What Jumping Spiders Teach Us About Color

- You're not looking at a yellow ball. Your brain might think you're looking at a yellow ball, but look closer. The screen you are viewing this on displays

color

s 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 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'm looking at a yellow ball, aren't I? (light and mysterious music) After all, my eyes are not so different from that camera.

The human retina only has cones sensitive to red, green, or blue wavelengths of light. To perceive other

color

s, we have to integrate the inputs from those three types of cones. When yellow light enters my eye, it stimulates my red- and green-sensitive cones, although not as much as pure red or green light would. With the red- and green-sensitive cones equally excited, my brain tells me I'm looking at yellow. This is how our technology, our cameras, screens and projectors can trick our brain into seeing an entire rainbow of colors using just three wavelengths of light by activating our three different types of cones in different ratios.

So is the ball really yellow? What does yellow mean? Many people would say that color is the wavelengths of light that an object reflects. In other words, as Aristotle thought, color is a property of the object. But when looking at the same ball on a screen, your eyes only perceived red and green light, but your brain still perceived it as yellow. So it is also possible, as Galileo believed, that color is not a property of an object at all, but a phenomenon of the mind. But whose mind? Because we are not the only animals that can see the world in color. - Many times we do not 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 to us, and not to them, because they can't see orange or red as anything other than green. -Then maybe we should start seeing the world from more than just the human perspective. Doing so could

teach

us why color vision evolved in the first place. - Honestly, we're on the lower end of the spectrum. We are a step ahead of our household pets, perhaps, if they are cats or dogs, but not as good as many groups of animals out there: butterflies, birds, fish, lizards and

jumping

spiders

.

In reality,

jumping

spiders

are harmless creatures. They never actually grow large enough to pose much of a threat to humans. Of course, if you were a small insect, yes, you would absolutely have to be afraid of a jumping spider. - They will take down prey that is sometimes two or three times larger than their own body. - The Chinese word for jumping spider literally translates to flying tiger, and that's how I like to think of them, the kind of little undergrowth cats. - Jumping spiders are everywhere. They are in your backyard. They're probably in your kitchen. - There are about 6,000 species of jumping spiders known.

Some are some

what

hairy, others are some

what

shiny, striped, spotted, red, green, blue. Pretty much anything you can imagine. It's as if each one is a small work of art. - As a group, spiders are not known for their vision. I mean, most species are nocturnal, and for many, their webs act as a kind of extra sensory organ, so they just don't need to see as well. But jumping spiders, as active diurnal hunters, are 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 things like motion detection and light sensitivity into 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 main eyes, and those are the really big eyes on the front of the face that make jumping spiders look a little cute, if you're willing to say. A spider always looks cute and those eyes are built like no other eyes in the animal kingdom. - It turns out that the way jumping spiders perceive color has a lot to do with the anatomy of those main eyes. - They are actually built very similar to a Galilean telescope or binoculars.

That large lens that you see from the outside of the animal is one of the two lenses of these eyes, and between those two lenses is a long tube filled with liquid. At the end of that liquid-filled tube is a second lens, and what that lens does is magnify the image that that first lens projects along that long tube, thereby increasing the ability to see details on the retina. which is located just below it. - And when it comes to seeing details, it's hard to beat a jumping spider. - For most animals, the larger the eye, the better it works.

Jumping spiders absolutely break this rule. Secondary eyes can see the world as well as the best insect eyes there are, better than the world's largest dragonflies, whose entire head is consumed by one eye. Primary eyes can see the patterns of the world better than a lap dog, a house cat, an elephant, and almost as well as a sharp-eyed pigeon, but what they can see is a very narrow portion of the world. It's like you have your thumb extended with your arm extended. - And it is only in that narrow portion of the world where jumping spiders can see fine details and colors. - So, the jumping spider's secondary eyes give them a full 360-degree view of the world.

Now, imagine that most of it is in black and white. When you see something moving, you can turn to look at it. And now, whatever is really curious to you, you can add it to this world of black and white vision with fine details and colors. But you can only do it moment by moment. So you're really painting in additional details about the color and pattern that you wouldn't be able to see otherwise. It's a wild world to try to immerse yourself in. (bee buzz) - While scanning a scene with their primary eyes, some species of jumping spiders add much more color information to their world than others.

Most jumping spiders, including this one, are dichromatic, meaning they have two types of color-sensitive cones in their retinas, just like dogs and most other mammals. - And by comparing those two types of cells and how they respond to ambient light, they get a rough understanding of color. They can distinguish between UV, violet, blue and green. - But some types of jumping spiders are trichromats with three types of cones, like humans, and others are tetrachromats, like birds. (light music) The strange thing is that all of these species with expanded color vision are not necessarily close relatives. - In the case of jumping spiders, we have great variation, even among closely related groups, in their ability to see the colors of the world.

