Today, I've decided to focus on eyes. Almost everything has eyes, but not all eyes are equal. We are gifted with the eyes that are best suited for our survival. When you're traveling through the park, have you ever stopped and taken a good look at the marvelous eyes that are looking back at you and wondered what that animal can see? Let's take a look at some of the eyes that can be found in the park and see what we can learn about them.
Our eyes are situated on the front of our face and deeply seated within our facial bones. Unlike our eyes, frogs have bulging eyes. Frogs have no necks, like humans, so they can't turn their heads. Bulging eyes allow them to see in many different directions at once. Humans have upper and lower eyelids, but frogs also have a transparent inner eyelid that moves from one side to the other, rather than up and down. This third eyelid is called a nictiating membrane. It helps protect the frog's eyes when it's underwater. In order to close it's outer eyelids, a frog uses muscles to suck it's eyeballs deeper within the sockets. The most interesting thing about a frog's eyeballs is the fact that they actually use them to eat. Frogs can't chew their food or swallow very well. They use their muscles to suck their eyeballs back into the sockets and blink. The motion of the eyes bulging, being sucked in, bulging, and being sucked in, helps move the food along. Frogs can't gauge distance very well because their eyes are too far apart to be able to focus together.
Slugs have tiny simple eyes on the ends of their tentacles. Their eyes are very simple and primitive, so they can't see very well at all. They're much better at smelling and can detect the aroma of food from as far away as one to two meters. Slugs and snails are pretty much the same creature. The biggest difference between them is the fact that snails carry their houses around on their backs. Having tiny eyes on the end of stalks allows a snail to withdraw all of it's body inside of it's shell, with the exception of the tips of it's tentacles, and still be able to see as well as a snail can see. Many slugs and snails have eyes that contain a rudimentary lens, so they can see vague pictures of things, but they can't see color and they can't focus their eyes. Instead, they use their tentacles to feel, smell, and taste their way through the world.
Snakes can see in two different ways. They can see with their eyes, but their eyesight isn't as clear as ours. They can see color, but the range of color they can see isn't as broad as humans and the colors they can see aren't as well defined. They have trouble seeing things that aren't moving, so when they're flicking their tongue in and out, they're really trying to smell what they think they see in order to determine what it is. They actually have a telescoping lens in their eyes that works a bit like the zoom lens on your camera, but it doesn't improve their sight a great deal. Most snakes are really near-sighted, which means that they can see things that are really close to them, but don't see things that are farther away. They have a natural filter over the lens of their eyes that filters out UV waves, much like our sunglasses do. Their eyes are also covered by a clear scale. They need this because they don't have any eyelids at all. Snakes cannot close their eyes. Just the thought of that is enough to drive me crazy. When a snake sheds it's skin, it also sheds this clear scale. You can generally tell when a snake is getting ready to shed it's skin because this clear scale turns very milky-looking. Snakes see much better at night than they do during the daytime because the rods inside of their eyes are far superior to ours. Pit vipers can also see pictures drawn by heat signatures. They have tiny pits on each side of their heads that contain thousands of heat-detection cells. These heat receptors are ten times more sensitive than the best artificial infrared sensors that we can build and they can repair themselves if damaged. Even if you cover a pit vipers eyes up, it can see a warm blooded animal standing in front of it.
Dragonflies have huge compound eyes that cover almost all of the tops of their heads. They have the largest compound eyes of any insect that we've ever found. Think about a compound eye as being a wall filled with television screens, each screen looking at the same object from a slightly different angle. Because their eyes are huge, sit up on top of their heads, and contain thousands of facets, dragonflies can see in all directions at the same time. No one has yet been able to figure out how a dragonfly's brain sorts through all of this information and makes sense of it, but it seems to work well for dragonflies. They're very sensitive to the slightest movement. Different kinds of dragonflies have slightly different visual perception. Some dragonflies can see more colors than humans and the colors they see are much sharper than what we can see. Dragonflies that are most active at dusk, need the ability to see in low-level light conditions more than they need to distinguish colors, so they've sacrificed a lot of their color vision in exchange for more light-collecting capacity.
