A new review of visual acuity compares the sight of 600 species, from mosquitoes to eagles
Ever wondered what the world looks like through a cockatoo’s eyes? How about a giraffe—or even a butterfly?
For a new study published last month in Trends in Ecology & Evolution, a team of researchers set out in search of answers. As lead author Eleanor Caves explains in a press release, humans have higher visual acuity than most members of the animal kingdom, who “see the world with much less detail than we do.” And in recent decades, researchers have been slowly teasing apart how clear (or blurry) each critter’s view of the world is.
To investigate the role of sight in the evolution of animal behavior and signaling—from the bright blazes of color on a butterfly’s wings to the zigs and zags of spider web patterns—Caves and her team compiled previously published estimates of visual acuity, or sharpness, for about 600 species. The list encompassed data for all walks of life, including mammals, birds, insects, fish, crustaceans and more. The study represents the most comprehensive database of such figures to date.
Scientists quantify visual acuity in cycles per degree, or the number of black-and-white parallel lines an animal can identify in one degree of their field of vision. Caves tells Live Science’s Yasemin Saplakoglu that for humans, the measure equates to the size of one’s thumbnail when the arm is extended as if in offer of a thumbs-up. At this distance, humans can see 60 cycles, or lines, per degree.
To determine a species’ cycles per degree, however, scientists measure the density of photoreceptors — cells that react to light — in an animal’s retina or conducted behavioral studies examining animals’ awareness of black-and-white stripes in their surroundings, Saplakoglu reports.
As the measure decreases, an animal’s (or individual’s) vision worsens: At less than 10 cycles per degree, a human is deemed legally blind. The majority of insects, however, are lucky to see even one cycle per degree.
Comparatively, Australia’s wedge-tailed eagle, one of the most sharp-sighted birds of prey, can see nearly 140 cycles per degree — enough to spot a rabbit from thousands of feet up in the air. Cats see the world in less than 10 cycles per degree. (Nautilus’ Elizabeth Preston notes, however, that felines have better nighttime vision than humans, as perceptions of color and light are different than acuity.) Meanwhile, cleaner shrimp see around 0.1 cycles per degree.
Overall, there was a 10,000-fold difference between the most sharp-sighted and the most blurry-eyed species included in the study.
Once researchers compiled these measurements from published values, they entered them into a software program called AcuityView. The technology allowed the team to view digital images as they might appear to various animals. Lower cycles per degree resulted in blurrier scenes, suggesting, for example, that a spider web’s intricate design serves as a warning for birds to change their flight path while remaining virtually invisible to house flies and other insect prey.
Although the doctored images allow humans to visualize the levels of detail seen by different animals, Caves tells Saplakoglu they are not wholly representative of what an animal sees, as post-processing influences how the brain interprets visual data.
The world is not irredeemably blurry to all animals with low acuity; instead, Caves explains, “[the software] just tells you what visual information is available. You can’t use information that you never received; if acuity is too low to detect a certain detail, it’s probably not something that your brain can then work on further.
Written by: Meilan Solly
JUNE 6, 2018
About Meilan Solly
Meilan Solly is a graduate of the College of William and Mary/University of St Andrews Joint Degree Programme. In summer 2017, she served as Smithsonian Magazine’s American Society of Magazine Editors intern. Previously, she interned at Kiplinger’s Personal Finance and served as editor-in-chief of The Saint, St Andrews’ student newspaper.
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