Jumping spiders are, in some ways, reinventing the ability to see color over and over again in different ways. - That makes jumping spiders very special. I mean, let's consider primates. Old World monkeys, apes, and humans have trichromatic color vision, but we also share a common ancestor. So our color vision probably evolved only once and then stuck. This is where jumping spiders really shine. The ability to see red, for example, has evolved several times in jumping spiders. Researchers know this because they've figured out how different groups are related, and by this I'm mainly referring to jumping spider fan Wayne Maddison. - Oh my God, Havaika.

Fantastic! Beautiful male. Ah, it's been 30 years since I saw a Havaika in the flesh. - He is absolutely Mr. Jumping Spider. His expertise is really about jumping around spider taxonomy. - Work on the evolutionary tree of jumping spiders. The evolutionary tree of life is basically the path of genetic descent that unites us all. - The position of different species on this evolutionary tree can tell us how they came to acquire the traits they have. For example, if most jumping spiders don't see red, but two species in two very different parts of the tree do, those two species likely developed those abilities independently. - In which case, then we can start asking questions like what is driving that evolution?

Do they have similar ecologies? Do they hunt similar prey? And try to really understand what selective forces are driving these expanded color vision systems. - It could help them find food or distinguish tasty prey from prey that could harm them. - Because, of course, many small insects are brightly colored and some of them use those bright colors to advertise that they are toxic. - Another possibility is that seeing a world richer in color could help animals, from lizards to spiders, choose better mates. To test these ideas, 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 all the major branches of the jumping spider family tree. - One of our first things is to simply prioritize where to go and what to look for. And so there's a lot of sampling in a lot of places. - Let's see who lives here. - It's kind of like "Pokémon GO", except the Pokémon are real, they're smaller than your pinky nail, and they're very good at hiding. - Some jumping spiders have evolved to adapt perfectly to a particular situation. So, for example, there are jumping spiders that are specialists in eating termites that you will only find around termites.

There was a species that we only found in piles of bones in South Africa. Who knows what he was doing there, but we quickly learned that this was the only place in the environment where we would find him. And that's part of the fun. I really feel like it's kind of a treasure hunt. (light music) - Leaving no stone unturned, the team returns to their laboratories with hundreds of spiders representing many different species. They want to discover which species have expanded color vision, how each species does it, and why. It's actually a difficult question to know how animals can see color.

We can't just wire our brains to see what they see. Then, how do we do it? - We start using a technique called microspectrophotometry. It's a very long word. What it simply means is a microscope combined with a device that measures different wavelengths of light, a spectrophotometer. - Researchers take ultrathin slices of jumping spider retinas and then measure the wavelengths of light absorbed by individual cone cells. With enough measurements, they can tell whether a species is dichromatic, trichromatic, or tetrachromatic, and which wavelengths of light its cells detect best. But that's not the whole story. - Having this knowledge of what is in the retina tells us what is or is not possible for these animals to see, but it does not really tell us what they see or how they could use it.

Therefore, the gold standard for establishing that an animal can see color is to do so behaviorally. - In other words, we somehow need to ask spiders what they can see and then understand their answers. It's hard to figure out what goes on inside a spider's mind. It's no surprise that it takes a group of expert zoologists to do it. But our own minds can be just as complicated, and yet we rarely talk to mental health experts to help us interpret our thoughts and emotions. Whether you're feeling depressed, anxious, or just stressed out and want to talk to someone, a therapist allows you to see things from a different perspective, and that's where today's sponsor, BetterHelp, comes in.

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You show the spider a screen with a moving shape that differs from the background in color, but not brightness, and see if the spider tries to follow you. - The problem with letting the jumping spider turn and respond is that it will, but it changes something about what it sees. So what we want is to really have some control over what the spiders can see at any given time. -The researchers then hold them in place with small magnets attached to their heads. - What we do is give them a ball to stop. They actually hold it with their feet and we can monitor how the ball moves on their feet so we know where they would want to go. - If the spider turns the ball to the left, it is probably trying to look to the right to follow the moving shape, and that is evidence that the spider can discriminate between the colors of the shape and the background. (fun music) Once the team knows which species can see which colors, the next question is how do they do it?

What is different about the DNA of these spiders that can see and discriminate more colors? If you ask Megan Porter, a lot of this comes down to genes that code for proteins called opsins. - The way animals achieve color vision is to have different copies of this opsin gene, and those variations are what produce proteins that are sensitive to different colors of light. The first technique that we usually turn 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 each gene that is expressed in that tissue. - This method provides researchers with an inventory of all the genes that are expressed, that is, all the genes that are copied from DNA and sent to make a protein.

