# Why Animal Eyes Look So Weird

## Метаданные

- **Канал:** Real Science
- **YouTube:** https://www.youtube.com/watch?v=V6UTU80vnjE
- **Источник:** https://ekstraktznaniy.ru/video/42089

## Транскрипт

### Segment 1 (00:00 - 05:00) []

My whole life, I've loved looking into the eyes of the creatures that I come across in the woods. Hi, buddy. Like this weird turtle that I found the other day. Or this frog that I found on my back porch. This guy also lives in my house. And I'm constantly looking at his piercing slit pupils and wondering why. And then also getting scared when they turn into giant saucers. Eye shapes. Those weird evolutionary little masterpieces seem to hold clues. puzzle pieces that tell us a story about how that animal lives, what it needs to see, what it fears, and how it survives. But the story, at least to me, isn't always apparent. Why do goats have those horizontal pupils? Why do cats have that sinister vertical slit? Why do geckos have multiple pin holes? And why do cuttlefish have the weirdest of all, a Wshaped pupil? And I guess my biggest question is how did camera type eyes evolve to work in a huge variety of conditions from bright to almost pitch black in the air and in the water and in a huge variety of animals. Round eyes or what biologists often call camera type eyes are found across all vertebrates and even some invertebrates. And there's a good reason for that. They're excellent at producing sharp, detailed images. Structurally, these eyes are typically spherical or nearly so with a cornea and a lens that work together to focus incoming light onto a retina packed with photo receptors. This setup is ideal for animals that rely heavily on vision for activities like hunting, navigating, or recognizing mates and rivals. So, how do they actually work? It's all about focusing light. The lens is at the core of this. The eye lens is a transparent flexible by convex structure inside the eye that helps focus light onto the retina, the light sensitive layer at the back of the eye. But maybe surprisingly, the cornea does more of the focusing than the lens, at least in terrestrial animals. The cornea is the clear dome-shaped outer layer at the front of the eye, and it does a lot to bend light due to the difference in its refractive index from air. To back up slightly, in a vacuum, light travels at its maximum speed of 300,000 km/s. In any other material, light slows down. The more it slows, the higher the refractive index. So when light travels from air into the cornea, which is mostly water, it slows down and bends. This initial bending begins focusing the light towards a single point on the retina. The lens then fine-tunes this focus, bending the light further to form a sharp image. In the human eye, about 2/3 of all focusing is done by the cornea and 1/3 lens. But things are different underwater. In water, there's not much difference in how light bends when it passes from water into the eye. For us, that's why it's blurry when you try to look around with your eyes open underwater. But fish and other aquatic animals have a way of dealing with this fact. In a fully aquatic animal, the medium behind the cornea is basically water. In front of it, there is water, which means the cornea doesn't really do much focusing at all. To learn more about the eyes of the animal kingdom, I spoke with Professor Dan Nlson, professor of functional zoology at Lond University in Sweden. So the lens inside the eye will have to do 100% of the focusing. And in fish in general, the lens is perfectly spherical. Now, normally this wouldn't be great because a spherical lens usually has aberrations, a type of optical distortion that happens when light rays passing through the edges of a curved lens focus at a different point than rays passing through the center. This causes a blurred or soft image, especially around the edges of what you're looking at. But the lens of a fully aquatic animal, like a fish or a dolphin, is special. The spherical lens does not contain a single refractive index, but instead a graded index which is achieved through structural and biochemical variations within the lens tissue. The refractive index varies gradually from the center to the edge. And the retina behind it is at the same distance to the lens all the way around. And the lens is also perfectly symmetric. uh it focuses not really by its surface but because it has higher refractive index or higher optical density in the center and lower towards the periphery. So rays that come into the lens gradually bend as they pass through the lens and they form a really nice aber totally aberration free focus on the retina. That is pretty neat. But this brings up an interesting conundrum

