Small cats, including the house cat, have vertical slit pupils, while lions and tigers look out through round pupils much like ours.1KQED. Hunter or Hunted: Why Animals Have Differently Shaped Pupils The shape seems tied to how close an animal’s eyes sit to the ground and how it hunts, rather than to night vision alone.2Berkeley News. Pupil shape linked to animals’ place in ecological web
Which Cats Have Slit Pupils and Which Have Round Ones
Animals that hunt by lying in wait and pouncing are the ones most likely to have vertical slits, especially if they hunt at night.3Live Science. Why Cats Have Vertical Pupils One comparison of 65 front-eyed ambush predators found 44 with vertical pupils, and 36 of those stood under about 42 cm (16.5 in) at the shoulder.4Berkeley News. Pupil shape linked to animals’ place in ecological web A house cat, at roughly 25 cm at the shoulder, sits squarely in that group.
Within the cat family, size largely draws the line. Lions and tigers have round pupils, like humans and dogs.5Berkeley News. Pupil shape linked to animals’ place in ecological web Mid-sized cats such as the Eurasian lynx have an in-between eye design.6PubMed. Pupil shapes and lens optics in the eyes of terrestrial vertebrates Their pupils are slightly oval and stretched vertically rather than true slits.7Popular Mechanics. Scientists Have Determined the Reasons Behind 3 of the Most Common Types of Animal Pupils The earliest cats likely had vertical pupils, and round pupils evolved separately more than once in bigger lineages.8Elsevier / iScience. Evolutionary insights into Felidae iris color through ancestral state reconstruction
The pupil shape follows eye height and hunting style, so no single body-size number flips a cat from slit to round. Some wild small cats, such as the sand cat or the Pallas’s cat, are reported to have round-looking pupils, so this is a strong tendency with exceptions.
You can check your own cat. Look in bright daylight near a window, when the pupil is nearly closed and the slit is easiest to see. In a dim room the pupil opens into a rounder shape, so a cat can look round-eyed in the dark even though it has slits.
How a Slit Helps a Small Cat Judge Distance
An ambush hunter gets one leap, so it must judge depth very accurately to land on its prey.9Live Science. Why Cats Have Vertical Pupils The same link between ambush hunting and vertical slits shows up in snakes, lizards and mammals.10PubMed. Insights into the adaptive significance of vertical pupil shape in snakes Slits are also common in other short ambush hunters like geckos and many snakes.
Besides comparing the two eyes’ views, eyes can use how blurry something looks to judge its distance.11PubMed Central. Blur and Disparity are Complementary Cues to Depth A vertical slit keeps a wide opening top to bottom, so things nearer or farther along the ground blur in a measurable way while vertical lines stay crisp. These blur cues work much better for short animals than tall ones.12Berkeley News. Pupil shape linked to animals’ place in ecological web A cat crouched in grass therefore gets a strong distance signal across the ground ahead of it.
A tall, narrow opening is wide top to bottom and narrow side to side, so blur from distance differences spreads mostly in the vertical direction. Vertical edges such as grass stems or a table leg stay sharp, which helps the cat keep track of the shape of its prey. Horizontal features such as the ground itself blur in a graded way, and the amount changes with distance.
For a crouched cat, the ground ahead stretches away as a series of horizontal bands at increasing distance. Each band blurs by a different amount, so the cat can read how far the prey sits along that surface. A cat standing upright on a tabletop would get a weaker version of this signal, because the cue works best with eyes low.
Front-facing eyes give the two-eye cue, and the slit strengthens the blur cue. Together they let a small cat strike without moving its head and giving itself away.
Why Lions and Tigers Have Round Pupils
Taller animals can’t use the extra distance information that blur provides.13KQED. Hunter or Hunted: Why Animals Have Differently Shaped Pupils A lion’s eyes, roughly a meter up, gain little from a slit. Big cats also more often stalk and run down large prey, often in daylight or twilight.14Big Cat Rescue. Cat Eyes | Big Cat Rescue Much of a lion’s hunting happens at dusk, dawn, and in dim light, and the cats rest through the hottest, brightest hours. They benefit more from balanced, all-around vision than from extreme light control.
