Cats have vertical pupils, sheep have horizontal ones — The "square pupils" of sheep and goats are the optimal solution for herbivores

The Evolutionary Tale Beginning with "Those Eyes Are Scary!"

Anyone who has ever looked directly into the eyes of a sheep or goat has likely been startled at least once. Their pupils are not "round" but elongated horizontally into a rectangle—resembling the barcode reader of a scanner. However, that shape is not a horror effect or urban legend; it is a "design for escape" honed for survival in grasslands. IFLScience

What Horizontal Pupils Excel At: Viewing the Horizon Panorama "Brightly"

Ruminants like sheep and goats, which chew cud in their rumen, are often targeted by predators. Thus, "early detection" becomes crucial. While having eyes on the sides of their heads significantly broadens their field of vision, horizontal pupils make that "wide field of view" even more practical.

Martin Banks, a vision researcher at UC Berkeley, explains that horizontally elongated pupils facilitate the intake of light from all directions, aiding panoramic vision. IFLScience also highlights that, combined with side-placed eyes, they enable "almost 360-degree" peripheral vision, and the more horizontal the pupils, the better the lateral visibility. IFLScience

Not Just "Wide": Reducing Glare from Above and Making the Ground Easier to See

The key point of horizontal pupils is "wide horizontally = thin vertically." The narrow vertical aperture helps reduce glare from strong overhead light (sunlight) and maintains visibility of the ground, which is crucial for herbivores not only to spot predators but also to avoid losing sight of the "escape route ground." Phys.org

Solving the "Seeing the Ground While Running" Problem: Sharpening Horizontal Contours

In comments featured on IFLScience, Banks notes that the vertical thinness of the pupils can sharpen "horizontal contours." This means it's easier to discern the edges of ground undulations and obstacles, allowing better foot placement during sprints or sharp turns. IFLScience

When Grazing, Eyes Spin: Maintaining Pupils "Level with the Ground"

Here's the interesting part: if the pupils were "just horizontally elongated," the moment the head lowers to graze, the pupils would tilt along with it, diminishing the advantage of being "horizontally aligned with the ground."

However, observations show that when grazing animals like goats and antelopes lower their heads, their eyeballs rotate to keep the pupils level. Additional observations indicate that these herbivores can rotate their eyes more than 50 degrees per eye, a much larger range than humans. Phys.org

Cats Have "Vertical," Sheep Have "Horizontal": Do Pupil Shapes Depend on Occupation (Ecology)?

Why "vertical" or "horizontal"? Research introductions indicate that analysis of 214 terrestrial animal species shows a strong correlation between pupil shape and ecological niche (predator or prey, hunting style, etc.). Vertical slits are common among ambush predators active day and night, while horizontal ones are prevalent among herbivorous prey animals. Round pupils are associated with "active foragers" who chase their prey. Phys.org

The explanation also includes that vertical slits accommodate brightness differences from dark to daylight while aiding distance estimation (using cues like stereopsis and blur). Additionally, the tendency for "smaller, ground-level predators to have vertical slits" is noted. Phys.org

There Are Exceptions: The "Mongoose" That Breaks the Rules

However, nature is not always orderly. An IFLScience article cites the mongoose as an exception to the general rule (predators = vertical, prey = horizontal), possessing "sheep-like pupils" despite being highly predatory. Researchers' comments that "this animal doesn't fit the explanation" suggest there is still "room" in the evolution of pupil shapes. IFLScience

Reactions on Social Media: Scary, Cute, Rational... Reasons for Divided Comments

The square pupils of sheep and goats are a "must-share" topic that regularly goes viral on social media. Reactions generally fall into three categories.

1) The "Like Devils" or "A Bit Scary" Camp

Because the shape deviates from human norms, it intuitively appears "alien." Close-up photos, in particular, have a strong impact, often eliciting phrases like "horror" or "fantasy monster-like."

2) The "Too Rational, So I Like It" Camp (Enthralled by Functional Beauty)

On the other hand, once the reasons are understood, opinions can flip. "Scary" becomes "rather clever." On Reddit's science threads, explanations like horizontal pupils being for peripheral vision and maintaining level pupils even when the head is lowered are casually shared. For example, the following exchanges occur. Reddit

“In combination with their eyes being on the sides of their head, this gives them almost 360° vision.” Reddit
“Their eyes will also rotate in their sockets… so the pupils will always remain level to the ground.” Reddit

3) The "What About Cats? Humans?" Comparison Camp

Once the topic gains momentum, a "chain of comparisons" inevitably begins. Questions like "Are cat's vertical pupils for depth perception?" "Do big cats have round pupils?" "What are human round pupils suited for?" arise, leading to an understanding of pupils as an "ecological calling card." Research introductions also point out that vertical pupils are common in small ambush predators, while larger predators may have round pupils, making "size-related differences interesting." Phys.org

Conclusion: Square Pupils as "Survival Tools of the Grasslands"

The horizontally elongated pupils of sheep and goats, despite their peculiar appearance, are the result of highly practical optimization. They allow for early detection of danger with a wide field of vision, reduce glare from above, maintain visibility of ground contours, and keep the pupil orientation even when grazing.
Next time you see sheep or goats at a farm or zoo, try imagining their lives through the "shape of their eyes." The fear might just transform into an appreciation for functional beauty. IFLScience