The Sensational Reason Behind Cats’ Vertical Pupils

There's a sensational reason behind cats' vertical pupils, as revealed by a study published in the journal Science Advances. It turns out that vertical pupils are most commonly found among predators that ambush their prey, especially nocturnal or crepuscular ones. However, the researchers didn't stop there; they also explored how the peculiar, horizontal and rectangular pupils observed in goats and sheep are related to cats' slit pupils. Furthermore, we'll show you the advantages of round pupils and who else exhibits them.

From this, we’ll uncover how our pets benefit from the advantages of vertical pupil.

The vertical pupil is a weapon

It was observed as early as the 1940s that predators typically have vertical pupils, while their horizontal-pupiled prey includes animals they hunt. Yet, until recently, no one dared to delve deeper. However, Martin Banks, a vision researcher at the University of California, Berkeley, and his team were ready to peer into the souls of a total of 214 terrestrial species, compiling a database in the process. After analyzing the data, they were able to correlate the shape of the eye with the animal’s feeding habits and active periods.

Ambushing predators, such as many species of cats and snakes, typically had vertical pupils, especially when leading a nocturnal lifestyle. The reason behind this correlation likely lies in the mechanics of the eye. “Ambush predators need to be very good at depth perception” – explained Martin Banks to Live Science.

There are two ways to measure depth without motion: stereopsis, where the brain creates an image from the projections of the two retinas, and the blur of objects behind and in front of the focus point to determine distance. It is believed that the side-to-side movement used in stereopsis is easier to measure with vertical lines and contours than with horizontal ones. However, for estimating horizontal distances, researchers suggest that method two is used by cats and other similar predators. To maximize blur, the pupil needs to widen from top to bottom, hence the ideal shape being the slit.

The pupil of the prey

On the other end of the spectrum, goats, sheep, horses, and some frog species have peculiar square pupils. In the database created by Banks and his colleagues, out of 42 herbivorous prey animals, 36 had horizontal pupils. To understand why, they created a computer model of a sheep’s eye and measured its optics. The results showed that the horizontal pupil minimizes the input of information from above and below while maximizing the images visible from the front and back. Coupled with the eyes being positioned on the sides of the head, this also contributes to panoramic vision.

But that’s not all! It’s remarkably intriguing that these pupils also minimize the blurring of horizontal contours, providing a sharper image of the ground and the area in front of the animal. This is crucial for an animal that needs to react immediately, meaning it must flee if it senses a threat.

However, during the development of the theory, they encountered a problem. The animal’s eyes must remain aligned with the horizon; otherwise, they wouldn’t sense any benefits. However, field studies revealed that these animals all rotate their eyes — one in the clockwise direction, the other in the counterclockwise direction — to keep their pupils aligned with the horizon while grazing.

The mystery of the round pupil

If simplifying the vertical pupil for hunting and the horizontal for escape, what advantage does the round pupil offer? Banks says there isn’t a clear answer to this question. Animals with round pupils are typically active predators or foragers, seeking food day and night. However, it’s undeniable that in this case, the correlation between pupil shape and characteristics wasn’t as strong as in previous cases.

To somewhat clarify the issue, researchers constructed a family tree of cats and dogs to gauge whether the various pupil shapes evolved only once in these families or multiple times independently. The results indicated that they evolved multiple times independently. Banks added that besides depth perception, the shape of the pupil is naturally related to color vision and pattern recognition, so they also want to expand their knowledge in this area.