Why Do Animals Have Differently Shaped Pupils?

Why do cats have vertical-slit pupils and humans round ones? Let's find out.

Why Do Animals Have Differently Shaped Pupils?
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You might have noticed that most animals' eyes are very different from our own. From vertically slit to horizontally w-shaped, there is a massive variety of them around.

But why? How many different types are there and for what purpose?

Let's find out.

RELATED: 7 FASCINATING ANIMALS YOU'VE PROBABLY NEVER HEARD OF

What animals have the best eyes?

There are many animals that can claim that they have the best eyes in the world. But this is a relevant term as it depends on what the animal needs its eyes for.

But, according to sites like India Today, some of the best animal eyes in the world are as follows: 

  • Eagles and Falcons (widely considered the best)
  • Birds of Prey
  • Owls
  • Cats
  • Prosimians
  • Dragonflies 
  • Goats
  • Chameleons 
  • Mantis Shrimp

What animal has weird eyes?

There are many strange eyes to be found in the animal kingdom. But this is, of course, a very human-centric concept.

After all, to a snake (if they have a concept of weird), the human eye would look strange. However, from our perspective, there are some very strange eyes to be found around the world.

But, let's take the chameleon as an example: 

"Chameleons have very unique eyes — some of the most unique in the entire animal kingdom. Their eyelids are joined in a circle around most of the eye, leaving only a pinhole for them to see through. A chameleon's eyes can move independently of each other, and each eye can move a full-360 degree." - The Science Explorer

Why do animals have differently shaped pupils?

The shape and form of animal pupils are directly related to how they eat, where they spend their time and, if a predator, how they hunt. Prey species will tend to prioritize great peripheral vision over depth perception and aquatic animals will have very different pupils to land-based animals. 

It really does depend on the evolutionary past of the animal in question. We'll go into it in more detail further down the article. 

"House cats have elongated pupils while their taller lion cousins have round pupils. For prey species with horizontally elongated pupils, the shape allows them to receive the most possible light from in front and behind, giving them a panoramic view of the world that helps them to detect predators." - mnn.com

Can humans have slitted pupils?

In the vast majority of cases no. We have evolved as active hunters who tend to hunt during the day, and we stand upright.

For this reason, our pupils are round like other large predatory mammals like lions and wolves. 

However, there are some rare genetic disorders where humans can develop pseudo-slitted pupils. Called coloboma, the pupil doesn't form correctly when the baby is in the womb. 

This can lead, in extreme cases, to a non-round, sort of slitted pupil. 

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What are the different kinds of animal eye pupils?

The animal kingdom is an incredibly diverse and interesting place to see many different evolutionary adaptations to various lifestyles and environments. One particularly interesting difference between animals is the shape of their pupils

Below we have highlighted 8 of the main pupil types you will find around the world. This list is not exhaustive and is in no particular order. 

1. Daytime-hunting large predators tend to have round pupils

animal eyes round pupil
Source: madaise/Flickr

Round pupils, like yours, are also pretty common in other animals like dogs and large cats. Animals with these kinds of pupils tend to be active hunters who chase down prey mostly during the day (diurnal).

They also tend to be a feature of predators whose eyes are usually held high off the ground.

However, this is not cut and dry. Large cats and dogs like lions or wolves can and do also hunt at night. This is through a combination of retinal adaptations (tapetum lucidum) and their increased stealth abilities in low-light levels. 

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If an animal has round pupils it tends to mean that their field of view is not particularly wide. As predators tend to need better depth perception, this is something of a trade-off.

However, animals with these kinds of pupils tend to be top predators with very few, if any, predators themselves. For this reason, good peripheral vision is less of an evolutionary necessity -- for obvious reasons. 

Round pupils do not provide the kind of accurate vision that some other pupil shapes have, but the sheer size and strength of their owners, a tendency to hunt in packs,  or an animal's intelligence, vastly overcome any limitations round pupils have.

In other words, they can make last-minute corrections during a hunt, or in our case, kill at a distance. 

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2. Vertical slit pupils tend to belong to ambush predators

animal eyes vertical pupils
Source: Dany Sternfeld/Flickr

Vertical slit pupils tend to belong to smaller ambush predators that hunt relatively close to the ground. These include animals like small cats, foxes, and crocodiles, to name but a few.

