Human vs. Animal Eyes

The differences between human and animal eyes are just as interesting as the similarities. Eye shape can transcend class of animal, eyeballs don’t always align with body size, and pupils come in a variety of shapes. But what about the eyes of a predator? Do they differ in humans vs. animals?

The Psychopathic Stare

Believe it or not, research exists for what’s called “the psychopathic stare.” A 2019 study assessed two groups of incarcerated offenders, one with psychopathic traits and one without. Psychopaths spent less time looking at the eyes — and eye area — of others. If you’ve ever stared into the eyes of a psychopath, they tend to look through you, not at you. An unnerving glare meant to intimidate, and it’s effective!

 

According to PsychCentral, the psychopathic stare has the following common characteristics:

• a coldness, with a lack of warmth, empathy, or compassion
• wide-eyed, with more of the white of the eye showing
• reduced blinking
• a focus that feels predatory or threatening
• dilated pupils
• heightened intensity
• eye contact or fixation held for longer

Do those same predatory traits carry into the Animal Kingdom?

Unlike human predators, animals don’t kill for hatred, dominance, or sport. They may have a similar drive — the instinct to kill — but it’s unfair to attach human emotions to their ability to hunt and feed their family.

My, What Big Eyes You Have

Eyes come in many different shapes and sizes. We share our round human eyes with bears, foxes, birds, and turtles. Larger mammals such as cows, horses, deer, and moose have horizontal oval-shaped eyes. Snakes and bobcats have a more vertical oval shape.

Pupils

The evolutionary path of an animal, and the manner in which they hunt or defend themselves from predators, has a huge impact on pupil shape.

The five most common shapes are:

• Circular (like us)
• Horizontal slits
• Vertical slits
• W-shaped
• Crescent-shaped

Large animals and powerful predators like lions, wolves, and humans usually have circular pupils. A circular pupil allows light to pass through the retina, thus controls the amount of light entering the eye. It is characteristic of a species who hunts higher off the ground, moves in packs, and are intelligent hunters. Humans fall into this category.

We, and other predators such as dogs and wildcats, can’t contract our pupils as tightly as animals with slit-shaped pupils, so we don’t cope as well with sudden changes in light. However, we do have even focus across the entire visual field, which helps us to survey our surroundings.

Horizontal slit pupils are commonly found in prey animals, like horses and sheep, while vertical slit pupils are typical of smaller hunters, such as domestic cats, foxes, and snakes.

W-shaped pupils are exclusive to cuttlefish.

Crescent-shaped pupils are found in sea life, like stingrays, catfish, dolphins, and Orca.

Some frogs have heart-shaped pupils, while geckos have pupils that look like pinholes arranged in a vertical line.

In the Science Mag journal, scientists report a strong linkage between the shape of an animal’s pupil and its way of life.

“If you’re the kind of animal that gets hunted,” says Banks, “you’re very likely to have a horizontal pupil” and to have your eyes on the side of your head. That makes sense, he says, because it gives prey animals a panoramic view, so they can best scan all directions for danger.

But then the scientists wondered if this theory held true if the animal’s pupils were parallel with the horizon. Animals like horses and sheep are continual grazers. When the researchers watched the animals in action, they discovered something remarkable.

“When they pitch their head down, their eyes rotate in the head to maintain parallelism with the ground,” says Banks. “And that’s kind of remarkable, because the eyes have to spin in opposite directions in the head.”

Hunting Style vs. Pupil Shape

Prey animals often have horizontal slit pupils, which provide a wider field of vision and thus, an advantage in spotting approaching predators. Slit pupils are found in goats, sheep, and antelopes — animals that need to be aware of their surroundings and able to flee in a split-second. The drawback of a wider visual field means their vision clarity isn’t as crisp.

Some reptiles and amphibians (like frogs and snakes) also have horizontal slit pupils that benefit them, as much of their time is spent on the ground peering upward for predators.

Smaller predatory animals who hunt close to the ground, like domestic cats and foxes in pursuit of rabbits, chipmunks, mice, moles, and other rodents, have vertical slit pupils. The vertical pupils allow highly focused vision and provide an accurate distance to help the predator know exactly when to strike. These types of smaller predatory animals are often nocturnal hunters, so they’ve evolved an extra set of muscles to drag the pupil into a narrow slit during the day to prevent damage by bright light — i.e., the Sun — preserving vision and protecting the eye.

Pretty cool, right?

Crescent-shaped pupils are another amazing adaptation of evolution. Crescent-shaped pupils are usually found in marine life, such as string rays, catfish, and flatfish. The shape of the pupil provides a wide visual field, helping with the search of predators and prey. The most notable thing about the crescent shape is it reduces light distortion by water, which gives marine life a huge advantage in their environment.

The cuttlefish’s W-shaped pupil evolved from horizontal-shaped pupils to allow the eye to open wider in dark environments like deeper water or nighttime. Their uniquely shaped pupils allow light to enter the eye from multiple angles and provides greater vision underwater, boosting image contrast and distance.

Eye Size

Human eyes are typically 24mm in diameter — same size as a dolphin eye. The largest eyes belong to the colossal squid at 280mm, while the blue whale’s eyes are only 150mm.