### Segment 2 (05:00 - 10:00) [5:00]

for animals that live both in the water and on the land like sea lions. An aquatic eye with a spherical lens on land would not work well. Nor would an eyeball built for air work well underwater. So, what do they do? Yeah, seals simply have a rather flat cornea. They've tried as much as possible to have a flat cornea because the problem is that refraction will change when they're in water and when they're above water, which means you can't really have a sharp eye in both conditions unless you do something else. Professor Nilson says this best in his book, Animal Eyes. The eyes of baby seals are like limpid pools. Not because of the purity of their souls, but because the corneas are flat. So they don't rely on the cornea for light focusing really. But even if they are just relying on the lens, doesn't this still pose a similar problem? It does. But seals have a remarkable trick up their sleeve. They can change the shape of their lens. Like many other mammals, seals have muscles called siliary muscles that allow them to change the shape of their lens, a process called accommodation. This helps them adjust focus depending on whether they're looking at something near or far, or whether they're underwater or on land. Humans can do this, too, but seals can do it to a much greater degree. This ability helps them maintain relatively sharp vision in both environments. Something humans can't do without goggles or masks. And all of these different tactics to focus light from the cornea and lens is for one ultimate goal to form a clear, sharp image on the retina. Inside the eye, the retina plays the role of the camera sensor. It's curved and lined with rods and cones. Rods help you see in low light, while cones handle color and fine detail. When light hits these cells, it triggers a chemical reaction that generates electrical signals, which we know as seeing. But before any light reaches any of this, it first must pass through the gateway of the eye, the pupil. Some of the simplest eyes in nature operate much like a pinhole camera, where the pupil does much of the work. The nautilus is a perfect example. It has no lens at all. Instead, it relies on the pupil to form an image directly on the retina. The image is dim, soft, and monochromatic. To add more light to see better in dim conditions, the pupil could be widened. But that comes at a cost. If you open the pupil fully, then the rays that come in at the periphery of the pupil are not bent in exactly the same way as those that come in through the center, which means the image is not as sharp. So, a wide pupil during the day would yield blurry vision, and a tight pupil at night would not let in enough light to see anything. Many animals have to deal with this balancing act. The eye needs to constantly adjust the pupil size to optimize vision depending on conditions, and some animals take it to the extreme. Enter the harpy eagle. The harpy eagle has eyes that are the same size as ours despite a much smaller head. The retina is densely packed with photo receptors and two phobia per eye. And their pupils can open much wider and can constrict much tighter than ours, giving them both sharper daytime focus and superior low light vision. Plus, they have powerful iris muscles, enabling them to adjust this pupil size rapidly. This rapid adjustment is crucial for navigating the varying light conditions of their rainforest habitat, where sudden transitions from shadowed canopy to bright clearings are common. This incredible dynamic range and adaptability contributes to their visual acuity being an estimated 3 to four times sharper than that of humans. Scientists estimate that they can detect prey at distances of approximately 200 m or 650 ft or more. But their vision is not limitless, and despite their big pupil, they still can't hunt at night. Other more nocturnal birds of prey have found a way around this, but the trade-offs are even more striking. So, if you're night active, you really need to maximize the size of your pupil. So, you need to have a very large lens and a fully open pupil. So, you use the full diameter of the lens. If you then add a complete retina to see as large a visual field as possible, you end up with a huge eye and eyes are expensive and heavy, especially for birds that have to flower in them as well. So what they do is they sacrifice peripheral vision by simply removing the peripheral parts of the retina. So they get a kind of tubular eye. This is what owls use. They've maximized sensitivity and have as good a vision as possible in the forward direction. But this comes with a price. Their eyes can't move in their sockets. So when an owl wants to look around, it has to move its head.