The team can then determine where each of these genes is expressed, in which eyes, and in which parts of the eye. - And we do it using a sophisticated technique called immunohistochemistry. - Researchers basically create shiny molecular tags specific to each protein they are interested in. - And then by looking for which parts glow the right color, we can determine where each opsin is expressed, where the protein is located. - The team is especially interested in genes that are expressed in the retinas of the main eyes. These are the genes most likely to be related to changes in color vision.

This process of asking which species have expanded color vision and how they do it has led to some surprising discoveries. 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 how widespread this ability would be. After measuring 45 species along the evolutionary tree, the team has already found up to 12 independent changes in color vision. In evolutionary terms, jumping spiders appear to be constantly evolving new, expanded forms of color vision, and different species have acquired their new visual abilities in very different ways. Take, for example, the ability to see red.

Most jumping spiders only have green- and UV-sensitive photopigments in their retinas. But some species became red-sensitive when their green-sensitive opsin gene was accidentally duplicated in the genome, and the new copy began to evolve, changing 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 happening again and again in jumping spiders. - But other jumping spiders see red completely differently. Instead of developing new photopigments, they added an internal filter to some of their green-sensitive cone cells, which cuts off green light and forces those cells to respond only to longer wavelengths, like red. - Basically they can create two types 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 develop expanded color vision in the first place? That's the question Lisa Taylor tries 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 could be harmful. - This is why many prey in the environment announce their toxicity with bright colors and, in particular, 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 avoiding red prey that tastes bad. - In this experiment, all the prey are termites.

Some have a little red paint on their back and some have a little gray, and this does not affect their behavior in any way. They still move naturally and spiders really like to eat termites. - Red termites are also treated with a compound called Bitrex, which is actually the most bitter substance known. - And yes, it turns out that spiders find it disgusting too. In this way we can simultaneously and independently manipulate color and palatability. - In other words, researchers can make termites red and bitter, gray and tasty, or if they want to mess with spiders, gray and bitter, or red and tasty.

The first part of the experiment is the training phase. Basically, the spiders can choose from a small buffet of termite prey, each in its own petri dish. - In three of the Petri dishes, they get a termite painted red and with a bitter taste, and then in the other three Petri dishes, they get a termite painted gray and very tasty. As they interact with this prey, they constantly learn and it is constantly reinforced that every time they attack something red, they get a bite of bitter-tasting termite, and every time they attack something gray, they get a bite of tasty termite.

The first spiders to perform this experiment were Habronattus pyrrithrix, and we started with them because we know they have good color vision that extends to long wavelengths. - Habronattus pyrrithrix is ​​one of the species that can see red using a red filter in front of some of its green-sensitive cones. - Our data so far suggests that spiders are very good at learning the rules. - And once they learn the rules, the real experiment begins. The spiders hunt for all their food in an environment similar to the termite buffet where they were trained, except for half the spiders there is a big difference.

Some of the termites are still bitter, but all are gray. The colored signals have disappeared. Now, the question is: do spiders that have color cues available, in other words, those for which bitter termites are still red, fare better? - So our data so far shows that they do better when they have access to those color cues, they lay eggs earlier, and they also weigh more at the end of the experiment if they are in the treatment group where they have access to color cues. - One hypothesis for why primates developed expanded color vision is to distinguish ripe from unripe fruit, or new, tender leaves from older, tougher ones; in other words, distinguishing good food from bad, kind of like what these spiders do. - Here we have this kind of evidence in a jumping spider, and we will test it repeatedly in other species of jumping spiders that have different forms of color vision. - The team predicts that spiders with expanded color vision will use color cues to their advantage.

Therefore, they will do better when color can tell them which prey tastes bad. Species that cannot see red will not benefit from warning colors or training. If the data support these predictions, these will be some of the first experiments in any species to reveal an evolutionary advantage in seeing and discriminating more colors. But feeding behavior can't be the whole story, because the spiders had more surprises in store. - For example, there is a genus of jumping spiders in Central America called Mexigonus, where the males and only the males sport incredibly bright red colors on parts of their body that they use during courtship. - We think for sure that the female should pay attention to red, distinguishing it from other colors.

They must have red color vision in some special way. - And it turns out that at least according to our measurements, they do not have the ability to see red. They simply have UV- and green-sensitive cells in the main retinas of their eyes. - I don't mind at all being proven wrong. It usually means something more exciting, because it means, oh my gosh, there's something new and cool in the world, right? And you have learned something new. - So what is going on here? To help answer that question and perhaps understand why some spiders show each other colors they can't see, it's time to revisit the jumping spider's retina. - Instead of a single retina like ours, they have a stack of translucent retinas on top of each other. - One thing we believe this stratification does is correct a problem that optics presents to the retina.

