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| A dung beetle atop a ball of dung it made. Bernard Dupont, CC BY-SA 2.0. How far can a dung beetle see? You’d probably think they can see a few feet or a few dozen feet (or a few dozen meters) at the most, but this is a bit of a trick question. The Ancient Egyptians considered scarabs—one species of dung beetle, Scarabaeus sacer—to be sacred. Dung beetles are also famous for rolling manure into a ball larger than themselves and pushing it across the landscape with their hind legs to bury in a hole where they lay their eggs, since dung is what they eat. Periodically during this excursion they stop to crawl on top of the ball and do a little dance. What they are actually doing is using clues to navigate. These signs include their surroundings, the sun, polarized light from the moon, the orientation of the Milky Way, and some star clusters. Much like us, the relatively small dung beetle can see some of the galaxy we live in. The farthest stars we can see with the naked eye are about 16 thousand light years away, but we can also see the collective light of the Andromeda Galaxy, which appears as a faint cloud and is 2.5 million light years from us—that’s 13 million trillion miles (21 million trillion km). Theoretically we could see a supernova that’s 13 billion light years away, if it’s bright enough. How far into the Milky Way we can see is difficult to say, but a dung beetle can at least see enough of it to orient itself—perhaps a couple of thousand light years. The beetles see stars as fuzzy blobs, but the light is brighter to them since they’re more sensitive to dim light than we are. Nearly every animal can see the sun, which is only eight light-minutes away—about 93 million miles (150 million km). You can see that it’s not really the distance that matters, it’s how bright the light source is, but since our rods can detect a single photon, if that photon comes from across the universe, that’s technically how far we can see. Besides dung beetles, some birds also use the stars and constellations to navigate. At the other end of the spectrum, about the smallest things we can see are the largest bacteria. Pea aphids can use their eyesight to avoid a type of bacteria that is deadly to them. They have no defense against this bacteria and will die if infected. The bacteria lights up in ultraviolet and the aphids can see this, so they avoid it like the plague.[1] Our relatively complex eyes evolved slowly over hundreds of millions of years. Many early types of eyes still exist in some animals and they range from patches of photoreceptors to cup-shaped dents to pin-hole camera-like eyes to eyes with lenses and retinas. The more primitive forms of sight can only detect the presence and absence of light. As sight became more advanced the animal could tell the direction of the light, then came the ability to make out shadows, and resolution gradually improved. Some creatures gained the ability to see color, ultraviolet, infrared, polarized light, and—in the case of the mantis shrimp—circular polarized light. Our way of seeing color using receptors in our eyes may not be the only way. Some non-mammalian vertebrates, like fish, detect brightness and colors using receptors in their pineal glands in their brains, although it’s not yet known if or how this affects vision. While we tend to look at this as a progression from the primitive to the advanced, that’s not the case. That implies eyes evolved towards a goal, which doesn’t happen. Evolution only makes random changes—it tinkers, if you will—and the changes that work better are usually the ones that survive. Mutations modify what is already there and natural selection causes bad mutations to vanish, while the good ones spread through a population. Each creature’s eyes adapt to its own needs and environment. If an advantageous mutation appears in one lineage, it won’t appear in another unless it develops independently or, in very rare cases, crosses over through horizontal gene transfer. While more advanced eyes progressed through various stages, that doesn’t mean only advanced animals have them. There is a protozoa that appears to contain an eye with a lens and a retina. Nature is unpredictable. The eyeless roundworm C. elegans has photoreceptors that are 50 times better at catching light than ours are. At some point geckos or their ancestors lost their rods, so their cones evolved larger and are now 350 times more sensitive to light than ours. Our eyes work pretty good in a generalized way. Birds seem to be able to detect colors better than us. Eagles and other birds of prey can see much sharper at farther distances than we can. If we had raptor vision we would be able to stand on top of a ten-story tall building and see an ant walking on the ground. And you could read this book from the other end of a football field. They have sharper vision because their foveae are deeper than ours, acting as a telephoto lens. They actually have two foveae that are denser with receptors with thinner capillaries in front blocking less of the view, as their retinas are backwards like ours. It’s thought that the central foveae is for seeing prey in the distance, while the other is for focusing close up. But there’s a trade off. Eagles have sharper vision because their photoreceptors are smaller and densely packed, but this has reduced their sensitivity, so they can’t see much at night. At the other end of the spectrum, many animals, such as lions, hyenas, cats, and dogs, have sacrificed distance acuity and some of their color vision to be able to see well at night, though they’re probably sensitive to movement in the distance. And some prioritize close-up vision. If you had the eyes of a rhesus monkey, you could read this if it was less than an inch in front of your face. According to Phillip Pickett, a veterinary ophthalmologist at Virginia Tech, on a scale of one to ten, with rats at one and raptors at ten, our vision would be about a seven.