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 Senses
Fruit 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.
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[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.
[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.
 | | A brittle star on an octocoral skeleton. They climb to get higher in the
current. NOAA. |
Most creatures have some way of
perceiving and responding to light. Many can do it without having any eyes.
Brittle stars have photoreceptors scattered across their
bodies. Some can only tell whether they’re in light or darkness, but others can
detect different contrasts of light, enabling them to seek shelter under a dark
shape in the distance. Sea urchins can also see without having any eyes. They
have clusters of photoreceptors in their tentacle-like tube feet and use their
own shadows to discern the direction of light.[1]
Similarly, the hydra can see without eyes. This freshwater
predator is basically a tube-shaped stalk with tentacles on one end. They can
grow up to two inches in length (5 cm), but can stretch themselves to eight
inches (20 cm). They use their minimal sight to detect and shoot their prey
with harpoon-like stingers.
Scorpions have eyes, but they can also detect ultraviolet
light with the waxy cuticle covering their bodies, making their exoskeleton a
sort of eye. Creatures that can see with their skin include octopuses, a
chameleon, a gecko, a wall lizard, a sea snake, a fish, a pond snail, a
caterpillar, new-born pigeons and rats, and fruit flies. Even earthworms can
detect light with photoreceptors in their skin. We do too, although we’re not
aware of it. These receptors launch immediate repairs when our skin becomes
sunburned.[2]
I See You
Many animals that we wouldn’t expect to be able to see, actually
do have eyes. Chitons, the tide pool mollusk that looks like a flattened slug
protected by a series of eight armored plates, have hundreds of tiny eyes built
into their shells that have retinas and aragonite
crystals as lenses. This relative of limpets and abalones can actually see the
shadow of an eight-inch fish (20 cm) that’s six and a half feet away (2 m). And
these are animals that mainly consist of
a snail-like foot that can grip rock faces. Their brains are just a simple
ganglion—a group of neuron cell bodies—but on seeing an approaching fish, the
chiton clamps down on the rock. Even though these eyes seem primitive, they
evolved in just the last 10 million years, so they’re pretty new,
evolutionarily speaking. Other chitons that don’t have lenses or retinas are
still able to detect small changes in brightness.[3]
Some species of starfish have between five and 50 eyes that
are on the tips of their arms. They only see in black and white, but, judging
from the position of their eyes, it’s likely they can see all around them for a
distance, detecting things up to a dozen feet (nearly 4 m) away, including the
surface of the water and whatever is right in front of them. They most likely
use their sight to stay on or near the reef.  | | A scallop with blue eyes and a close-up from another scallop. Top: Rachael Norris and Marina
Freudzon. Bottom: Matthew Krummins, CC BY 2.0. |
Scallops also have dozens of eyes—some have 200 of them—that
protrude from their mantel between their shells. They move around a lot, so
their eyes are quite useful. Oysters and mussels don’t have them, but then
they’re mostly immobile. Scallop eyes are on the end of tentacles and protrude
from under their mantle in a line along the edge of each of their shells. Some
have red eyes, but many have blue eyes. Their eyes also have pupils that expand
and contract simultaneously. Light passing into one of their eyes reflects off
of a curved mirror and onto two retinas that detect different things, but it’s
thought they’re mostly looking for movement. When they see large enough
particles drifting by, they open their shells to investigate, probably by using
their sense of smell.
Scallops can detect large objects, but their visual system
is so slow that it’s probably not much use in detecting predators. Although the
eyes of one species—the venomous crown-of-thorns starfish (Acanthaster planci)—are good enough to see predators. They also use
their sight to hunt for prey, and they’re fast enough to chase it. It’s
possible that all sea stars with eyes can detect the bioluminescence of other
nearby starfish and they might even be able to communicate with each other
using flashes of light.[4]
Even box jellies,
which are nearly transparent, have 24 eyes distributed among four eye stalks.
With eight of their eyes, which are similar to ours, they can probably see
silhouettes at least 26 feet (8 m) above the water. This helps them hunt prey
and the navigate mangrove swamps they sometimes live in.[5]
Scientists have found thousands of creatures that produce
their own light. They include fireflies and mushrooms, but most of them live
deep in the ocean, such as some sharks, fish, jellies, crustaceans, and
octopuses. It’s thought that 80 to 90 percent of sea organisms luminesce. These
creatures use light to communicate, attract mates, attract prey, camouflage
themselves, ward off predators, and to attract bigger predators that eat their
predators, among other things. The lights range from blue to green, but in
barbeled dragonfish it can be red. Bioluminescence has evolved independently at
least 50 times. Genetic engineers have transferred the ability to glow in the
dark to other creatures, such as plants, marmosets, rabbits, cats, and dogs.
Oysters don’t have ears, but they hear sounds through a different
organ called a statocyst. We don’t know how things sound to them, but it
probably wouldn’t be like how our brains interpret sounds. Still, they can hear
breaking waves, water currents, the approach of predators, thunderstorms, and they’re
particularly sensitive to man-made noise pollution. They use the sounds to
decide when to clam up, feed, and spawn. Also, oyster larva navigate towards
the sound of snapping shrimp, which helps lead them to reefs. Scientists found
that mussels and hermit crabs can also hear[6],
and there are likely many other sea creatures that can. We don’t yet know how
noise pollution affects them[7],
but it can destroy the statocysts in octopuses, squid, and cuttlefish, making
them permanently deaf and unable to move or hunt—effectively killing them.[8]
Also, since many people like to eat them alive, we can
imagine what that experience might be like for them. Just as we can’t know what
it’s like to be a bat. We may never truly know, but we can get a better idea
the more we learn about them.