### Segment 3 (10:00 - 15:00) [10:00]

They can turn their head an astonishing 270°. Humans are lucky if we get 180° of rotation. Owls tubular eyes certainly seem unusual, but with them they can see incredibly bright and sharp images even under moonlight. And they aren't the only animals with tubular eyes. And you can actually see similar things in deep sea fish for a different reason. These deep sea fish, they point these tubular eyes upwards and they look for silhouettes against the downwelling light. And that's why they can actually see prey looking upwards. If they look sideways or down, it's way too dim to actually see anything at all. And other predators have them, too. Yeah, there are a few really weird examples. Worms and marine worms. Really large predatory awful things. They have tubular eyes as well. It's just basically prey detectors. They're not interested in seeing anything else. They burrow in the sand and look up with their tubular eyes and if something passes above, they'll jump up and grab it. Quick side note, if you want to hear the full conversation with Dr. Nilson, head over to Nebula now. In the full interview, you can hear even more tangents about some of the weirdest eyes of animals we couldn't fit into this video. Okay, back to the video. So, some predators have tubular eyes, but certainly not all of them, and not all predators even have circular pupils. If you have a cat, you've probably wondered about their piercing, vertically slit pupils. And conversely, if you've ever hung out with a goat, you've probably seen their somewhat eerie, horizontally slit pupils. What's the deal with these shapes? We noticed a very striking correlation between animals that have the eye on the side of their head. They almost always have a horizontal slit pupil and they're almost always prey animals. This is Marty Banks, professor ameritus of optometry and vision science at Berkeley University. We only found one animal that had horizontal slits and had their eyes in front of the head, and that's the mongoose. And if you ever seen one, it really looks weird. To see an animal that has pupils like this in front of their head really looks weird. Okay, so we noticed that. And um then we noticed that of the animals that had vertical slits, they were all predators. So we say, "Wow, what's that about it? The these different ecological niches seem to drive pupil shape. " This gets a little complicated, but the horizontal ones are easier to explain. So let's start there. Okay. So, their eyes are on the side of their head and they are very likely to be prey animals, not predators. And so, if you're a prey animal on the ground, what do you need to see? You got to see around you in order to see someone sneaking up on you. Okay. So, then how do I maximize my field of view along the ground? The eyes on the side of the head certainly increase the field of view and the pupil. Having the horizontal slit allows you to get more light in from behind you and from in front of you. Okay, so that seems simple. Horizontal pupils allow more side to side light in to detect predators on the ground. The horizontal shape helps sharpen contrast between the ground and the horizon, making it easier to spot movement from a distance. The unique shape also enables them to keep the ground in focus while moving, even when running away quickly. But there's one problem with this. theory. What happens when the animal tilts its head to do things like eat grass, the thing they do like all the time? If the pupil rotated with the head, the sharpness would be misaligned with the ground, and that would defeat the whole purpose of detecting predators. To think about this conundrum, Marty took a trip to the zoo and I neighbor's farm. So, I literally went down to the children's zoo with a video camera, and it turns out when goats lower their head and raise their head, the pupil stays aligned with the ground. So, the eye literally is rotating in the head. It turns out we can do this, too, but it's very small movements. We can do it a couple of degrees. These animals can do it like 40°. It's huge. And there are other strange things about the eyes of certain unullet. If you look closely, you sometimes see this lumpy dark structure at the top edge of the pupil. This is called the corpora negra. It acts a bit like a built-in sun visor or awning. When the sun is high, especially during midday, the corpora shades the top portion of the pupil, reducing glare and protecting the sensitive retina from being overwhelmed by direct light from above. that helps preserve contrast and clarity in the terrain below, right where they need to focus their attention. So, if horizontal slits are optimized for certain prey animals, why do certain, but not all, predators need

