Why Do Cats' Pupils Get Big?

Beautiful grey and black cat with pupils that gotten bigger

It is said that the eyes are the window to the soul and that includes the big, beautiful eyes of our feline friends. As any cat lover knows, cats’ eyes are incredibly expressive.

They squint, they open wide and notably, their pupils can get really big.

So Why Do Cats' Pupils Get Bigger?

The reasons given by experts and scientists who have researched the subject vary from health to emotional state to available light and as a reaction to their current circumstances. In this blog, we’ll answer the question of why cats pupils get big.

Cats' Pupils Get Bigger When It's Dark

Calico cat displaying pupils that have gotten bigger in the dark
Humans' pupils dilate in low light to allow us to see better. Cats’ pupils do the same, except measurements indicate that their pupils can widen a lot more than ours.

Under normal resting conditions, a cat’s pupils appear to be narrow black slits. Cats’ retinas are very sensitive, and that narrow slit prevents too much light from shining on the retina during daylight hours. This protective pupil configuration is seen in many animals, including domestic cats.

Cats’ eyes, specifically, also contain large lenses and curved corneas. And their eyes contain more rods, which are the photoreceptors responsible for helping eyes see at night, than humans and other species. So, expanding the pupil allows more light in and then cat's many rod photoreceptors are able to process that limited light in a way that best helps cats see in dim and dark conditions.

To learn more about if and how cats can see in the dark, visit our blog on the topic here.

Cats Pupils Dilate When They Hunt

Infographic for why do cats pupils get big
Cats are what is known as crepuscular animals, which means they tend to be active at dawn and dusk. Cats, as we all know, are also natural born hunters. So, you combine the need to hunt for food and the practice of doing so in low light and you have a recipe for large pupils. The interesting thing, however, is that cats’ pupils will dilate when they hunt even if full light is available.

Dilating their eyes even in situations of adequate light allows cats that are hunting to collect as much information, including small movements of their prey, as possible.
Cats’ Pupils Get Bigger When They are Surprised
Whether you're a human or a cat, surprises can be both good and bad. And until your cat decides whether whatever surprise they just experienced is good or bad, their eyes will likely dilate.

A cat dilating their eyes when they are surprised is simply a way for the cat to immediately take in as much visual information about a potential threat as possible. Once the surprise has worn off and they recognize the surprise is just your hand going in for a belly rub, their pupils are likely to return to normal.
Cats Pupils Dilate When They’re Scared
Small cat with pupils that have gotten big in responsed to being scared
As noted by Catster, large pupils can also indicate fear. If your beloved feline’s pupils are slightly widened, she’s a little scared, but if they’re huge, she’s terrified.

Whether your cat's afraid of a loud noise, a stranger coming through the front door or a large dog outside, their pupils will likely remain dilated until they make a determination about the potential threat. Similar to their pupils getting bigger when they are surprised, a cat that is scared needs to make a determination about reacting to the potential threat. The more visual information they have the better they are able to react.

So, if for some reason your cat’s pupils suddenly get bigger for no apparent cause, you might want to speak softly to them and calm them down the best you can.

Anxiety Can Also Cause Cats' Pupils to Dilate

Similar to fear of a loud noise or sudden stranger, unknown anxiety can also cause cats’ pupils to dilate. It’s easy to determine the reason your cat’s eyes have gotten bigger if there is an easily recognizable sudden threat. But it’s also a good idea to be aware of dilation when there is no easily identifiable threat as it may be a sign of anxiety in your cat.

Chronic stress can be damaging to cats just as it can humans. So, this would be something you would want to watch out for and address and for which you might need veterinary assistance.

Beautiful brown and white cat using the furry masterpieces cardboard cat house to relax and calm down
One of the best ways for your cat to relax after being started or frightened is to have a place they feel safe and secluded from predators or whatever scary thing frightened them. Our Cardboard Cat Houses with Scratchers offer cats the best of both worlds. Our cat houses are fully enclosed but offer strategically placed play and peep holes, which also give your cat the ability to survey their surroundings. And, as all of us cat companions know, cats love warm spots. So cardboard cat houses, particularly, help reflect your cat's body heat back into the space to help keep them warm, cozy and calm.

Dilated Pupils Can Also Indicate Aggression

Using the state of your cat’s pupils to determine if your feline is feeling truly aggressive is a little tricky. As indicated, cats' pupils dilate for any number of reasons, many of which have nothing to do with anger or aggression. A cat’s pupils can also narrow to slits when they are aggressive. This is often accompanied by squinted eyes. They squint their eyes in a potentially aggressive situation in order to better protect them.

But it also can be the case that cats pupils get bigger when they are feeling aggressive. So, it’s best to look at the overall signals your cat is sending in making this determination. If their pupils are dilated in conjunction with hissing, growling and, particularly, with their ears flattened against their head, that could mean your cat is about to be aggressive.

And as any cat owner knows, this is not the time to try to handle or pick up your cat. Best to try to calmly eliminate whatever is likely causing the aggressive reaction.

Perpetually Dilated Pupils Can Be a Sign of a Medical Issue

Pupil dilation in cats can happen for a variety of reasons, and pupils will typically go back to normal soon. However, in some sever cases, as noted by Vet Help Direct, if your cat's pupils remain dilated, it could be a sign of a medical condition.

Eyes that remain abnormally dilated could be a sign of hypertension, anisocoria (pupils in different sizes), deteriorating vision, chronic pain, ocular tumors, iris atrophy or medication side effects. If you think your cat’s pupils are not reacting as he or she should, you might want to consult with a veterinarian.

Excitement Just Might Be the Best Reason Your Cat's Pupils Get Bigger

Don’t worry. It’s not just bad stuff that makes your favorite feline’s pupils go big. If you’re about to give her a treat or have taken out her favorite toy so you can play with it, your cat’s pupils will often go big in anticipation. Fun times are coming, and you’re both going to enjoy it!

So, your cat’s eyes, including their pupils, are truly windows to their soul. And what a beautiful view it is.

Frequently Asked Questions

What does a cat's pupil size mean?

Cat's pupil size fluctuates based on circumstances, their emotional well-being and available light. When there pupils look like black vertical slits, that is often an indicator that they are relaxed. However, when they get excited or nervous and want to take in as much information as they can, their pupils will become dilated and appear large and round.

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