The shape of the pupil allows for a greater expansion of it to let more light in when needed. This is an adaption for predators who tend to hunt in a variety of light conditions including at night-time. 

For nocturnal hunters, getting as much light as possible into the eye is of paramount importance when hunting. As they tend to belong to ambush predators (either because of their small size or metabolism) they need to wait for their prey and be incredibly accurate when they strike to make a successful kill.

These kinds of pupils, therefore, are great for judging short distances very precisely. 

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3. Horizontal pupils tend to belong to prey animals

animal eyes horizontal pupil
Source: Hans De Bisschop/Flickr

Horizontal pupils, more usually oblong-shaped (sometimes called rectangular), tend to feature in prey animals like sheep, goats, and horses. These curious-looking pupils provide an excellent, wide field of view.

This, including their physical positioning on the head, provides the animals with an excellent means of scanning their surroundings for potential threats -- like predators. These kinds of pupils are also excellent for keeping the ground in sharp focus to aid the prey animal during times of flight.

"The rectangular pupil gives equines and ruminants a horizontal panoramic vision of what is happening on the ground -which is where predators could potentially be- allowing them to have a diaphanous vision from the front to the back of the visual." - Juan Pascual.

Their eye positioning and pupil shape allow prey animals to see both towards the front and back of their bodies. It also provides a relatively good frontal vision to check for obstacles when fleeing predators.

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4. Horizontal slit pupils tend to be found in prey reptiles and amphibians

animal eyes horizontal slit
Source: TJ Gehling/Flickr

Horizontal slit pupils are another pupil shape possessed by some prey-like animals. They tend to be found in reptiles and amphibians like frogs, toads, snakes, and octopus. 

This type of pupil allows the possessing animal to spot the vertical motion of predators in order to take evasive action when required.

They can also extend wide to let plenty of light into the eye during day or night. 

5. Crescent pupils tend to be found in stingrays, catfish, and flatfish

animal pupils crescent
Source: Bryce Bradford/Flickr

Commonly seen in animals like stingrays, flatfish and some catfish, crescent-shaped pupils are very strange indeed. These interesting pupils provide animals with enhanced vision by decreasing the effects of light-distortion by water. 

Some have postulated that crescent-shaped pupils help exclude light more effectively than others, especially circular pupils. 

They also provide their owners with a wide field of view to scan for predators or prey items. Studies have also shown that they can help boost contrast too.

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"We found that this pupil shape preserves a small depth of field while limiting light flux to the retina. This pupil shape will also decrease the effects of lenticular spherical aberration, provide a larger visual field, provide a higher theoretical resolution limit, enhance contrast at high spatial frequencies, and provide information on the sign and degree of visual defocus relative to a reflective object." - Christopher J. Murphy. 

6. W-shaped pupils can be found in things like cuttlefish

animal eyes w-shaped pupils
Source: neil banas/Flickr

W-shaped pupils are another interesting animal pupil shape. They tend to be found in animals like cuttlefish and are thought to be a modified form of the horizontal-split pupil

During low light levels, these pupils tend to open up to form a more circular shape but revert to their characteristic w-shape during strong light. The shape enables light to enter the eye from many different directions and is also thought to boost image contrast and distance vision. 

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7. Geckos have vertical beaded pupils

animal eyes beaded pupil
Source: James Niland/Flickr

Vertical beaded pupils can be found in animals like geckos and some fish too. These pupils can decrease to very thin vertical slits with multiple pinholes appearing in bright light. 

Each of the "beads" works together to help the animal perceive distance as well as allowing it to hunt in many different environments. 

They tend to be found in animals who are either nocturnal or active both day and night, and who also do not stand too far off the ground. 

8. Insect eyes don't actually have pupils!

animal eyes insects
Source: JamesDPhotography/Flickr

As insects tend to have compound eyes, you won't be surprised to hear that they don't actually have pupils. That being said, some do have dark spots called "pseudo-pupils". 

These are "a dark spot that moves across the compound eye of an invertebrate as it is rotated, caused by absorption and reflection of incident light by the ommatidia." - Wiktionary.com

Insects eyes, for the most part, tend to be solid rather than the fluid-filled eyes of most higher animals. 

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