### Segment 4 (15:00 - 20:00) [15:00]

vertical slits? To find out, we can start to look at certain clues. The ones with vertical slits tended to have eyes from the front of their head. They tended to be predators. either active at day and night or at night. They're ambushed predators, so they have to lay still. They can't be moving around, and they tend to be short. Imagine a small cat, a lone hunter, active at dawn and dusk, stalking in tall grass, and waiting for the exact right moment to pounce. The prey, small mammals, reptiles, and even birds. The hunter is quick, but success depends on precision. There's only one shot. To get this right, the cat needs to know exactly how far away the target is. In the open savannah, a lion might solve this using motion parallax. While the lion chases, the prey moves one way, the background moves another, and it becomes possible to estimate distance from that relative movement. But for ambush predators that stay perfectly still, that doesn't work. So, what other tools are available? First, there's defocus blur. When the eye focuses on a particular object, that point is sharp, and everything closer or farther away appears blurry. Your eyes and your brain can use the amount of blur to infer distance, especially for nearby objects. Next, there's stereopsis. Two eyes, two slightly different views, and one combined sense of depth. This binocular vision is common in predators, including cats. It's especially powerful for close-range targeting, perfect for the final pounce. So, without motion parallax, small cats will need to optimize for these other two cues. But this is easier said than done. Because cat eyes are horizontally aligned, like most eyes are, stereopsis is great for detecting disparities along the horizontal axis, but not the vertical axis. But when a hunter is close to the ground, the differences it needs to detect are largely along this vertical axis. like the vertical outlines of small prey as they move through that dense foreground. So we thought, okay, so we need an eye that images vertical contours very well and doesn't care so much about horizontal contours. How do you do that? Well, it turns out the vertical set pupil is perfect for that. By narrowing the pupil this way, it increases the sharpness of features that are like this, but not like this. So vertical pupils optimize stereopsis or rather expand the functionality of stereopsis to include more verticality. What's the best way to optimize for blur? And the way you get a lot of blur and make blur a really good cue is to open up the pupil. Photographers know that if you taking a picture and you want to have a sharp image of a face and a blurry background, you open up the aperture in your camera. If you want the background and the face to be sharp, you stop down the aperture of the camera. That's called depth of field. So, our idea was we need a short depth of field for blur to be a useful cue to distance. Well, you open up the pupil. But wait, if we open up the pupil, then that's going to hurt stereopsis. Hm. Oh, let's open up the pupil vertically and stop the pupil down horizontally. It's a perfect compromise. So, a vertical pupil is open enough to create a shallow depth of field and allows for better focus on vertical subjects. On top of this, a vertical pupil can shrink and expand to a much greater degree than a round pupil. A human pupil can grow to 15 times its smallest size. A cat pupil can grow to 135 times its smallest size. So, a cat has a much wider range of light sensitivity, which is critical for hunting in both the day and the night. And cats are not the only ones that exploit this vertical pupil orientation. Geckos have them, too, but they're a little different. Yeah, geckos have these they have these slit pupils with little indents in them. So, when they're fully closed, they actually form a series of tiny pupils, which they of course can only use in daytime when there is plenty of light. And that helps them estimate distance, which is really important to geckos. If the image is in focus, they will get one coherent image. If it's out of focus, and depending on how much it is out of focus, it'll produce a number of separate images displaced. By measuring how far these images are displaced, they can judge distance. Once you start noticing weirdly shaped eyes, you'll see them everywhere. And sometimes we aren't fully sure what's going on. This is the case for the cuttlefish who has a crazy looking Wshaped pupil. So, let's say you have a W-shaped pupil and you're looking at a single point of light. If you're well focused, if your eyes focused at that distance, the light will go through that W-shaped pupil and form a point on the retina. But if the eyes are out of focus, the blurred point of light

### Segment 5 (20:00 - 25:00) [20:00]

doesn't just smear. But if your eye is out of focus, your eyes focus too far. Then the image of the point on your retina is a W. If you're focused too near, then it's an M. It flips. And the size of the M or the W is proportional to how out of focus you are. So the speculation is that these animals use that Q to focus their eye. If they see the M shape, they go, "Oh, I have to relax my focus. " And if they see the W shape, oh, I have to increase my focus. But that's not the only possible explanation. Looking at the overall shape, you can see that it's oriented overall like a horizontal pupil. So, I asked Dr. Nilson this. So, do you think that the W and M focusing theory fully explains the shape of the cuttlefish pupil? I'm not sure actually if it may fully explain it, but we don't know. But we did some investigations of it and make made some modeling and computations and we can see that pupil shape actually cuts out more light from above the horizon than it does from below the horizon. So, it evens out the naturally uneven distribution of light. And there's even more going on. Like a cat's pupil, the W shape in a cuttlefish's eye can expand fully to let in more light, which is perfect for seeing in the dark. But that's just one piece of a much more complex puzzle. Despite lacking cones for color vision, cuttlefish can detect polarized light. When you combine that with their skin's ability to reflect polarized light just as easily as it changes color, you get a sophisticated way for them to send visual signals to each other. This is how a cuttlefish might be seen by another cuttlefish with just its monochromatic visual system. But this is how it might look with the polarization visual system. Horizontally polarized light would appear as bright white showing the facial stripes which are used in communication. But we're still piecing all of this together. Some theories even suggest that the shape of the pupil is not only about vision, but about camouflage. Two perfectly circular pupils are easy to spot, but that natural wavy structure might help cuttlefish blend into their surroundings. And they're not the only ones using this camo strategy. And then there are a number of really peculiar pupils in a number of fish, often flatfish. And they are the kind of freely pupils, lots and lots of little branches that cover the part of the eye. And that possibly has to do with camouflage. And they are often predators and they I guess they don't want their prey to see them because then their hunting success will drop in order to camouflage themselves as predators. They conceal their circular pupils which otherwise if the body is really camouflaged the pupils are a giveaway. It's clear that eye shape isn't just a physics problem about building the most efficient camera. That's maybe part of it, but the form and function of eyes are shaped by the environment an animal lives in. The incredible variety of eye shapes we see reflects the wide range of habitats and behaviors found in the natural world. Take the undeniably weird but kind of adorable four-eyed fish. Looking closer, you'll see that it only has two eyes, but each is divided by a thin band of tissue with the top half adapted for seeing in the air and the bottom half for underwater vision. Each section has its own lens shape and focus, so the fish can keep an eye out for insects above while still watching for danger below. It looks dumb, but it's a pretty clever setup for a fish that lives right at the surface. And what's amazing about it all is that camera type eyes have evolved independently many times over. Eyes are often used as an example of intelligent design, but complex eyes can evolve quickly. The 1994 paper by Dr. Nilson titled A pessimistic estimate of the time required for an eye to evolve addresses the question of how long it would take for a complex cameralike eye to evolve through natural selection. Contrary to the notion that such an evolution would require an impractically long time, their model suggests that it could happen relatively quickly in evolutionary terms. From a simple patch of light sensitive cells to a working eye, the whole process might take around 365,000 years. In evolutionary time, that's barely a blink. The study supports the idea that complex structures like the eye can evolve through small successive changes, each giving a selective advantage. Far from being the product of a single blueprint, the eye has emerged again and again through evolution's patient trial and error. shaped not by perfection, but by what works well enough to survive. This is the nuance of the natural world that I love to explain. I love engaging with people and having discussions about evolution. However, in any video like this that contains predatory animals like owls

