Look a little closer
In bright light, a cat's pupil narrows until it looks like a black vertical line. That shape is not merely decorative. The pupil is the opening that controls how much light enters the eye, and research links vertical pupils with the lifestyle of ambush predators that approach prey close to the ground.
A vertical pupil can close to a very thin slit in daylight and open wide in dim conditions. This gives cats a large range of control over incoming light as they move between bright days and darker hours. Light control alone, however, does not fully explain why the slit runs vertically.
The other clue is distance. Cats can judge how far away an object is by comparing the slightly different views from their two eyes and by noticing how parts of a scene blur. A vertically open pupil may help keep important vertical edges useful for binocular comparison while preserving blur in another direction as an additional depth cue.
That combination is especially useful to a small predator crouching near the ground before a short, precise pounce. In a study comparing many land animals, vertical pupils appeared frequently among shorter ambush predators. Domestic cats fit that pattern well.
Not every hunter has vertical pupils. Larger cats such as lions and tigers have round pupils. Pupil shape is therefore not a simple badge that says predator or prey; it reflects a mixture of eye height, activity pattern, eye position and hunting strategy.
A cat's narrow pupil is more than a tiny aperture. It can be understood as part of a visual system tuned to manage changing light and read distance at the exact moment a ground-level ambush turns into a leap.
