When it comes to details, Human Eyes see the World in Higher Resolution than Most Animals

Compare to many animals, human eyes aren't so good at distinguishing colours or seeing objects in dim light, but when it comes to visual acuity humans can see fine details that most animals can't.

Researchers at Duke University compared hundreds of species by the sharpness of their sight and found a huge difference between the most sharp-sighted and the most blurry-eyed species, with humans ranking near the top.

Across the animal kingdom most species "see the world with much less detail than we do," said Eleanor Caves, first author of the study and a researcher at Duke.

The study measured visual 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.

But researchers can't ask a camel to identify letters on an eye chart😂😂. So, they estimate visual acuity based on an animal's eye anatomy - such as the spacing and density of light-sensing structures or using behavioural tests.

The limit of detail that human eyes can resolve is about 60 cycles per degree, 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 of Australia can see 140 cycles per degree, more than twice the limit of human visual acuity. Eagles can spot something as small as a rabbit while flying thousands of feet above the ground.

A household scene as viewed by various pets and pests. Human eyesight is roughly seven times sharper than a cat, 40 to 60 times sharper than a rat or a goldfish, and hundreds of times sharper than a fly or a mosquito. Image Credit: Eleanor Caves.

But apart from some eagles, vultures and falcons, the results show that 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," Caves said.

Humans can resolve four to seven times more detail than dogs🐶 and cats🐱, and more 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 most sharp-sighted and the most blurry-eyed species. They also created a series of images showing how different scenes might appear to animals with different acuities.

The images show animal patterns that, while easy for some species to see, may be imperceptible to others, or only recognizable from a short distance.

For example, Take the patterns on a butterfly's wing. Scientists have debated the function of their spots and stripes. One common assumption is that they warn birds and other predators to stay away. It has also been proposed that they help butterflies to attract potential mates.

The image on the left shows the wings of a map butterfly as they might look to a jay looking for a snack, and on the right, to another member of its kind, such as a rival or potential mate. Image Credit: Eleanor Caves.

The research determined that the wing patterns may be apparent to many birds, but to others of their kind their wing patterns 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 visual acuity to send secret messages that sharper-sighted species can read but others can't.

For 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 with their delicate webs, like the window stickers used to keep birds from flying into the glass. Another idea is that they lure insect prey.

This is a spider web as seen in bird vision (left), and fly vision (right). The zigzags on the spider's web send a secret message to birds that their insect prey can't see, even from less than a foot away. Image Credit: Eleanor Caves.

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 six feet, they are virtually invisible to house flies and other small insects that might blunder into the spider's sticky traps.

It seems the decorations help spiders alert birds to webs that might be in their flight path, without blowing their cover with the creatures they might be trying to catch for lunch.😲😲

Being said that these images don't represent accurately what animals actually see, because while the eyes take in visual information, the brain must also make a sense of it.

It is likely that certain things may be sharper or easier to detect due to the enhancement and other forms of post-processing that occur once the visual information is relayed to the brain, but the images definitely give researchers a sense of what visual information the brain has to work with.

"The point is that researchers who study animal interactions shouldn't assume that different species perceive detail the same way we do," Caves said.

Source: Duke University