How Animals See the World

 

See through the eyes of cats, birds, fish, and snakes.

By Elizabeth Preston Photo by Dr. Klaus Schmitt March 20, 2014

Some animals, including your pets, may be partially colorblind, and yet certain aspects of their vision are superior to your own. Living creatures’ visual perception of the surrounding world depends on how their eyes process light. Humans are trichromats—meaning that our eyes have three types of the photoreceptors known as cone cells, which are sensitive to the colors red, green, and blue. A different type of photoreceptors, called rods, detect small amounts of light; this allows us to see in the dark. Animals process light differently—some creatures have only two types of photoreceptors, which renders them partially colorblind, some have four, which enables them to see ultraviolet light, and others can detect polarized light, meaning light waves that are oscillating in the same plane.

“None of us can resist thinking that we can imagine what another animal is thinking,” says Thomas Cronin, a professor at the University of Maryland who studies visual physiology. But while guessing animals’ thoughts is a fantasy, looking at the world through their eyes is possible.

Drag the slider to the right to see an animal’s view; to the left to see a person’s view.

Cat

“We will never know what a cat would experience,” says Dan-Eric Nilsson, a zoology professor at the University of Lund in Sweden and coauthor of the book Animal Eyes. But we can come close to seeing what it sees. Unlike humans, cats are dichromats; they have only two kinds of cones in their retinas. They see similarly to humans with red-green colorblindness, Nilsson says. To model a cat’s vision, one has to pool everything that’s red or green into one color.

The cat’s eyesight has a lower resolution than our own, which means it sees objects slightly blurrier than we do. Human vision is among the sharpest of all animals, thanks to densely packed cones at the center of our retina. Nilsson says cats’ daylight vision is about six times blurrier than ours, which is not depicted in the image above. However, cats have more rods than humans, so by moonlight, the advantage is reversed. 

Bee

Bees are trichromats like humans. But instead of red, green, and blue, their three types of photoreceptors are sensitive to yellow, blue, and ultraviolet light. The ability to see ultraviolet light lets bees spot patterns on flower petals that guide them to nectar. In fact, Nilsson says, bees perceive so much of the ultraviolet range that “they could potentially see more than one color of ultraviolet.”

Unlike human eyes, which have only one lens, bees have compound eyes composed of thousands of lenses that form a soccer-ball-like surface; each lens produces one “pixel” in bees’ vision. That vision mechanism comes at a price—bees’ eyes have extremely low resolution, so their vision is very blurred. Nilsson calls this design “the most stupid way of using the space available for an eye.” If humans had compound eyes that performed as well as our real ones, he says, they’d each have to be as wide as a hula hoop.

This image doesn’t show the fuzziness of a bee’s eyesight—if it did, there wouldn’t be much for us to look at. But the photograph does capture the ultraviolet vision that we lack.

Bird

Unlike humans, birds are tetrachromats. Their four types of cone cells let them see red, green, blue, and ultraviolet together. A few birds of prey have sharper vision than humans, Nilsson says. A large eagle sees with about 2.5 times the resolution that we do.

If Nilsson could truly get inside the head of another animal, “birds would be interesting,” he says. But we can neither sharpen our resolution past human limits nor see ultraviolet light—we don’t have the photoreceptors and brain neurons to make it happen. We can use binoculars to see the distant detail that an eagle would discern, and cameras that convert ultraviolet light to a color visible to our eye, but without such technology “there’s no way of allowing a human to really experience what the world would be like to a big eagle,” Nilsson says.

Rattlesnake

Rattlesnakes have low-resolution color vision during the day and plenty of rod cells for a boost at night. But what sets rattlesnakes apart is their ability to sense infrared light. Similarly to vipers, pythons, and boas, the rattlesnake has special sensory tools called pit organs—a pair of holes on either side of the snout between the eye and the nostril. Suspended in each pit is a thin membrane that detects heat, says David Julius, a physiology professor at the University of California, San Francisco. Julius discovered that a neural receptor, TRPA1, present in the nerve cells connected to this membrane is responsible for snakes’ ability to transform infrared light into nerve signals. In humans, the same receptor triggers our pain response to certain spicy foods such as wasabi and mustard. But in snakes, it responds to the heat of nearby prey.

The rattlesnake’s brain merges the information from the pit organs with information from the eyes so that a prey’s thermal image is overlaid on the visual one. Julius says it’s actually not hard for humans to approximate what the snake sees: Just look through an infrared camera.

Cuttlefish

Seeing through the eyes of a cephalopod such as a squid, octopus, or nautilus requires a major stretch of the imagination. These sea creatures evolved their eyes separately from vertebrates, so their vision process is very different from ours. For example, cephalopod eyes have no blind spot. And the pupil of a cuttlefish is shaped like a W, making it look especially alien as it pursues prey in the ocean.

Despite their hunting prowess, cuttlefish have blurrier vision than us. “They couldn’t read the fine print on a newspaper,” says Thomas Cronin. “They could only read the headlines.” And even though they have incredible color-changing skills—going from beige to blood-red or striped in the blink of an eye—cuttlefish are totally colorblind.

Cuttlefish eyes have one photoreceptor that lets them see in shades of gray, Cronin says. Another pair of photoreceptors detects polarization. Humans’ only experience of polarized light comes when we wear sunglasses that reduce sun glare by filtering out one orientation of light waves. But unlike cephalopods, we don’t have photoreceptors to detect whether light is polarized or not.

Cuttlefish produce polarization patterns on their skin that they may use to communicate. Looking at one another, cuttlefish would see shades of gray with the polarization information overlaid, not unlike the rattlesnake’s infrared sense.

“I think it’s reasonable to put ourselves in the head of a dog or a cat or a monkey,” Cronin says, “because their brains are similar to ours.” But something like a cuttlefish is so evolutionarily distant—its brain and perceptions are so unlike our own—that we can never know what it experiences. “I don’t think we can put ourselves in their heads.” But, he adds, “Imagining it is fun.”

Elizabeth Preston is the editor of Muse, a magazine about science and ideas for kids, and author of Inkfish, a blog about science and cephalopods for everyone. She has also written for Slate and National Geographic.

Bird and Bee photography © Dr Schmitt, Weinheim, Germany.