How does having a non-circular pupil affect the way an animal sees?



How does having a non-circular pupil affect the way an animal sees?

In: Biology

The pupil of an animal is almost always directly related to their place in the food chain and how they hunt/flee.

If you take a domestic cat for example, they tend to sneak up on their dinner then strike! Their long vertical pupils and ability to dialate their eyes help them see in the dark but also helps to better measure distance.

The horizontal pupils on pray animals is very interesting however! When you look at goats or sheep their eyes are on the sides of their heads as they need to be watching around themselves at all times. The position and horizontal views give them a near panoramic view of their surroundings. When a predator is spotted they must be able to clearly see forward those their eyes are positioned to the side of their head. With their unique pupil shape they can focus ahead and get a very good gauge on distance for an effective escape! This would be like turning off a wide angled view on your phone camera to better focus on the closest items to youA (!)


I got lots of questions I did not know the answer to and have spent my morning reading to try to find some answers! I will add I am in no way a proffesional, I just have awful eyesight as was always fascinated by this.

Vertical Slit Pupils – This is common on ambush predators and specifically those low to the ground. Think foxes, cats, snakes and crocodiles. The view from a vertical pupil is going to be elongated vertically with less coming in from the horizon, they also have an ability to dialate more then a circular pupil allowing better sight in low light conditions. This is believed to be seen on predators that are low to the ground because they have a naturally shorter line of sight, and it works to increase depth perception for gauging distance better. Ambush preditors depend on catching their prey quickly, as they are not nearly as adept at a chase.

Circular pupils – We have the most experience with this, as it seen on humans, canines and big cats. Predators that are not low to the ground have a naturally further line of sight, taking away the need to have virtical pupils. This allows a mix of the horizontal and vertical view, but does not seem to have any specialized niche beyond the different level of dilation and thus sight in low light across species. As nature says, if it ain’t broke breed it.

Horizontal pupils – This is common in large prey animals, think goats, horses and moose. This is going to elongate the view of the horizon, shortening the vertical view but allowing for more sight around the body. This is also seen on animals whose eyes are placed to the side of their heads, which still allows them accurately see forward facing, which is vital for running away. It is also believed this gives a steady view of the horizon both when the head is low to graze and the when the head is upright. Another fun fact, most all animals for horizontal pupils have special combs in their eyes to limit light coming from above, stopping the sun from blurring their view of the horizon. The best way to sneak up on a goat is indeed a parachute!

W shaped pupils – Cuttlefish have this specifically as a F*** You to all the research done. It is widely believed to be a cosmic prank, specifically designed to give marine biologists headaches. Not a lot of research has been done, but it appears the W shape allows for better sight in the low light conditions of the Cuttlefishes habitat. Simulations of the eyesight within their habitat shows increased depth perception of shadows allowing for better navigation. It is also theorized to aid in their camouflage.

TLDR and overly simplified version:

Vertical Slit – Like a phone screen

Circular pupil – A standard screen TV (as in a square for all you youngins)

Horizontal Pupils – A widescreen TV

W Shaped Pupils – A bad mushroom trip

The iris shape affects the degree of focus similar to a radar antenna’s shape.

Air Traffic Control radar uses a [wide antenna]( which generates a vertical fan shaped beam. Anything in a exact direction but at whatever altitude can be detected. The Vertical resolution shows the exact direction. The Horizontal (altitude) direction isn’t as important.

A Height Finding radar uses the opposite – a [tall, narrow antenna]( The fan beam sweeps up and down to detect the exact altitude of the target, which is somewhere in that direction.

A circular antenna generates a narrow cone beam, like in weather and aircraft missile tracking – it gives the exact vertical and horizontal position.

The Predator’s vertical slit eye will have higher resolution vertically, to better resolve the exact elevation of a target before leaping. The Prey’s horizontal slit eye will have higher resolution horizontally to detect the direction of a threat in the grass / bush / tree / sky to run away from.

They’re all trade offs to detect things. Note that the predator’s vertical slit tends to go round (better vertical and horizontal) when it prepares for an attack.



The way I understand it is like this: herd animals like goats etc have square pupils, because they need a wider range of view. The need to see what predators could threaten them.

Predators have vertical slit like pupils because it helps control the amount of light that enters the eye. That iris configuration can allow way more light into the eye, which helps these animals be nocturnal.

Animals with round pupils, like humans, need good vision at a range that is moderately far to close up. They need to avoid things like diffraction, and they don’t need to have the ability to allow a huge amount of light into the eye, because they are diurnal.

Small predatory animals, like lizards, cats, and foxes, have slit pupils to see high and low for hunting. Mid sized, vegetarian animals that live on the plains have horizontal pupils on the sides of their heads to keep watch, such as goats and horses. Octopuses are the exception, being predatory and underwater. Most other animals have round pupils.

The shape of cuttlefish eyes may also help them “filter” light coming down through the water column to give the light reflecting off objects in front of them. I will try and find the article I read when not on the clock.

Edit: source

It’s six years old so entirely possible there is a newer understanding

In elementary school we had a boy named Carlos, he had reddish hair and freckles skin. He also had eyes shaped like cat eyes. Some mean Kids called him “Carlos the cat eyed kid”. But I always wanted to ask him how the world looked, if he saw differently than me. I wonder how Carlos is today. I bet he is super handsome with his striking cat eyes,?which were actually super cool looking.

horses for example have slit pupils that provides them with natural polarizing filter which helps to spot predators in high grassland. downsite is that those eyes filter out any reflections on water. that’s why even shallow water puddles look like bottomless holes for them.

One of my cats has the vertical pupil and the other doesnt, why would that be?

I’ve wondered this as well, do they have anamorphic vision?

Argh this post funny but where the answer at…goats eyes freak me right out, if I know the reason I may hate them less. Poor loves.

Predator animals (meat eaters) don’t run away as much, they chase! So when you’re chasing you need really good sight from far away. Like a hawk! This is why the pupils in their eyes are vertical. They can focus in (push hands together side to side to show the pupil being squished)

Plant-eaters, sometimes called prey animals need to see wide spaces. They never know where the meat eaters will come from to try to eat them! So the pupil for them is squished down like this (push hands top to bottom to show pupil being squished) so they can see all around them a lot better.

You and I, we are humans. See we have a circle pupil (draw a circle with finger in the air) because we are both a meat eater, and a plant eater. We need both to survive! So when we are running away, we have good sight of all around us, up and down. But when we are chasing animals or hunting, we also need a very good sight from far to be able to see the animal we want to catch.

It doesn’t really. You can make an Iris of any shape and if it’s properly focused you won’t notice. The bokeh of unfocused light will take the shape of the iris however.