It's called chromatic aberration. - Most optical materials, such as these glass prisms, refract or bend short-wavelength light, such as blue and ultraviolet, more strongly than long-wavelength light, such as red. That's chromatic aberration. Lenses do this too. In photographs taken with older camera lenses, you can often see a band of color around the high-contrast edges. A camera sensor is a single flat layer of photosites. Therefore, it is essential to get the different colors of light to focus on the same plane. Modern camera lenses correct chromatic aberration using complicated optical designs with many lens elements. - But the other solution is to place different color sensitive cells at the appropriate depths behind the lenses, so that the colors they are sensitive to are in proper focus. - That's exactly what jumping spiders' retinas do, and this brings us one step closer to understanding what red could mean to a spider that can't actually see red.

In the jumping spider eye, 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 dichromatic. They only have two types of cone cells. So why do they have four layers in the retina? - Generally, we call the lower two levels, or those furthest from the lens, levels 1 and 2. Both are usually sensitive only to green light. And with a retina like that, an object in that world could appear with a different focus at level 2 than at level 1. - Researchers in Japan have shown that jumping spiders can use this focus discrepancy to perceive depth and depth. distance in their surroundings. - But this system carries a responsibility.

It only works if you use a single color of light, like green. If you start mixing other colors of light, for example red, the system generates errors. Essentially, colors like red could create this perception of being near or close to the receiver, and that would provide a totally different perceptual experience for the viewer. - Therefore, the red coloration of one jumping spider may not appear red to another jumping spider, but instead creates a kind of illusion of depth. But why would a male spider benefit from displaying an optical illusion? - A characteristic of jumping spiders is that females tend to be quite aggressive towards their potential mates.

In fact, they may often eat the male instead of allowing him to mate with them. So these men, when they dance for women, in many cases they are actually dancing for their lives. -If a female thinks a male is closer than she actually is, that could deflect the attack away from her, or perhaps confuse the female in other ways. -If she cannot understand the male's display, she may pay attention to him longer, and this could result in better results for the male at the end of the courtship. - And amorous male spiders might not be the only ones exploiting these deep delusions. - Then imagine a red dam.

We could look at it and say, "It's probably toxic," and it warns the birds that it's toxic." But another possibility is that it's red simply to make it look closer to a jumping spider, so it has a better chance of escaping. We also see small insects with red and blue patterns, which would create a really complicated visual illusion that might just baffle you and require a period of timelonger before judging this distance. Even a fraction of a second can really matter. - But in this little game of cat and mouse, a spider that could see and discriminate red from green would be much harder to fool, and this could be another surprising benefit of color vision, one that actually has nothing to do with color. - And what we really need to ask whether this hypothesis is plausible or not are really good, high-resolution measurements of the distances of things in their eyes, including the retina and the lens, from living animals.

It is not obtained simply from samples preserved on microscope slides, but there is another way. (light music) (air hissing) Using a particle accelerator called the Advanced Photon Source, researchers have begun collecting high-resolution X-ray videos through the spider's exoskeletons. - This has never been done before. It's on X-ray, so we can see through its eyes and we can see these eye tubes moving. - If the movements of the spider's retina change the shape or length of the eye tubes, that will affect what they are able to perceive. - It would change the way they experience depth. It would change the way they experience color.

Previously, this information had only been collected from dissections. So we're very excited to get super-high-resolution video of the inside of the spider's head as it makes complicated visual movements. - Unfortunately, a few months after its initial testing, Advanced Photon Source was closed for updates that will take over a year to complete. So it looks like we'll have to wait a little longer to get some of the answers the team has been looking for. - We know that the retinas can move and that they can have a path of between 50 and 60 degrees. They can not only be moved in the horizontal plane, but also in the vertical plane, and in fact can be rotated 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 is this connection between focus, depth and color that makes these spiders so intriguing. - Opens up all kinds of questions about what color it 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. (soft music) If our understanding of their visual system is correct, the experience of color for jumping spiders might even be three-dimensional in a totally different way than how we see the world.

And we haven't even talked about his other senses, such as his ability to communicate through vibrations. When you think about it, you realize that the universe we humans perceive, even with all our technology, is only a small part of what exists. (light music) - If we owe anything to the world, it is to allow the world to be experienced in the fullness of itself. I think this is one of the tragedies of extinction: the loss of an often 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, as Aristotle thought, or something that exists only in the mind that perceives it, as Galileo believed? Maybe it's not an either/or question. - My belief is that color is something that arises 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 felt about the world and those who feel it. - It is that dance that unfolds over millions of generations that created the colorful world we inhabit and shaped the countless ways in which we and our other life forms experience it. (light music) (graphics beeping) Come to me.

Okay, not that far. Ah! (Derek laughs) They're not called jumping spiders for nothing. Yeah come on.