Why Do Cats' Eyes Get Big?

Cats aren’t the best at expressing their emotions, leaving us to decipher their eccentric behaviors. But when Fluffy’s eyes nearly double in size as you wave her favorite wand toy or walk down the hallway with a laundry basket in tow, it’s downright terrifying. These moon-sized pupils seemingly appear out of nowhere, so why do cats’ eyes get big?

Cats’ eyes get big (dilated pupils) because it’s a cat’s response to negative emotions. A cat’s dilated pupils can signal arousal (ambush incoming), frustration, aggression, surprise, and fear. When a cat’s eyes are black, her butt is shimmying, and she seems laser-focused, she’s about to pounce!

You might remember learning about dilated pupils in 8th-grade science class by dimming the lights, flipping them back on, and snapping a photo. For cats, it’s not so much a dark versus light debate; it’s about instincts and emotions. To learn why cats’ eyes get big, read on!

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Cats & Dilated Pupils

A cat’s eyes are very similar to humans and other mammals, though the tight vertical slits and golden-ember shades are quite distinct. Those bold, full-moon eyes are called ‘dilated pupils.’

Before we get into the why of feline pupil dilation, we’ll dive into the how. There are three main parts to a cat’s eyes that are visible to the naked eye:

The iris: The iris is the gold, pale blue, or copper-colored portion with one essential responsibility — detecting and responding to light changes. In dim lighting, like when a cat’s hunting for prey at dusk, the iris will cause the pupil to dilate to allow more light in. When lights are blindingly bright, like when sunbathing, the iris will constrict the pupils.
The pupil: The pupils are the black portion centered in a cat’s eye with one job — guiding the perfect amount of light into the retina for visual acuity. The retina absorbs the scene ahead, like a squirrel darting across the yard, and sends the signal to the brain.
The sclera (eye white): The eye white is something you’ll rarely see in kitties unless they lend you a glaring side-eye or look far left or right without turning their heads. If you see the whites, back off — Fluffy’s vying to pounce at anyone crossing her path!

From the most fundamental scientific perspective, a cat’s pupils will only expand (dilate) in dim lighting — which is a possible explanation, though certainly not the only one. This theory doesn’t consider the psychological and emotional factors that control a cat’s every move.

As terrifying as those bulging pupils seem from afar, they often appear with playful stalking and striking in well-socialized cats. The video below shows how quickly dilation can progress:

Why Are My Cat’s Pupils So Big?

Any experienced cat owner will recognize those intimidating, massive pupils and mutter the phrase, ‘Uh-oh.’ A cat whose pupils phase out those beautiful blue irises isn’t about to nuzzle in your neck or lick your hand affectionately; they’re in defense mode and ready to attack!

If you see a cat’s pupils widen against a sternly-focused backdrop, here’s why:

Defense Mode

Even the most extroverted cats become startled from time to time, whether it’s due to the Fourth of July fireworks or introducing a ‘strange’ cat to the household. A fluff ball in ‘defense mode’ will feel threatened and use whatever means necessary to protect herself, including:

Swatting at the invading threat
Hissing or growling
Biting with its dagger-like teeth (only in extreme circumstances)

As hilarious as it might be to ‘toy’ with a defensive kitty, don’t press your luck. A cat who feels cornered without an escape route won’t hesitate to use her claws and teeth to fend for herself and escape this danger. This response is typical if you spook your cat as she’s in the litter box, taunt her as she lounges behind the couch, or a bobcat approaches your screen door.

However, your cat may also dilate her pupils when a legitimate threat enters her territory. These massive pupils can signal a predator in the area (like a coyote), a foreign cat infringing on her property, or a strange knock on the door.

Excitement

The line between excitement and aggression is undeniably fuzzy in the feline stratosphere, at least from your perspective. How can you possibly distinguish whether those unusually wide pupils are playful or defensive?

Look at the other behavioral cues and what led to those ever-growing pupils. If your cat raises her butt in the air and wiggles it to and fro, she’s fulfilling her predatory instincts in a more light-hearted way. Fluffy’s pupils might expand as she:

Sneaks up on you from behind the couch or door
Watches your toes or hands wiggling beneath the blanket
Goes full ‘attack mode’ as you cross paths with her
Sees a wand toy waved or laser shined in her direction

Don’t worry. Even though those pupils also indicate aggression and a serious ambush attack, a playful kitty will ‘take it easy’ on you. Your cat may gently gnaw your arm instead of digging her teeth, swat with her paws (not her claws), or gallop off to say, ‘Chase me!’

To See Better in Dim Lighting

Of course, the scientific explanation is sometimes right. While cats have ‘night vision’ and can see in 1/6th the natural light, they can’t find their way around a pitch-black room. A cat’s pupils might expand as daylight turns to dusk, you draw the shades, or you shut the bathroom door.

These widened pupils will allow light remnants in to give your kitty some visual clarity as she explores this mysterious environment. A nocturnal cat who’s most active at 3 a.m. might experience dilated pupils as she stalks unsuspecting prey, pounces on that realistic-looking mouse toy, or identifies some movement under the covers.

It’s All in the Eyes: Other Hidden Messages

Have you ever glared into a stranger’s eyes and thought, ‘He seems nice,’ or, ‘She looks angry?’ Cats’ eyes are also quite expressive about how they’re feeling, albeit not intentionally.

When you see these changes in your kitty’s eyes, here’s the message Fluffy’s sending:

Slow Blinking

Sometimes, you’ll catch your cat’s gaze as she’s lounging on the sofa across the room or curiously picking her head up mid-sunbathe. Look into her eyes lovingly and watch for the slow-motion blink. A slow eye-shut signals trust and vulnerability; a cat with her eyes shut won’t notice a menacing coyote approach, making her a ‘sitting duck’ in the wild.

What it means: I love you!

Fluffy might not understand those ‘I love you’s’ that you toss into her direction, but you can express this sentiment in other ways. The video below explains four ways how:

Half-Shut Eyes

Cat eyes at half-mast (half-shut) send a very similar message to the heart-warming slow-blink. When a cat squints her eyes, she’s expressing more positive emotions: Calm, contentedness, trust, and downright fatigue. Fluffy is likely on the verge of plopping her head down and snoozing for the next few hours.

What it means: I’m sleepy!

Constricted (Narrow) Pupils

When a cat’s pupils are more pinpricks than pizza pies, it represents the opposite emotions: Excitement or aggressive arousal. Those narrow, slit-shaped pupils suggest that your cat’s readying to hiss and attack. Or she’s merely too grumpy to play or cuddle at the moment.

What it means: Stay away!

Conclusion

A cat’s ultra-wide pupils are normal. She doesn’t know her eyes are widening as she sneaks up on your toes, poking out from the blanket. Or how you know to yank your foot away just before she ambushes.

But the dilation can be worrisome if one eye dilates (anisocoria). These unevenly-sized pupils can be a sign of something more sinister, like:

An eye condition (glaucoma, corneal ulcer, or iris defect)
Cancer in the eye
A brain or nerve injury
Feline leukemia virus (FeLV)

Sudden anisocoria can lead to permanent blindness if left untreated — take your cat to the vet!

Sources

Pam Turner

Pam is a self-confessed cat lover and has experience of working with cats and owning cats for as long as she can remember. This website is where she gets to share her knowledge and interact with other cat lovers.