A round pupil blurs evenly in every direction, so the image stays balanced across the whole scene. A big cat scanning a wide landscape for moving prey benefits more from even sharpness than from a distance cue that works mainly at ground level. For a tall animal, distance can come from other cues, such as the two eyes’ comparison and the size of the prey.
How Slits Handle Bright Sun and Dark Nights
Round pupils close with a ring of muscle, but slits have two extra muscles that squeeze the sides together.15PubMed Central. Why do animal eyes have pupils of different shapes? As a result, a house cat’s pupil can change its opening area about 135-fold, while a human’s changes only about 15-fold.16PubMed Central. Why do animal eyes have pupils of different shapes? Geckos’ slits go further still, about 300-fold.17PubMed Central. Why do animal eyes have pupils of different shapes? A cat can open very wide for night hunting yet shut nearly closed at noon.
In a round pupil, the ring squeezes the opening evenly like a drawstring. In a slit pupil, the extra muscles pull the two edges together like a zipper, so the opening can close almost completely. A circle can only shrink so far before the ring of muscle bunches up.
Engineers have copied the idea. A camera built with a cat-like switching pupil avoided washed-out images in light about 30 times brighter than a round opening could handle.18Science Advances. Feline eye–inspired artificial vision for enhanced camouflage breaking under diverse light conditions Some researchers think slits may also help reduce color fringing in the image.19PubMed. Pupil shape in the animal kingdom: from the pseudopupil to the vertical pupil
Kittens’ Eyes and How Pupils Develop
Kittens’ eyes point more outward at first, and this decreases during the first 2 months.20PubMed Central. Eye alignment in kittens The slits also tilt inward slightly over the first two months, settling around 14 degrees.21PubMed Central. Eye alignment in kittens Kittens raised in darkness developed an abnormally large tilt, so normal light exposure seems to help a kitten’s eyes develop correctly.22PubMed Central. Eye alignment in kittens So very young kittens can look slightly wall-eyed, which usually corrects itself, and their pupils may seem less sharply controlled in the first weeks. A kitten whose eyes still look misaligned past a few months is worth a vet check.
Kittens are born with their eyes closed and open them in roughly the first couple of weeks. The eyes at first look blue and cloudy, and vision sharpens gradually over the following weeks.
When a Cat’s Pupil Shape Signals a Health Problem
Health problems can make a cat’s pupils unequal in size, stuck wide open or narrowed, or oddly shaped.23Vetster. What Does The Size And Shape Of A Cat’s Pupils Mean? Causes include glaucoma, inflammation inside the eye called uveitis, retinal detachment, a dislocated lens, iris thinning, and eye tumors.24Vetster. What Does The Size And Shape Of A Cat’s Pupils Mean? In one reported case, a cat with lymphoma developed D-shaped pupils because cancer cells invaded the iris.25PubMed. ‘D-shaped’ and ‘reverse-D-shaped’ pupil in a cat with lymphosarcoma Call a vet promptly if you notice these changes.26Vetster. What Does The Size And Shape Of A Cat’s Pupils Mean?
Before the visit, look for redness, cloudiness, squinting, or a changed eye color. Note any bumping into things or hesitating on stairs, which suggests vision loss. A photo taken in steady light can show the vet the difference.
Speed matters. Conditions like glaucoma can damage sight within days, and pupil changes can be the first sign of illness elsewhere in the body, as with lymphoma.
The vet needs to examine the whole eye, because the causes range from minor problems to serious disease and the shape alone can’t reveal which one it is. A vet can measure eye pressure, look inside the eye with a light, and check how each pupil reacts to light. These tests separate conditions like glaucoma from inflammation, which need different treatments.