Those black slits are your cat’s pupils. Pupils are openings in the middle of an eye that lets light in.
Under certain conditions, which you can read about below, those slit-shaped pupils may become big round-shaped pupils.
The slit pupils are called constricted pupils.
The big round pupils are called dilated pupils.
Ambush predator animals who lie in wait to pounce upon prey are observed to commonly have vertical-slit eye pupils. [1] Our Domestic Cats are such predators.
Vertical-slit pupils are believed to enhance an ambush predator’s ability to estimate the distance to prey so as to know when to pounce. [1]
Prey animals such as goats, are observed to commonly have horizontal slit-shaped pupils, which is believed to afford them a panoramic view that helps them spot predators, and to assess the territory in front of them when they are running to escape a predator. [2]
Researchers also found that distance of eyes above ground can affect pupil shape. Small animals whose eyes are closer to the ground commonly have slit pupils. On the other hand, larger prey animals, who also happen to be chase predators rather than just strictly ambush predators, commonly have round pupils. Lions are an example of this larger predator. [1]
Why Do My Cat’s Pupils Get Big and Round (dilated)?
As a response to the level of light
Animals with pupils that have a narrow slit shape experience a significant expansion in pupil size when the pupil dilates, so as to greatly increase the amount of light entering the eye, and thus enhancing the animal’s ability to see in dim light. [3]
Cat pupils are capable of expanding in size many times greater than the expansion human eyes are capable of. [2] This is a contributing reason why cats see so much better than we can in dim light
While your cat cannot see in total darkness, in dim light your cat can see six times better than humans. Those big round pupils help them do that. [3]
The iris is the round part of the eye that encircles the pupil, and it is muscles in the iris that expand and contract the size and shape of the pupil. [3]
Another contribution to your cat’s excellent vision in dim light is the tapetum lucidum, a structure in your cat’s eye that essentially magnifies the light available to the eye. This layer of tissue is reflective, and some of that light bounces back out through the pupils. That is why your cat’s eyes glow in the dim light. [4]
If the eye is exposed to very bright light such as the flash of a camera, even more light may bounce off the retina, and the blood vessels in the eye can cause “red eye” in photographs. [4]
Cats are not nocturnal. They are crepuscular, meaning they are evolved to hunt at dusk and dawn. [5]
Since your cat cannot see in total darkness, it may be a good idea to have small plug-in lights around the house so your cat can find the litter box at night. Since litter boxes are so very important to cats, you may want to visit my blog post How to Set Up Your Cat Litter Box.
If you care to discover even more interesting and amazing things about your cat’s eyes, you should visit my blog post Can Cats See Things Up Close?
As a response to what is going on in a cat’s brain
Eye dilation may occur during a period of intense concentration, or excitement, or some other state of mind. These mind states may be due to either positive or negative circumstances. [6]
Your cat’s pupils may dilate when the kitty is chasing after a dot from a laser pointer, or frightened by a stranger in the house, or anticipating being given a treat.
Pupil dilation may also occur if your cat is under stress from illness or injury. If your cat’s pupils are dilated continuously or almost continuously, or if one eye dilates and the other does not, you should consult with your veterinarian.
Why Does My Cat Blink At Me is my blog post that explores another example of a possible link between your kitty’s eyes and their brain
For Additional Information:
Please visit my blog post Learn About Cats
References:
[1] Sciences Advances, “Why do animal eyes have pupils of different shapes?”, 08/07/2017, …………….Martin S. Banks, et al
[2] Berkeley News, a magazine of University of California Berkeley, Pupil shape llinked to animal’s place in ecological web, 08/07/2015, Sarah Yang
[3] Merck Veterinary Manual, “Eye Structure and Function in Cats, 07/2018, Kirk N Gelatt, VMD, DACVO
[4] Carnegie Museum of Natural History article, “MEOWFEST: WHY DO CAT EYES GLOW IN THE DARK?”
[5] Marine Parker. ETHOLOGY AND BIOLOGICAL RHYTHMS OF THE CAT. Life Sciences [q-bio]. Université de Strasbourg, CNRS, 2018. English. ⟨NNT : ⟩. ⟨tel-02274459⟩
[6] frontiers in Neural Circuits, Neuromodulatory Correlates of Pupil Dilation, 09/03/2018, …………………Rylan S. Larsen, Jack Waters