Scientists compared hundreds of species by the sharpness of their sight.
They found a 10,000-fold difference between the most sharp-sighted and themost blurry-eyed species, with humans ranking near the top. Photo by EugeneOliverScientists compared hundreds of species by the sharpness of their sight.
They found a 10,000-fold difference between the most sharp-sighted and themost blurry-eyed species, with humans ranking near the top. Photo by EugeneOliverSHARE THIS STORYShare this story on facebookShare this story on twitterShare this story onredditShare this story on linkedinGet this story''s permalinkPrint thisstoryROBIN A. SMITH @DUKERESEARCHDURHAM, N.C. -- Compared with many animals, human eyes aren’t particularlyadept at distinguishing colors or seeing in dim light. But by one measure atleast -- something called visual acuity -- human eyes can see fine detailsthat most animals can’t, Duke University researchers say.
A new study of animal vision compared hundreds of species by the sharpnessof their sight.
In a paper published in the May 2018 issue of the journal Trends in Ecology& Evolution, the researchers compiled previously published estimates ofvisual acuity for roughly 600 species of insects, birds, mammals, fish andother animals.
Across the animal kingdom, most species “see the world with much lessdetail than we do,” said first author Eleanor Caves, a postdoctoralresearcher at Duke.
A household scene as viewed by various pets and pests. Human eyesight isroughly seven times sharper than a cat, 40 to 60 times sharper than a rat ora goldfish, and hundreds of times sharper than a fly or a mosquito. Imagecourtesy of Eleanor CavesA household scene as viewed by various pets and pests. Human eyesight isroughly seven times sharper than a cat, 40 to 60 times sharper than a rat ora goldfish, and hundreds of times sharper than a fly or a mosquito. Imagecourtesy of Eleanor CavesThe study measured acuity in terms of cycles per degree, which is how manypairs of black and white parallel lines a species can discern within onedegree of the field of vision before they turn into a smear of gray.
Researchers can’t ask a camel to identify letters on an eye chart. Instead,they estimate visual acuity based on an animal’s eye anatomy -- such asthe spacing and density of light-sensing structures -- or using behavioraltests.
The limit of detail that human eyes can resolve is about 60 cycles perdegree, which helps us make out road signs and recognize faces from afar.
Chimpanzees and other primates can pick out similarly fine patterns.
A few birds of prey do better. For instance, the wedge-tailed eagle ofAustralia can see 140 cycles per degree, more than twice the limit of humanvisual acuity. Eagles can spot something as small as a rabbit while flyingthousands of feet above the ground.
But apart from some eagles, vultures and falcons, the results show that mostbirds see fewer than 30 cycles per degree -- less than half as much detailas humans.
The same goes for fish. “The highest acuity in a fish is still only abouthalf as sharp as us,” Caves said.
Humans can resolve four to seven times more detail than dogs and cats, andmore than a hundred times more than a mouse or a fruit fly.
A person who sees less than 10 cycles per degree is considered legally blind. Most insects, it turns out, can’t see more than one.
Overall, the researchers found a 10,000-fold difference between the mostsharp-sighted and the most blurry-eyed species.
The researchers also created a series of images showing how different scenesmight appear to animals with different acuities, using a software packagethey developed called AcuityView. The software takes a digital photo andstrips away all the spatial detail that may be too fine for a given animalto distinguish.
The image on the left shows the wings of a map butterfly as they might lookto a jay looking for a snack, and on the right, to another member of itskind, such as a rival or potential mate. Image courtesy of Eleanor CavesThe image on the left shows the wings of a map butterfly as they might lookto a jay looking for a snack, and on the right, to another member of itskind, such as a rival or potential mate. Image courtesy of Eleanor CavesThe converted images reveal animal patterns that, while easy for somespecies to see, may be imperceptible to others, or only recognizable from ashort distance.
Take the patterns on a butterfly’s wing. Scientists have debated thefunction of their spots, stripes and splotches.
One common assumption is that they warn birds and other predators to stayaway. It has also been proposed that they help butterflies check out orseduce potential mates.
The researchers determined that the wing patterns of, say, the map butterflymay be apparent to many birds, but to others of their kind their wingpatterns are likely a blur, even from just a few inches away.
“I don’t actually think butterflies can see them,” Caves said.
Some animals may use such differences in acuity to send secret messages thatsharper-sighted species can read but others can’t, Caves said.
A spider web as seen in bird vision (left), and fly vision (right). Thezigzags on the spider’s web send a secret message to birds that theirinsect prey can’t see, even from less than a foot away. Image courtesy ofEleanor CavesA spider web as seen in bird vision (left), and fly vision (right). Thezigzags on the spider’s web send a secret message to birds that theirinsect prey can’t see, even from less than a foot away. Image courtesy ofEleanor CavesFor instance, orb-weaver spiders decorate their webs with white silk zigzags, spirals and other designs whose function has been debated.
One theory is that they keep larger animals from accidentally colliding withtheir delicate webs, like the window stickers used to keep birds fromflying into the glass. Another idea is that they lure insect prey.
But images of spider web decorations as they might appear to differentspecies suggest that, while birds can spot them from as far away as six feet, they are virtually invisible to house flies and other small insects thatmight blunder into the spider’s sticky traps.
It seems the decorations help spiders alert birds to webs that might be intheir flight path, without blowing their cover with the creatures they mightbe trying to catch for lunch.
The converted images the team produced don’t represent what animalsactually see, the researchers caution. That’s because while the eyes takein visual information, the brain must make sense of it.
It is likely that certain things may be sharper or easier to detect thanksto edge enhancement and other forms of “post-processing” that occur oncethe visual information is relayed to the brain, Caves said. But the softwaregives researchers a sense of what visual information the brain has to workwith.
“The point is that researchers who study animal interactions shouldn’tassume that different species perceive detail the same way we do,” Cavessaid.
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