation makes the original question
even more intriguing. Why evolve expanded color
vision in the first place? That's the question Lisa
Taylor is trying to answer. For a visual predator,
like a jumping spider, better color vision could
mean finding more prey. It could also mean avoiding
prey that might harmful. - [Lisa] And so, a lot of prey
in the environment advertise their toxicity with bright colors, and particularly with
long wavelength colors, such as red and orange. So we're testing the idea that the ability to use color vision will help
these spiders learn to avoid and continue to avoid
red prey that taste bad. - [Derek] In this experiment,
all the prey are termites. Some have a dab of red
paint on their backs, and others have a dab of gray, - And this doesn't affect
their behavior in any way. They still move around naturally, and the spiders really
like to eat termites. - [Derek] The red
termites also get treated with a compound called Bitrex, which is actually the most
bitter substance known. - And yeah, it turns out
that the spiders also think it tastes disgusting. So we can simultaneously and independently manipulate
color and palatability. - In other words, the researchers can make the termites red and
bitter, gray and tasty, or if they want to mess with the spiders, gray and bitter, or red and tasty. The first part of the experiment
is the training phase. Basically, the spiders get
to choose from a tiny buffet of termite prey, each one in
its own little Petri dish. - [Lisa] In three of the Petri dishes, they get a red-painted,
bitter-tasting termite, and then the other three Petri dishes, they get a gray-painted,
very tasty termite. As they interact with this prey, they are constantly learning and it's constantly being reinforced that whenever they attack something red, they get a mouthful of
bitter-tasting termite, and whenever they attack something gray, they get a mouthful of tasty termite. The first spiders to go
through this experiment are Habronattus pyrrithrix,
and we started with them because we know that they
have good color vision that extends into the long wavelengths. - [Derek] Habronattus
pyrrithrix is one of the species that can see red using a red filter in front of some of its
green-sensitive cone cells. - [Lisa] Our data so far suggests that the spiders are really
good at learning the rules. - And once they learn the rules, then the real experiment begins. The spiders hunt for all
their food in a setup just like the termite buffet
where they were trained, except that for half of the spiders there's a big difference. Some of the termites are still
bitter, but they're all gray. The color cues are gone. Now, the question becomes do the spiders that have
color cues available, in other words, the ones
for which bitter termites are still red, do they do better? - So our data so far show
that they do fare better when they have access to those color cues, they lay eggs sooner, and that they're also heavier
at the end of the experiment if they're in the treatment group where they have access to color cues. - One hypothesis for why primates evolved expanded color vision is to distinguish ripe from unripe fruit, or tender new leaves
from older, tougher ones, in other words, telling good
food apart from bad food, kind of like what these spiders are doing. - Here, we've got this kind of
evidence in a jumping spider, and we're gonna repeatedly test that in other jumping spider species that have different forms of color vision. - The team predicts that spiders with expanded color
vision will use color cues to their advantage. So they'll do better
when color can tell them which prey items taste bad. Species that can't see
red won't get any benefit from the warning colors,
or from the training. If the data support these predictions, these will be some of the first
experiments in any species to reveal an evolutionary
advantage to seeing and discriminating more colors. But feeding behavior
can't be the whole story, because the spiders had some
more surprises in store. - For example, there's a
genus of jumping spiders in Central America called Mexigonus, where males and only males sport incredibly bright red colors
on parts of their body that they use during courtship. - We thought for sure the female has gotta be
paying attention to red, distinguishing it from other colors. They've gotta have red color
vision in some special way. - And it turns out that at
least by our measurements, they don't have the ability to see red. They just have UV and
green-sensitive cells in those principal eye retinas. - I don't mind being proved wrong at all. It usually means something more exciting, because it means that, oh my
God, there's something cool and new in the world, right? And you've learned something new. - So what's going on here? To help answer that question and maybe understand why
some spiders are displaying to one another with colors they can't see, it's time to revisit the
jumping spider retina. - Instead of just one retina like we have, they have a stack of translucent retinas right on top of each other. - One thing that we think
that this layering does is to correct for a problem that the optics present to the retina. It's called chromatic aberration. - Most optical materials,
like these glass prisms, refract, or bend short wavelength light, like blue and UV, more strongly than long wavelength light, like red. That's chromatic aberration. Lenses do this, too. In photos taken with
vintage camera lenses, you can often see a fringe of color around high contrast edges. The sensor in a camera is a
single flat layer of photosites. So getting the different
colors of light to focus in the same plane is critical. Modern camera lenses correct
for chromatic aberration using complicated optical designs with lots of lens elements. - But the other solution is to put different color-sensitive
cells at the right depths behind the lenses, so that the colors that they're sensitive
to are in proper focus. - That's exactly what
jumping spider retinas do, and this gets us one step
closer to understanding what red might mean to a spider
that can't actually see red. In the jumping spider
eye, the cells sensitive to shorter wavelengths are
generally closer to the lens, and those sensitive to longer
