The image on the left shows the wings of a map butterfly as they might look to a bird looking for a snack. The image on the right is what the butterfly might look like to another member of its kind.Duke/Eleanor Caves
Scientists have put together a series of images that let us understand how animals might see the world. Many animals have senses far superior to that of humans - bats can hear with ultrasonic accuracy and sharks can smell blood diluted in water kilometers away - but humans tend to dominate over animals in one sense, visual acuity.
Now scientists have developed images that demonstrate the way different animals actually visually perceive the world around them. Visual acuity is measured by “cycles per degree,” that is the number of black-and-white parallel lines an animal can see in one degree of its vision.
The number of cycles per degree informs how sharp objects appear and can change according to how far away the viewer is from the object. Scientists have studied more than 600 species and created visual acuity for each one.
The study found that the range of visual acuity in the animal kingdom can be classified into four orders of magnitude, with humans just below the top. Humans see about 60 cycles per degree, a similar number to many other primates.
Birds top the list with crustaceans almost blind
But hunting birds such as eagles, vultures, and falcons see about 120 cycles per degree which allows them to spot tiny prey from thousands of feet in the air. Fish, on the other hand, see no more than 40 cycles per degree. A human considered legally blind would see around 10 cycles per degree.
Insects see roughly 1 cycle per degree and crustaceans just a fraction of that. Though just because an animal experiences a low number of cycles per degree, it doesn’t mean that it doesn’t have other ways to experience the world.
Many insects, though they have poor eyesight, have extremely good motion detection. Which means while a fly can’t see you very well, it definitely knows you are there.
Images might help us understand animals' communication
The report into the research in Trends in Ecology & Evolution doesn’t try to tell the whole story of animal vision, rather it allows us just a glimpse into the animal world via sight. To create the images, Eleanor Caves, a postdoctoral researcher at Duke University and lead author on the paper, along with her colleagues developed a software called AcuityView.
The software allows them to reduce the spatial detail in photos to only what a given animal can perceive. The resulting images allow the viewer to think through the ways animals might use signs and movement to communicate with other members of their own species as well as others.
Cave proposes that some species may not even be able to see the colorful designs that they have evolved to display, but that these are a tool to communicate with prey and or potential food sources. For instance, some orb-weaver spiders adorn their webs with silk decorations.
Some believe this is done to deter birds who have a high enough acuity to notice them and avoid the web. Another theory is that the decorations are woven to lure prey, however, so many insects don’t have the acuity to see such minute objects that they would not even know what has happened to them from the moment of hitting the web to being digested. The study has the potential to open doors to more research into the ways animals communicate.