### Segment 6 (25:00 - 29:00) [25:00]

harpy eagles, lions, or cats, and their natural hunting behavior, I have to be exceedingly careful. Showing any amount of predation has gotten my videos demonetized in the past. The algorithm simply flags the video as unsuitable for advertisers, blocking me from getting paid. You can appeal the decision to get a human to review your case, but this can take days or weeks. And during that time, all of your money is being essentially taken away from you. This happened recently in my Lion video, and since then, I have been tiptoeing around trying not to anger the AI algorithm. What a world we have found ourselves in. If this infuriates you as much as it does me, you can watch all of my videos uncensored over on Nebula. Nebula is the streaming platform created by and for independent content creators, and it's a place that isn't concerned with the demands of corporate advertisers. It provides a space for us to share the best versions of our videos and provides us a level of stability. And for viewers of Nebula, it's the best place for incredible original content that you can't get anywhere else. Like my Nebula original series, Becoming Human and Archaeology Quest. Two shows about our ancient past. One serious, one kind of silly. And if you want to dig deeper into this episode, I've uploaded the extended interviews with the scientists featured here, completely adfree and only on Nebula. These conversations get into the nuance, the tangents, and the fascinating off-mic stuff that didn't make the final cut. Think of it like a video podcast. On Nebula, there is so much original content that pushes the boundaries, like 17 pages, a featurelength documentary that dives into one of the most shocking controversies in modern science. It explores how a single biology paper just 17 pages long shook the entire scientific community led to a full-blown congressional investigation and became known as the scientific Watergate. It's got ethics battles, media frenzy, and real congressional testimony. But more than that, 17 pages asks the question, what happens when science, politics, and human bias collide? And what does truth even mean when public perception takes over? And it's only available on Nebula. Nebula is also a better viewing experience for everyone. There are no ads, lots of content that gets published to Nebula first before YouTube, and tons of exclusive content. Sometimes this is extended episodes or behindthescenes content. And right now, we're offering gift cards to Nebula for you to give to friends or family members. This is a great lastinute gift idea. It's a totally digital instant gift that will give your friends and family so much original content without having to worry about yet another subscription. One year's subscription is only $36. Or to give your friends or family members access to everything Nebula will ever have to offer, you can gift a lifetime subscription for $300. Lifetime subscriptions, whether it's for yourself or as a gift, help fund original series. If you've ever supported this channel back when our bundle deal was a thing, now is a great time to come back and support me directly through Nebula. So, for less than the price of a cup of coffee per month, you can get access to all of Nebula's originals along with the Real Science full catalog without any ads. You can also easily download videos to watch on the go, all for just $3 a month. And if you're like me and are growing increasingly sick of these gigantic corporations that rule our life and the everinccreasing encroachment of AI, Nebula is the place for you. It's a small company that supports creators directly. A company that takes no VC funding, is not beholden to shareholders, and has no plans to try to take over the government. So go to nebula. tv/realcience TV/realcience or click the link in the description to get 40% off a yearly subscription for the incredibly low price of $3 per month.