Muskrats have teeny-tiny eyes and very small ears. Because their eyes are so poorly developed, they have very poor vision, but they make up for their lack of sight by having a highly developed sense of smell and hearing. A muskrat's eyes are located on the top of it's head. This allows the muskrat to see above the surface of the water when it's swimming.
Coyotes are hunters. They can see six times better than humans. Because they're primarily nocturnal, their eyes are designed for seeing well in low light conditions. They have huge pupils to let in lots of light and their retinas contain more light-collecting rods. Their night vision is excellent, but like the dusk hunting dragonflies, they sacrificed a lot of their color vision in order to have superior night vision. They have what's called dichromatic vision, which means they can only see in two colors. To a coyote, all green things appear to be shades of white and gray. Think about the wetland now and how green it is. A coyote cannot see any of that green, just white and gray. They can see some color though, but their ability to detect color is very limited. For instance, red, orange, and yellow all appear as various shades of yellow. So, a coyote looking at the wetland sees white, gray, and yellow. Imagine that in your brain. Spooky! Even though a coyote can see well under conditions where we wouldn't be able to see at all, what they do see is slightly blurry and they're very nearsighted. Being able to see in the dark means giving up the ability to see most colors and produces a slightly blurry picture. This is okay, though, because a coyote's sense of smell and hearing are very highly developed. Becca Gorodyskyi photographed this coyote at Marlake.
Most people think that bats are blind, but that's not true at all. There's a considerable difference between what a fruit bat can see and what a true bat, or microbat, can see. Fruit bats eat fruit and nectar, so they have some color vision. They also have UV sensitive cones in their eyes that allow them to locate UV reflecting flowers. Microbats, the kind that live in the park, can only see in black and white, but they also have UV sensitive cones in their eyes that improve their vision in low-light conditions and help them avoid predators, gauge their height above the ground, and navigate using landmarks. Fruit bats don't echolocate, but microbats do. Echolocation is like sonar. The bats send out a signal and then read the echos that bounce back at them. So, they're able to see two different ways, one using their eyes and the other using their ears.
Bald eagles can see what's in front of them and what's beside them at the same time. They have an inner eyelid, or nictitating membrane, over their eyes, like frogs, that sweep across their eyes from side to side, both cleansing and protecting the lens. Eagles have color vision. They can see things approximately four times better than a human and can spot tiny rodents and rabbits from a mile away.
Look at where an owl's eyes are. They're huge and smack dab in the middle of their faces. Owls have the ability to turn their heads all of the way around so that they can look backwards and see what's coming up behind them. Humans can't do that. So, if birds have such good eyes, why do they frequently fly into poles and things? Scientists theorize that this happens because the birds are busy looking down, trying to find yummy things to eat, instead of looking forward, in the direction that they're flying. When birds fly into glass windows, it's theorized that the light is just right to reflect the sky and trees in the glass. Instead of seeing the glass, the bird sees empty sky ahead. To help prevent birds from flying into your glass windows, try sticking a few interesting decals on the glass to break up the reflection. This horned owl was photographed by Adele Freeland.
Spiders have a lot of eyes. Some spiders have as many as eight of them. Even though they have a lot of eyes, very few of them can see very well. Most of them can't see anything more than small differences in light and dark. Instead of using their eyes, these spiders use their sense of touch to sense vibrations, and their sense of taste to navigate around and find their food. The lenses in spider eyes are even better than the lenses on most of our cameras, but their receptor cells are so coarsely-grained that the picture they get is extremely poor. Even though some spiders have crystals in their eyes that reflect light, that only makes the image brighter, not clearer. Net-casting spiders have better low-light vision than cats and owls, but their vision clarity is almost non-existent. An interesting thing about this light-sensitive membrane is the fact that it's destoyed every day and recreated every night. Not all spiders have poor eyesight though. Jumping spiders and wolf spiders hunt during the day and have different eyes that perform different functions. Their side eyes are used to detect movement, although the resulting picture is very blurry. After their side eyes detect the movement, their middle eyes lock onto it and zoom in on it, creating a clear, focused image. Their side front eyes are used to determine how far away their prey is.
There you go! Wild creatures all have different eyes that perform in different ways. The next time you visit the West Hylebos Wetland, see how many different kinds of eyes you can spot. Eyes are amazing!
Teri I. Lenfest
Comments