Eye size doesn’t always align to the size of the animal. Some smaller eyes belong to the rattlesnake, ferret, squirrel, and bullfrog, which are all 8mm in diameter. Dogs’ eyes are typically between 9.5mm and 11.6mm in diameter, depending on the breed.

 

Color Vision

With different habitats, prey species, and various predators, one would presume animals evolved to possess varying levels of color vision, but that’s not always the case. Plenty of myths surround animals’ color vision. For example, some say owls are the only bird that can see the color blue. Not true. Why would peacocks display such a vibrant blue plumage if potential partners couldn’t see it? The colors and patterns on their tail are a vital part of finding a mate, and female peacocks clearly appreciate blue hues. 😉

Crows have better vision than humans. While we see combinations of three primary colors — red, green, and blue — crows perceive combinations of four colors, blue included. Where they falter is with a specific hue of yellow. In fact, Professor Sugita in Japan helped produced a trash bag in that special shade of yellow to keep crows out of the city garbage. If the pigment is off at all, the crows won’t be fooled, so the exact recipe is a closely guarded secret. This particular nuance of yellow has the effect of blocking one of crow’s four primary colors.

Genius! I love when science works to help solve a problem without harming animals.

Geckos have excellent color vision — 350 times better than humans — even in dark environments like rainforests. Pigeons actually have one of the best color visions of all animals, and can even see UV light. Their vision is so remarkable they’re often used in search and rescue missions at sea.

Contrary to popular belief, dogs do see color, just not as well as humans. Canine vision may be slightly tinged with grey and they struggle to differentiate red and green. Other than that, they’re fine. So, stop calling Fido color blind! 😉

Bird Vision

A study conducted at Cornwell University focused on how corvids and other birds perceived ultraviolet light. While humans can detect red, green and blue light, most birds have a fourth cone that allows them to more acutely detect short wavelength colors near the ultraviolet range.

 

 

“The ability to simply detect UV isn’t enough though (in fact humans are sensitive to UV light), you must also have the ability to transmit that part of the spectrum. While our eyes filter it out, rendering it invisible to us, birds have special oil droplets in their cones that allow for the passage of UV light, while limiting its damage. This allows for greater contrast of the environment, rendering what may look to our eyes as a flat wall of green vegetation, as a much more dynamic plane, enhancing a bird’s ability to fly through dense foliage.”

UV sensitivity is also important among many types of nectarivorous (nectar drinking) and frugivorous (fruit-eating) birds. Many fruits, for example, are coated in a UV-reflecting waxy substance that advertise their availability to seed dispersing birds.

Descriptive UV patterns in feathers opens an entire world of visual signaling that is otherwise hidden from us. Given the ways we might imagine crows would benefit from exploiting any one of these possibilities, it makes sense that they’d possess the same rich UV experience like other birds.

“While it’s true that most passerines are what we call UVS birds, corvids, like flycatchers and most raptors, are VS birds, meaning their visual system is biased toward the violet-spectrum and they are not considered especially sensitive to UV light. The low UV-detection abilities of corvids and many raptors, appears to offer a lifeline to smaller passerines, which exploit these visual differences in their plumage, allowing them to remain conspicuous to potential mates, while staying inconspicuous to their potential predators.”

Science hasn’t fully cracked the code of corvids yet. There’s still so much we don’t understand about their lives. What we do know is, CROWS possess two fully functional eyes, but they only use one eye at a time because their monocular vision is more powerful than their binocular vision. They also don’t see well at night.

During the day, crows can see up to 1.5 miles away!

EAGLES have the best eyesight in the Animal Kingdom. A bald eagle can spot and focus on prey up to 2 miles away. Though eagles weigh around 10 pounds, eagle eyes are roughly the same size as humans.

Healthy human vision is 20/20, but HAWKS have 20/4 or 20/5. Meaning, they have a field of vision of about 278 degrees, compared to our 180 degrees.

WOODCOCKS can see 60 degrees in the horizontal plane and 180 degrees in the vertical plane. This helps them spot predators while their beaks are underground searching for worms. Woodcock eyes are set farther back than any other bird, and their ears are below rather than behind the eye socket. Evolution pushed their large eyes farther and farther back, which caused the ears to shift positions.

PEREGRINE FALCONS have binocular vision eight times superior to that of humans. They can spot small prey more than a mile away (like crows; just sayin’). They’re able to maintain sharp vision even while diving at 180 miles per hour! A third eyelid protects their eyes during these plunges.

HUMMINGBIRDS can see colors beyond our comprehension, including ultraviolet. They can also see farther than humans and have a nearly panoramic field of vision.

VULTURES have keen eyesight. It’s believed they can spot a 3-foot carcass from upwards of four miles away!

Okay, owl lovers, this one’s for you…

OWL eyes contain a combination of rods and cones. Rods interpret light. Cones interpret color. The more rods an eye has, the better it sees in the dark. Owls have one million rods per square millimeter of the eyes, a density of five times the amount we do. Their extraordinary ability to spot prey in the dark is the equivalent of a human trying to spot a mouse a mile away with only the light of a matchstick.

There’s so much I couldn’t include in this article. Would you want a part II? Let me know in the comments. What’s your favorite fun fact?