Why do Animals have Non-Circular Pupils? The Effect of Pupil Shape on Vision

Cats have the most fascinating eyes: vertical slits that supposedly help them see better at night. But hold on, cats aren’t the only creatures with odd-shaped eyes—so do toads and sheep, with their horizontally bar-shaped pupils. And stingrays, with the crescent pupils, and even cuttlefish with their odd w-shape pupils! Why do these animals have such unique and bizarre shapes in their eyes? Do slits really help animals see better in low light? And why are humans stuck with the seemingly boring round pupils?

How Do Our Eyes See Light?

To properly understand why animals have different eyes, we first need to know how our eyes work. The eye has 6 main components: 

1. The cornea is the transparent, hard layer that covers the front of your eye and focuses light that enters your pupil. If you gently touch your closed eyelids, you can feel your cornea!

2. Behind the cornea is your iris. The iris is composed of coloured muscles that expand and contract to control the amount of light that enters the eye, depending on the brightness of your surroundings.

3. At the centre of your iris is your pupil, a small black circle where light enters your eye.

4. After entering the pupil, light passes through the crystalline lens, which further focuses light.   

5. At the back of your eyeball, where light is concentrated, lies the retina. It is responsible for converting light into nerve impulses.

6. The optic nerve transfers electrical signals from the retina to the brain, where you then ‘see’ the information.

Fig. 1: (left) A simple diagram of the front of the eye

Fig. 2: (right) A simple cross-section diagram of the eye

• Fun Fact: Myopia occurs when eyeballs become elongated, and light does not correctly hit our retinas.

Positioning of Eyes

Fig. 3: A diagram of 3 animals’ different fields of view

In figure 3, observe the different fields of vision between the three animals. This is primarily caused by the positioning of the eye, not the pupil. 

Monocular: the area of vision that is seen by only one eye.

Binocular: the area of vision that is seen by both eyes. 

Prey animals tend to have monocular vision, while predator animals tend to have binocular vision. Generally, cats have eyes that are more narrowly spaced than dogs, resulting in a smaller field of vision. But where both cats and dogs have front-facing eyes, rabbits have eyes on the sides of their heads, resulting in a large, but predominantly monocular, field of vision.

Stereopsis is the perception of depth formed using the minor differences in object positions in each eye. 

Close one eye and hold your thumb in front of your face, keeping your arm straight. Then, switch eyes. You’ll notice that your thumb moves ever so slightly. It is this small difference that your brain uses to calculate a more three-dimensional vision, allowing for more accurate depth perception. 

Types of Pupils

Pupil shapes are affected by 2 main factors: illumination and ecological niche. While there exists a plethora of different shapes and sizes of pupils, some of the 3 most common you’ll find are:

1. Vertically slitted pupils

2. Horizontal pupils

3. Round pupils 

Fig. 4: A collection of photographs depicting several kinds of animal pupils

Slit and Vertical pupils

Vertical slit pupils are most common in shorter predators that ambush prey, and nocturnal animals who hunt in very low light, such as cats, snakes, and foxes.

In addition to the circular pupil muscles humans have, slit pupils contain two additional muscles to allow for greater dilation and contraction. This allows animals better visibility in low illumination and makes them less likely to be ‘dazzled’ by bright lights.  

The vertical orientation of pupils helps vertical shapes (such as a deer’s tall body) stay sharp even as objects increase in distance. However, light that enters the pupils becomes increasingly blurred the further it is on the x-axis, resulting in horizontal shapes (such as the ground) becoming more unfocused than they would be along the y-axis. This creates an astigmatic depth of field which enhances blur gradients. 

Animals with vertical slits can use this blur gradient to identify how far away an object—or prey—is: the blurrier the surrounding area of an object is, the farther away it is. This gives animals better depth perception, allowing predators to accurately judge the distance between them and their prey. This is particularly important for small ambush predators, who rely on precision to catch their prey. It also reduces blur during motion, allowing clear vision for these fast-moving animals as they pounce.

• Fun fact: Non-venomous snakes often have circular pupils, while venomous snakes often have vertically slitted pupils. Can you think of why this is? 

Horizontal Eyes

Horizontal pupils are most common in grazing prey animals, such as deer, goats, or sheep. These rectangular pupils allow an increased area of light entry along the x-axis, expanding their peripheral vision. When the lateral eye placement of these animals is also considered, these creatures have an incredibly wide field of vision, approximately 320 to 340 degrees wide, making it hard to be snuck up upon by predators. 

In comparison, cats have a field of vision of approximately 200 degrees. Humans have even less, with their eyes being placed very close together, at 180 degrees. But because of the positioning of their eyes, the majority of grazing animals’ vision is monocular, and they tend to have poorer depth perception. According to an evolutionary perspective, this makes perfect sense. Animals near the bottom of the food chain don’t need to know exactly how far away a predator is—they simply need to be aware of the predator’s presence.

