Vertically oriented pupils give some reptiles the ability to gauge distances. Ambush predators, such as this crocodile, use the advantage to know exactly when to launch an attack.
Animals see the world through all kinds of different eyes: from simple ones that can only tell the difference between dark and light, to complex eyes that can see colors and perceive depth.
It was once thought that animals, including cats and dogs, could only see in black and white. However, scientists have proven that this is a myth. In all animals, including humans, the perception of color is determined by the presence of cells in the eye called “cone photoreceptors.” Cats and dogs have two kinds of cones, which are sensitive to blue and green light. This means that they have a useful level of color vision. The color vision level in other animals depends on the presence and types of their cones, too.
Now, however, researchers have just discovered that how animals see not only depends on their eyes’ physical attributes but on their natural habitats, as well: whether they live in water or on land—and if they are adapted to forests or wide, open landscapes.
Your cat doesn’t live in a shades-of-gray world. Reds and pinks don’t register for them, but blues and greens do. Even purple may appear more like a blue hue for cats.
Cones, rods and pairs of eyes
There is a lot we don’t know about animal vision. But what we are certain about is that almost all animals have eyes that sense light in their environments, even in dark habitats, such as deep in the ocean, where the only light source might be an odd burst of bioluminescence.
And although species across the animal kingdom have evolved various structures for sight, they all need photoreceptors, cells that transform light energy into electrical signals that can then be interpreted by the brain. Based on shape, there are two types of photoreceptors: cones and rods.
A cone cell will carry one of many pigments, each tuned to a specific range of wavelengths—matching a color—on the visible spectrum. A rod cell is built for contrast (black versus white) and contains a light-sensitive pigment called “rhodopsin.” The number and type of photoreceptors an animal has dictates which colors it can perceive. A human retina has roughly 6 million cones and 120 million rods. The cones are split into three kinds, or “spectral classes,” for trichromatic vision (around blue, green and red wavelengths).
Bioluminescence is the ability of living organisms to create their own light, using a chemical reaction. On land, the phenomenon is rare, limited to fireflies, glowworms, some mushrooms and a few other organisms. But in the ocean, bioluminescent animals create an underwater light show. In fact, 75 percent of deep-sea animals make their own light.
For example, in contrast with humans, butterflies and mantis shrimp have a dozen classes to cover a broader range of the spectrum (from deep ultraviolet to far-red light), which gives them hyperspectral vision.
But there’s more to sight than sensing light. Vision also requires neural circuits. The electrical signal generated by a photoreceptor is relayed via branching neurons that connect to other nerve cells, which amplify or dampen signals before they reach the brain. The brain then processes complex patterns in that visual information to detect the edges of objects and form an image of the outside world.
In vertebrates such as humans, light only hits photoreceptors after passing through the neural wiring. But in cephalopods (marine mollusks, including cuttlefish, octopuses and squid), eyes are wired more directly: behind light-sensitive cells.
Butterflies use color vision when searching for flowers. Unlike the trichromatic retinas of humans (blue, green and red cones; plus rods), butterfly retinas typically have six or more photoreceptor classes with distinct spectral sensitivities.
One reason animals have pairs of eyes is to calculate distance. Prey (such as deer) typically have monocular vision: an eye on each side of the head so two fields of view can be combined into a single large one, helping them watch out for hunters. But predators (such as lions) have binocular vision, with forward-facing eyes so fields can overlap for depth perception, helping them pinpoint food.
Colors, lands and unobstructed views
Scientists have long hypothesized that animal vision evolved to match the colors of light present in their environments. But this theory is difficult to prove. That’s because gathering data for hundreds of species of animals living in a wide range of habitats is a monumental task, especially when considering that invertebrates and vertebrates use different kinds of cells in their eyes to turn light energy into neuronal responses.
So to learn more about how various animals see and what they see, researchers from the University of Arkansas recently collated vision data for 446 species of animals spanning four phyla (a primary, biological taxonomy of animals that ranks above the class and below the kingdom). One of these phyla contained vertebrates (animals that have backbones), such as fish and humans. The rest contained animals that were invertebrates (those that do not have backbones), such as insects, jellyfish and squid.
Deer have monocular vision. Therefore, most of the sensory input from their eyes is two-dimensional, which is like looking at a photograph for us. While their depth perception is poor, monocular vision is well suited for detecting movement, usually by a predator.
The researchers found that animals adapted to the land can see more colors than animals adapted to the water. And animals suited for open terrestrial habitats see a wider range of colors than animals tailored for forests.
Environment, evolution and genetic mutations
The researchers’ study, which was published in the science journal Proceedings of the Royal Society B in May 2022, explains how environment, evolution and, to some extent, genetic composition influence how and what colors animals see. While animals do adapt to environments, their ability to do so can be physiologically constrained.
An animal’s capacity for detecting visual information depends on the wavelengths and intensity of light in each environment. But quantity and wavelength sensitivity of a family of retinal proteins, called opsins, govern the spectrum of light an animal sees—from ultraviolet to far-red light.
Lions have binocular vision and exceptional visual acuity, which helps them locate prey from great distances. Their color vision extends into the ultraviolet range, enabling them to perceive subtle differences which could indicate prey or potential danger. Additionally, their eyesight allows them to hunt at night when most other species cannot see clearly.
While vertebrates and invertebrates broadly use the same opsin cell types to see, they build these cells differently. This physiological difference—what biologists call ciliary opsins in vertebrates and rhabdomeric opsins in invertebrates—might explain why invertebrates are better at seeing short wavelength light, even when their natural habitats should select for vertebrates to also see short wavelengths of light.
The difference could be due to genetic mutations occurring in vertebrates but not invertebrates.
Sight, perspectives and views
Over the years, we’ve come to understand that landscapes are truly in our genes. So, it doesn’t surprise me that how we see the world is tied into those landscapes that we hold inside, that have become part of our very being, too.
There’s a common expression that someone “has an eagle eye.” That means that the person is very good at noticing things—even the tiniest details. An eagle’s eyesight is estimated to be four to eight times stronger than that of the average human. Eagles can see standard colors, as well as in the ultraviolet light spectrum.
I suspect that we’ll greatly benefit not only by learning more about how and what our fellow animals see, but also about their inner perspectives and views, as well.
Here’s to finding your true places and natural habitats,
Candy