wavelengths are farther away. But most jumping spiders are dichromats. They only have two cone cell types. So why have four layers in their retinas? - [Nathan] Typically, the bottom, or farthest away from the lens two tiers, we call those tiers 1 and 2. Those are both typically
sensitive just to green light. And with a retina like that, an object in that world might
appear in different focus in tier 2 than in tier 1. - [Derek] Researchers in Japan have shown that jumping spiders can
actually use this discrepancy in focus to perceive depth and distance in their environment. - But there is a liability
with this system. It only works if you're just
using one color of light, like green. If you start to mix in
other colors of light, for example, red, then
the system creates errors. Essentially, colors like red
might create this perception of being close, or being
looming towards the receiver, and that would provide a totally different perceptual
experience for the viewer. - So a jumping spider's red
coloration might not look red to another jumping spider, but instead create a
sort of depth illusion. But why would a male spider benefit from displaying an optical illusion? - One thing about jumping spiders is that females often are quite aggressive towards prospective mates. In fact, they can often eat the male rather than allowing
him to mate with them. So these males, when
they're dancing for females, are really actually
dancing for their lives in many instances. - [Derek] If a female
thinks a male is closer than he really is, that
could throw off her attack, or maybe confusing the female
pays off in other ways. - If she can't quite figure
out the male's display, she might stick around paying
attention to it for longer, and this might result in better outcomes for the male at the end of courtship. - [Derek] And amorous
male spiders might not be the only ones exploiting
these depth delusions. - So imagine a red prey item. We might look at it and
say, "That's probably toxic, "and it's warning birds that it's toxic." But another possibility is that it's red simply to look like it's
closer to a jumping spider, so that it has a better
chance of escaping. We also see small insects with red and blue patterns on them, which would create a really
complicated visual illusion that might simply baffle it, and require it a longer period of time before it judge this distance. Even a split second can really matter. - But in this tiny game of cat and mouse, a spider that could see and
discriminate red from green would be a lot harder to fool, and this could be another
surprising benefit of color vision, one that isn't
really about color at all. - And what we really need to ask whether, or not this
hypothesis is even plausible is really good high
resolution measurements of the distances of things in their eyes, including the retina and the
lenses, from live animals. - [Derek] This information
you can't just get from preserved specimens
on microscope slides, but there is another way. (light music)
(air whooshing) By using a particle accelerator called the Advanced Photon Source,
the researchers have started to collect high resolution X-ray videos through the spider's exoskeletons. - This has never been done before. It's in X-ray, so we can
see through their eyes, and we can see how these
eye tubes are moving around. - [Derek] If the spider's
retinal movements change the shape, or length of their eye tubes, that'll affect what they're
capable of perceiving. - It would change how
they experience depth. It would change how they experience color. Previously, this information has only been collected from dissections. So we're very excited to get
super high resolution videos of the inside of the spider's head as it's performing
complicated visual motions. - Unfortunately, a few months
after their initial tests, the Advanced Photon Source
shut down for upgrades that'll take over a year to complete. So it looks like we'll have
to wait a little longer for some of the answers the
team has been looking for. - We know that the retinas
can be moved around and that they maybe have between
a 50 and 60-degree travel. Not only can they be moved
in the horizontal plane, but in the vertical plane, and they can actually be twisted to change the orientation
of their field of view. - The question is how do
these movements affect what the spiders can focus
on, or how they sense depth, or even how they perceive color? It's this connection between focus, depth, and color that makes these
spiders so intriguing. - It opens up all sorts of questions about what color is in the first place. (light music) - It's already clear
that these spiders have a lot to teach us about color vision, how and why it evolves, and how many forms it can take even within a single group of animals. (gentle music) If our understanding of their
visual system is correct, the experience of color
for jumping spiders might even be three-dimensional in a way that's totally different
from how we see the world. And we haven't even talked
about their other senses, like their ability to
communicate through vibration. When you think about it, you realize that the
universe we humans perceive, even with all our technology, is just a sliver of what's out there. (light music) - If we owe anything to the world, it's to allow the world to be experienced in the fullness of itself. I think this is one of the
tragedies of extinction is the loss of oftentimes
a totally unique way of experiencing our world, a way of experiencing our world that we probably couldn't even imagine. - So color, what is it? Is it an intrinsic property of an object, like Aristotle thought, or something that exists only
in the mind perceiving it, like Galileo believed? Maybe it's not an either/or question. - My belief is that color
is something that emerges through the evolution of
the eyes that see the world and the world that the eyes see. Color as a thing emerges
through this dance, this evolutionary dance between what can be sensed about the world and those that are sensing it. - It's that dance playing out
over millions of generations that created the colorful
world we inhabit, and shaped the countless ways that we and our fellow
life forms experience it. (light music) (graphics beeping) Come to me. Okay, not that far. Ah! (Derek laughing) They're not called jumping
spiders for nothing. Yeah, come on.
