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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 eye shapes.

But why? How many different types are there and what purposes do these different shapes serve?

Let's find out.

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

What animals have the best vision?

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

According to 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 animals have 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.

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 the pupils in animal eyes 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 peripheral vision over depth perception, and aquatic animals will have very different pupils to land-based animals. 

It also depends on the evolutionary past of the animal in question. We'll go into this 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, such as 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. 

What are the different kinds of animal eye pupils?

The animal kingdom is an incredibly diverse and interesting place where we can see many different evolutionary adaptations to diverse 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 those of humans, are also common in other predators such as 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 usually tend to be a feature of predators whose eyes are usually held high off the ground.

However, this is not always true. Large cats and dogs, such as lions or wolves, can and do also hunt at night. They are able to do this through a combination of retinal adaptations (tapetum lucidum) which  give them increased stealth abilities in low-light levels. 

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 apex predators who face 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 this is made up for by the size and strength of their owners, a tendency to hunt in packs, or an animal's intelligence, which all help to overcome any limitations from round pupils.

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

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, among others.

The shape of the pupil allows for a greater expansion 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. This pupil shape also aids in judging short distances very precisely. 

For nocturnal hunters, getting as much light as possible into the eye is of paramount importance. This type of eye shape is also used by ambush predators (who tend to be smaller in size or have a slower metabolism) who need to wait for their prey and be incredibly accurate when they strike, in order to make a successful kill.

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 side of the head, provides the animals with an excellent means of scanning their surroundings for potential threats and 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.

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 such as frogs, toads, snakes, and octopi. 

They can be opened wide, to let plenty of light into the eye during day or night. This aids the allows the animal in more easily spotting the vertical motion of predators in order to take evasive action when required.

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 provide enhanced vision by decreasing the effects of light-distortion by water. 

Some have postulated that crescent-shaped pupils help exclude light more effectively than other shapes, 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.

"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 cuttlefish

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

W-shaped pupils are another interesting animal pupil shape. They are found in 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 in 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. 

7. Geckos have vertical beaded pupils

animal eyes beaded pupil
Source: James Niland/Flickr

Vertical beaded pupils can be found in animals such as geckos and some fish. 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. 

This pupil shape tends 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. Their compound eyes are made up of thousands of photoreceptor units, called ommatidia, that are packed close together. Each photoreceptor unit includes a cornealens, and photoreceptor cells which distinguish brightness and color.

However, some insect eyes do have "pseudo-pupils". These pseudo-pupils 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

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