Cat Health

Why Do Cats And Snakes Have Slits For Pupils?

No doubt you’ve wondered why cats and snakes have vertical slit pupils. Can they see colors, and can they see in the dark?

As it turns out, the pupils of your cat or snake look and work the way they do to best suit their way of life. According to research, vertical slit pupils are popular among nocturnal and ambush predators. Given that the two don’t really like each other It’s amazing what they both have in common.

The shape of the pupil is believed to help gauge distance for precision prey snatching.

cats pupils look like stright lines in bright light condtions

Indeed the shape and size of the pupils play a bigger role in these animals’ lives and the care you provide to these pets. So let’s explore and truly reveal what makes cats and snakes have slits for pupils.

Why Do Cats Have Vertical Slit Pupils?

If you look into the eyes of different animals, including cats and snakes, you will be amazed by the shape of their pupils. 

Did you know that the pupil’s shape determines an animal’s role in its habitat!

A pupil marks the hollow opening in the iris, and it’s the portal which allows light into the eye.

Pupils are usually black because most of the light that passes through them is absorbed. 

The size of pupils changes in response to brightness. But their basic form normally varies among species.

Your house cat, for one, is a twilight hunter with vertically elongated pupils (slits). In the dark, the vertical slits expand naturally, taking in the available light. When it is bright, the vertical pupils shrink into slits.

Cat pupils are so flexible that their maximum area is 135 times greater compared to their minimum area. Human pupils can only go up to 15 times.

Are There Any Advantages of Having Slits for Pupils?

According to science, this evolutionary adaptation presents several benefits for cats, snakes, and other reptiles.

Slit Pupils Help Judge Distance When Hunting

The unique manner in which the slit pupils take in light creates sharp, vertical contours. 

When your cat’s brain processes the images from each eye, the small but sharp differences between them help the feline judge the precise distance of its target

The same thing has been observed with other ambush hunters like snakes and crocodiles.

Both cats and humans use a technique called stereopsis to gauge distances. This technique allows felines to use binocular vision along with other visual cues to determine the distance to an object or prey. 

Vertical pupils are better at performing stereopsis because they are small

horizontally and tall vertically, allowing animals to predict distances more accurately.

The Cats eye shape help it with hunting pray

Split Pupils Can Dilate And Contract Better Than Circular Pupils

The most common benefit of vertical slit pupils is their ability to dilate and contract more compared to circular pupils. 

Think of a cat who is awake and running around in full daylight as

well as at night. Felines can see better than we do in these two extreme light environments.

Nocturnal snakes with slit pupils can effectively regulate the light that goes into their eyes. This feature helps them avoid blinding their eyes when hunting in unexpected sunlight.

Animals With Split Pupils See Colors In Different Light Conditions

The next benefit, according to research, is the ability of slit pupils to see different colors in different light conditions. This leaves a larger amount of the retina open to receive wavelengths of light that translate to different colors.

Color is usually produced when light bounces off an object. Colors are represented by different types of wavelengths, which hit the pupil at different speeds. Colors are normally easy to perceive in bright daylight. 

Color vision, however, diminishes when it gets dark. That’s how hard

it is for human beings to perceive colors in low light conditions.

In animals with slit pupils, however, this problem can be corrected. A pupil that constricts vertically ensures all the focal zones have some exposure, even in low light. 

Why Are My Cat’s Pupils Dilated?

Temporary dilation of the slit-like pupils is considered a normal physiological response to exciting situations or low light conditions for cats. What happens if the pupils are completely round?

Over the years, the domestic cat has evolved and adapted its natural habits like hunting. The lineages of these cats were nocturnal, and they took advantage of the dark to protect themselves from predators and feed on prey. This was possible thanks to the design of the cat’s eyes.

this cat is in low light condtions with dilated pupils and is about to pounce

Your Cat’s Pupils Maybe Dilated To Allow Light To Enter

Cats dilate their eyes as wide as possible to allow light to enter. Also, they have a membrane in the back of their eyes that allows light to be absorbed and reflected into the retina, helping them to see very well in the dark. 