Interestingly, grazing animals are able to keep their horizontal pupils parallel to the ground by rotating their eyes up to or over 50 degrees. This allows them to continuously scan their surroundings for threats, even as they bend their head to eat. Another benefit of horizontal pupils is that they reduce the entry of light from above and below the animal, protecting their eyes from the glare of the sun.

Fig 5: Photo displaying a cow’s ability to keep their pupils parallel to the ground, by Martin Banks, UC Berkeley

Round Pupils

And finally, back to us. Why do humans have round pupils? Round pupils can be found in larger predators whose field of vision is higher from the ground, such as lions, tigers, and wolves, or occasionally in smaller animals, such as raccoons. 

Unlike ambush predators who have vertical slits, animals with round pupils tend to chase their prey or be active foragers. This aligns with humanity’s own history, as a previously hunter-gatherer society.

Because the majority of our sight is binocular vision, humans have fairly accurate depth perception from two front-facing eyes. Unlike slitted pupils with two extra muscles, we cannot adapt as well to sudden changes in light, and are often dazzled by the sun’s bright glare. But because the shape of round pupils equally gathers light from all directions, there is no special orientation of the pupil, meaning that humans have an even focus across our entire vision, rather than just across the y or x-axis. This allows us to survey our surroundings with clarity and keep a wider range of objects in our focus at once, such as scanning surroundings, whether it be searching for a specific type of plant in the forest or tracking moving prey, even when they are not in the centre of the vision. Round pupils are simple, yet versatile.

Unique Cases

Now we are starting to move on to the truly wacky, yet amazing, features evolution has managed to come up with.

Crescent pupils are typically found in sea animals, such as sting rays, catfish, and flatfish.

The elongated shape of the pupil allows for a wider visual field, not dissimilar to horizontal pupils. However, it also reduces the effects of light refraction underwater, allowing animals to see with more accuracy, giving them an advantage over their circular-eyed prey.

However, it gets weirder! The cuttlefish have uniquely shaped pupils that are believed to have similar effects to the crescent pupil. In low light, these pupils appear circular, but take on the unique shape of a 'W' when in bright light, which allows for light from further angles to enter the eye. Furthermore, its curved shape reduces light more so from the upper field of view than it does to the lower field of view, helping them see in the dark and evening out the light gradient from the lack of sunlight underwater.

Fig. 6: A close-up picture of a cuttlefish’s eye. Source: National Shell Museum and Aquarium

Let’s move back to land animals. Geckos have vertically slitted pupils, with a string of dotted circles that appear in high-intensity light. Remember how humans have binocular vision to increase their depth perception? These several pin-like holes increase the gecko's depth perception through a similar concept. This helps them judge the distance between them and their fast-moving prey, without having to move, which may alert or startle their target.

Fig. 7: A close-up picture of a gecko’s eye. Source: asknature. 

Conclusion

Fig. 8: Two images depicting the contraction and dilation of a cat’s pupils. Source: Earth Unreal (2022), Facebook. 

• Slit pupils allow the eye to expand and contract more than a round eye, allowing animals to see better in the dark and not flinch against bright light, and result in a stronger blur gradient.

• Vertical pupils allow for better judgment of depth and distance. It is more common in predators to better hunt animals.

• Horizontal pupils allow for a panoramic vision and reduce glare from the sun.

• Circular pupils allow for an equal intake of light from all angles, providing a broader field of focused view, acting as a general all-rounder.

Closing Note

These are just a few of the many types of pupils animals have. Some frogs have curious heart-shaped pupils that researchers don’t fully understand, and the compound eyes of invertebrates have dark spots called pseudopupils.

So, have we gotten the short end of the stick? Not quite! We may not have a 340-degree field of view like animals with horizontal pupils, nor do we have crescent-shaped pupils to help see underwater, but what we do have is a pretty solid grasp of a little bit of everything. We have a wider area of focus than animals with slitted pupils, decent low-light vision, better depth perception than grazing animals with monocular vision, and many more features when you consider factors outside of pupils and eye placement. These are all traits that humanity has developed through evolution to fit our ecological niche.

So while round pupils may seem boring, there is so much going on inside our eyes, and a very good reason for our circular eyes.

Reference List

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Science ABC. (2015). Why do animals have such differently-shaped eye-pupils? [online] Available at: https://www.scienceabc.com/nature/is-there-an-evolutionary-advantage-to-different-shapes-of-eye-pupils.html.

TED-Ed (2022). Why do cats have vertical pupils? - Emma Bryce. YouTube. Available at: https://www.youtube.com/watch?v=jdrNjHPYKz4 [Accessed 14 Feb. 2023].

Wagh, M. (2022). Scientists Have Determined the Reasons Behind 3 of the Most Common Types of Animal Pupils. [online] Popular Mechanics. Available at: https://www.popularmechanics.com/science/animals/a39773523/animal-pupil-shapes/.