[2] Many top golfers, including Tiger Woods, Vijay Singh, Fred Funk, and Zach Johnson, improved their vision to 20/15 or better by getting laser eye surgery. Baseball legend Pete Rose once described how when batting he could tell each type of pitch by how the ball looked. For example, he said a slider looked like white circle with a red dot in the middle, because of the way the red stitches on the ball were spinning. Remember, these were balls that were approaching him at nearly a hundred miles per hour. As far as I can remember, I was nearsighted from a very young age. I remember, when in high school I first got glasses, being totally amazed that I could see individual leaves on trees. After getting tired of dealing with glasses and contacts, I got laser surgery—once in one eye and three times in the other because of some complications. I lost some close vision, but we lose that as we age anyway. It was amazing to suddenly be able to see so clearly. It’s been twenty-five years and I can still see clearly farther than anyone I know, though I now have more trouble close up. I occasionally point out ships or whales leaping on the horizon that my friends can’t see. But I’ll never see as well as an eagle, even though my eyes are larger than theirs. Much of what is good about our vision comes from our foveae. Even though they make up a tiny portion of our field of view, they provide our brains with more than half of the visual information. Primates have foveae, as do some fish, reptiles, and birds—such as chickens—but most other animals don’t have them. Our foveae give us sharp color vision, but it’s slow, which is why we can’t see the flicker of TV, computer, and cell phone screens, while other animals can. Eyes have been evolving for around 700 million years. Theoretically, the most advanced eyes could have evolved within half-a-million years, but because of the meandering routes taken, it actually takes much longer. Since eyes are very useful for survival, it’s not surprising that they’ve evolved many times. In fact the various types of eyes have evolved independently between forty and sixty times in a wide variety of animals, using nine different optical principles, including pinhole eyes, several kinds of compound eyes, two types of camera-lens eyes, and curved-reflector eyes.[3] Green algae have eyespots good enough to make out a vague image of their environment. Some mollusks have eyespots with spherical lenses. Scallops and limpets have reflecting mirrors behind their retinas—much like cats and owls. The marine copepod Pontella has an arrangement of three lenses, while another copepod, Copilia, has two lenses arranged like a telescope. Our eyes also have two lenses, but one is fixed and the other adjustable. Chameleons have eyes like telephoto lenses. When you think about it, vision is a pretty amazing sense—to be able to discern things off in the distance. Our other senses have ranges that are quite a bit closer to home. Weird SensesFruit flies have a structure between their eyes that both hears and smells…and it swivels. So, would you say they smell by rotating their ears, or do they hear through their nose? Or both? The star-nose mole’s nostrils are surrounded by 22 short tentacles that sense electric fields arising from sweat or mucus on the skin of their prey—usually worms—buried in earth or mud. Its tentacles are about five times as sensitive as our fingers. This electric sense is usually found in fish, although the fish don’t have tentacles. Giant squid and colossal squid have eyes the size of basketballs, but their bodies are about the same size a large swordfish—about the size and weight of five men—yet a swordfish’s eye is only about the size of a softball. One hypothesis for this is that the squids’ large eyes enable them to detect the faint glow of bacteria in the distance being disturbed by a sperm whale that is coming to eat the squid for dinner, the whale having picked up the squid on its sonar. Being able to see that faint glow warns the squid it’s time for evasive action, which is difficult since it has no where to hide and the whale swims faster, though it’s not as maneuverable. Crabs smell with their feet, which may be a good thing since they pee through their heads near the base of their antennae. That’s after they run it through their gills to extract extra salt. And Japanese swallowtail butterflies (Papilio xuthus) have eye-spots on their genitals enabling them to see what they’re doing, and both males and females have a very difficult time reproducing without them.
If you like this, please subscribe below to receive an email the next time I post something wondrous. It's free. [1] Cornell University press release, “Aphids use sight to avoid deadly bacteria, could lead to pest control,” ScienceDaily, September 27, 2018, https://www.sciencedaily.com/releases/2018/09/180927135145.htm, citing Tory A. Hendry, Russell A. Ligon, Kevin R. Besler, Rachel L. Fay, and Melanie R. Smee, “Visual Detection and Avoidance of Pathogenic Bacteria by Aphids”, Current Biology, 2018, https://doi.org/10.1016/j.cub.2018.07.073. [2] G.C., “Who’s Got Good Eyes?”, Discover Magazine, August 2001, p. 53https://www.discovermagazine.com/mind/artificial-sight. [3] Richard Dawkins, “Where d’you get those peepers?”, New Statesman & Society, June 16, 1995, vol. 8, pp. 29. An exceptional explanation of the many different types of eyes and how they evolved can be found in Richard Dawkins’ book Climbing Mount Improbable.
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