EVER wondered what your cat sees when it stares into your eyes? Wonder no more.
Researchers in the US have compared hundreds of species by the sharpness of their sight — and discovered that human eyes can see fine details that most animals can’t.
Compared with many animals, however, human eyes are not particularly adept at distinguishing colours or seeing in dim light.
In a paper published in the journal Trends in Ecology & Evolution, a team at Duke University in North Carolina compiled previously published estimates of visual acuity for roughly 600 species of insects, birds, mammals, fish and other animals.
They measured acuity in terms of cycles per degree, which is how many pairs of black and white parallel lines a species can discern within one degree of the field of vision before they turn into a smear of grey.
Visual acuity is based on eye anatomy — such as the spacing and density of light-sensing structures — or behavioural tests.
The limit of detail human eyes can resolve is about 60 cycles per degree, on par with chimpanzees and other primates.
The wedge-tailed eagle, from Australia, can see 140 cycles per degree, enabling them to see something as small as a rabbit while flying far above the ground.
But apart from some eagles, vultures and falcons, most birds see fewer than 30 cycles per degree — less than half as much detail as humans.
The same goes for fish. “The highest acuity in a fish is still only about half as sharp as us,” said first author Eleanor Caves. Across the animal kingdom, most species “see the world with much less detail than we do”.
Humans can resolve four to seven times more detail than dogs and cats, and more than 100 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 cannot see more than one.
Overall, the researchers found a 10,000-fold difference between the most sharp-sighted and the most blurry-eyed species.
The researchers also created a series of images showing how different scenes might appear to animals with different acuities, using a software package they developed called AcuityView. The software takes a digital photo and strips away the detail that may be too fine for a given animal to distinguish.
The converted images reveal animal patterns that, while easy for some species to see, may be imperceptible to others, or only recognisable from a short distance.
Some animals may use such differences in acuity to send secret messages that sharper-sighted species can read but others cannot, Caves said.
For instance, orb-weaver spiders decorate their webs with white silk zigzags, spirals and other designs. One theory is that they keep larger animals from accidentally colliding with their delicate webs. Another idea is that they lure insect prey.
But images of spider web decorations as they might appear to different species suggest that while birds can spot them from as far away as two metres, they are virtually invisible to house flies and other small insects that might blunder into the spider’s sticky traps.
The converted images the team produced do not represent what animals actually see, the researchers caution. That is because while the eyes take in visual information, the brain must make sense of it.
It is likely that certain things may be sharper or easier to detect thanks to edge enhancement and other forms of “post-processing” that occur once the visual information is relayed to the brain, Caves said. But the software gives researchers a sense of what visual information the brain has to work with.