During the daytime, a cat contracts its pupils and keeps them

closed more or less, depending on the brightness.

Cats May Have Dilated Pupils Due To Reflex, Mood, Or Health

In general, the pupils should not be rounded in shape. When they are dilated with a normal amount of light, this can imply that the cat is not relaxed.

Common causes of dilated pupils in cats related to feelings and emotions include:


When a feline is very nervous or excited, such as during a heat cycle, it is normal to present elliptical or round pupils. The same level of excitement may be caused by anxiety or stress.


The cat’s pupils often dilate in situations that produce happiness. An example may be when your cat has just eaten or when your cat feels safe in the house.


If your cat has dilated pupils and wide eyes, it may be feeling fear or be afraid of something specific. This could be due to the presence of a stranger or a loud noise.


Cats dilate their pupils when they stalk prey or are ready to launch an attack. If your cat looks at you this way, it’s probably because it feels threatened by something.

When Do Dilated Pupils Mean A Necessary Trip To The Vet

There are certain conditions and pathologies which have pupil dilation as a symptom. If the environmental light is normal, there are no stressors or other stimuli, yet the cat still has dilated pupils. Then it’s probably time to visit the vet.

We recommend seeing a vet because the cause of dilation may be head trauma, eye discomfort or injury, some type of cancer, feline leukemia virus, renal insufficiency, glaucoma, anisocoria,

poisoning, hypoglycemia, or uveitis.

Of course, only the vet can diagnose exactly what the symptom may be related to, as there are a lot of crossovers. For example, anisocoria in cats is when one pupil is dilated much more than the other. This could be the symptom of many different diseases.

Pupil dilation is not always a cause for concern. In most cases, it will be related to vision and the entry of light. This allows animals like cats and snakes to see colors in high and low light environments.

While We’re Talking About Cats, Reptiles, And Vets!

If you are a pet parent, you might have wondered why cats shed fur and reptiles like beardie shed skin. 

This is one of the bizarre things you will observe in addition to slit pupils.

Your feline friend will shed fur normally depending on the season, but this can also occur because of medical problems like:

sunburn, infection, stress, medication, allergies, and poor diet. 

You could reduce the shedding by brushing your cat regularly, providing it with a stress-free habitat, feeding it good quality food, and keeping it hydrated.

As for reptiles, such as bearded dragons, it is normal for them to shed their skin. This is so because the skin doesn’t stretch as the beardie grows older. So, they’ve got to shed it to give way for new skin. The beardies shedding process takes around 1 to 2 weeks. The skin usually becomes flaky and dull.

Why Do Snakes Have Vertical Pupils?

According to studies, some snakes have circular pupils while others have vertical pupils. These two pupil shapes also evolved several times in different snakes.

Sankes like the one in the image have vertical pupils for night vision and to figure out distance

Snakes with vertical pupils are mainly nocturnal, which means these special eyes help them stalk prey. This, however, doesn’t mean that slit pupils provide night vision.

Research conducted on 127 Australian snake species revealed that nocturnal snakes that ambush prey had slits for pupils. Diurnal snakes, however, had round pupils.

How vertical slit pupils work for snakes is almost the same as that of cats. 

The slits probably help ambush snakes judge the precise distance from their prey. Vertical pupils allow snakes to see a wide horizontal field. So, they don’t have to constantly move to focus on the prey.

Vertical pupils usually help snakes camouflage and stay hidden in the surrounding grasses.

Some people believe that snakes with slit pupils are poisonous. But there is no correlation between pupil shape and the absence or presence of venom.

Final Words

Cats and snakes have more control over the amount of light that gets into their vertical pupils.

They get a wide horizontal field of view that helps to improve night vision. Did you also know that cats can also reduce the amount of light even further by bringing their eyelids closer together? It’s a perfect setup for a night time hunter who loves to bask in the daytime sunshine.

Johni Barresto

Johni Barresto Is a father and animal lover. With a range of expertise in animal health, he decided to start Animal Heed. His passion is to share his knowledge to help animal owners worldwide. When not in front of his computer, he's out with his kids, teaching them the importance of animal care.

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