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[1]
Ed Yong, “Sea urchins use their entire body as an eye”, National Geographic,
May 2, 2011, https://www.nationalgeographic.com/science/article/sea-urchins-use-their-entire-body-as-an-eye,
citing Esther M. Ullrich-Lüter, Sam Dupont, Enrique Arboleda, and Maria Ina
Arnone, “Unique system of photoreceptors in sea urchin tube feet”, PNAS, 2011, https://www.pnas.org/doi/full/10.1073/pnas.1018495108,
https://doi.org/10.1073/pnas.1018495108.
And University of Gothenburg press release, “Sea urchins see with
their whole body”, ScienceDaily,
September 12, 2011, http://www.sciencedaily.com/releases/2011/06/110630111538.htm,
citing Esther M. Ullrich-Lüter, Sam Dupont, Enrique Arboleda, and Maria Ina
Arnone, “Unique system of photoreceptors in sea urchin tube feet”, PNAS, 2011, https://doi.org/10.1073/pnas.1018495108.
[2]
Wendy Zukerman, “Skin ‘sees’ light to prevent UV harm”, New Scientist, no. 2838, November 12, 2011, p. 20, and as “Skin ‘sees’
the light to protect against sunshine”, https://www.newscientist.com/article/dn21127-skin-sees-the-light-to-protect-against-sunshine/,
citing Nadine L. Wicks, Jason W. Chan, Julia A. Najera, Jonathan M. Ciriello,
and Elena Oancea, “UVA Phototransduction Drives Early Melanin Synthesis in
Human Melanocytes”, Current Biology,
vol. 21, no. 22, November 22, 2011, pp. 1906-1911, https://doi.org/10.1016/j.cub.2011.09.047.
[3]
Ed Yong, “Chitons see with eyes made of rock”, National Geographic, April
14, 2011, https://www.nationalgeographic.com/science/article/chitons-see-with-eyes-made-of-rock,
citing Daniel I. Speiser, Douglas J. Eernisse, and Sönke Johnsen, “A Chiton
Uses Aragonite Lenses to Form Images”, Current
Biology, vol. 21, no. 8, April 26, 2011, pp. 665-670, https://www.cell.com/current-biology/fulltext/S0960-9822(11)00305-8,
https://doi.org/10.1016/j.cub.2011.03.033.
And Anna Nowogrodzki, “Mollusc sees the world through
hundreds of eyes made out of rock”, New Scientist, November 19, 2015, https://www.newscientist.com/article/dn28520-mollusc-sees-the-world-through-hundreds-of-eyes-made-out-of-rock/,
citing Ling Li, Matthew J. Connors, Mathias Kolle, Grant T. England, Daniel I.
Speiser, Xianghui Xiao, Joanna Aizenberg, and Christine Ortiz,
“Multifunctionality of chiton biomineralized armor with an integrated visual
system”, Science, vol. 350, no. 6263, November 20, 2015, pp. 952-956, https://doi.org/10.1126/science.aad1246.
[4]
Laura Geggel, “Starfish Can See You …
with Their Arm-Eyes”, Live Science, February 7, 2018, https://www.livescience.com/61682-starfish-eyes.html.
And Christie Wilcox, “Sea Stars See!”, Discover Magazine, January 7, 2014, https://www.discovermagazine.com/planet-earth/sea-stars-see.
And Ed Yong, “Starfish Spot The Way Home With Eyes On
Their Arms”, National Geographic, January 8, 2014, https://www.nationalgeographic.com/science/article/starfish-spot-the-way-home-with-eyes-on-their-arms.
All three citing A. Garm and D-E. Nilsson, “Visual navigation
in starfish: first evidence for the use of vision and eyes in starfish”, Proc Roy Soc B, 281, 2013, http://dx.doi.org/10.1098/rspb.2013.3011.
[5]
Cell Press press release, “Through unique eyes, box
jellyfish look out to the world above the water”, ScienceDaily, April 30, 2011, http://www.sciencedaily.com/releases/2011/04/110428123938.htm, citing Anders Garm, Magnus Oskarsson, and Dan-Eric
Nilsson, “Box Jellyfish Use Terrestrial Visual Cues for Navigation”, Current Biology, April 28, 2011, https://doi.org/10.1016/j.cub.2011.03.054.
And Ed Yong, “Single-Celled Creature Has Eye Made of Domesticated Microbes”, National
Geographic, July 2, 2015, https://www.nationalgeographic.com/science/article/single-celled-creature-has-eye-made-of-domesticated-microbes,
citing Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi Gojobori, Curtis A. Suttle, Patrick J. Keeling, and Brian S. Leander, "Eye-like ocelloids are built from different endosymbiotically acquired
components", Nature, 2015, http://dx.doi.org/10.1038/nature14593.
[6]
Louise Roberts, Harry R. Harding, Irene Voellmy, Rick Bruintjes, Steven D.
Simpson, Andrew N. Radford, Thomas Breithaupt, and Michael Elliott, “Exposure
of benthic invertebrates to sediment vibration”, Proceedings of Meetings on Acoustics,
vol. 27, no. 1, 010029, January 5, 2017, https://doi.org/10.1121/2.0000324.
[7]
Andy Coghlan, “Oysters can ‘hear’ without ears”, New Scientist, no. 3149, October 28, 2017, p. 18, and the longer version “Oysters can ‘hear’ the ocean even though they don’t have ears, https://www.newscientist.com/article/2151281-oysters-can-hear-the-ocean-even-though-they-dont-have-ears/, citing Mohcine Charifi, Mohamedou Sow, Pierre Ciret, Soumaya Benomar, Jean-Charles Massabuau, “The sense of hearing in the Pacific oyster, Magallana gigas”, PLoS ONE, October 25, 2017, https://doi.org/10.1371/journal.pone.0185353.
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