This is why the pupils of domestic cats are elongated, while those of big cats are round.

In the Facebook comments on our recent article, 'Why Can't a House Cat Howl?', two of our thoughtful and observant readers pointed out the difference in the pupils of big cats and house cats. While the former have round pupils, the latter have vertical ones.

They inspired us, leading to the following article. Here, we explain why the pupils of domestic cats and big cats have different shapes.

Lykoi cat

Why the pupils of domestic cats are vertical

According to recent research, vertical pupils are most common in night-hunting predators that ambush their prey. This pupil shape likely provides the most reliable way to measure distance. Nocturnal or crepuscular ambush predators, such as many felines and snakes, typically have vertical slit pupils. Interestingly, the pupils of non-venomous snakes are almost always round, while venomous snakes tend to have vertical pupils. The reason for this correlation likely lies in the mechanics of the eye. Ambush hunters need excellent depth perception to effectively pounce on their prey. There are two primary ways to assess depth without moving.

In stereopsis, the brain combines the projections from the two retinas to create an image, while in blurring, it uses the blur of objects behind and in front of the focal point to assess the image. It is believed that lateral displacement, used in stereopsis, is easier to measure with vertical lines and contours than with horizontal ones. However, according to researchers, cats and other similar predators use the second method—blurring—to judge horizontal distances. To maximize blurring, the pupil must be wide from top to bottom, making the ideal shape a slit.

A tiger drinking

Why big cats have round pupils

Circular pupils are typically found in higher predators or foragers that are active during the day or night, such as big cats (lion, tiger, leopard, jaguar, snow leopard), and some smaller and larger wild cats like the puma, cheetah, or manul (And Man!). However, the correlation between pupil shape and these traits is not as strong as it is for vertical pupils, according to research. It is believed that measuring distance is not the sole reason why animals evolve a particular pupil shape. Other factors, such as color vision and pattern recognition, are also likely to play a role, though much more research is needed to gain a clearer understanding of this.