Eyeball Evolution

It was a Sunday afternoon, and I found myself sitting at my desk attempting to draw a realistic house cat. First, I sketched out a small mouse. Behind it, I started to draw a large, menacing house cat, clearly ready to pounce. Afterwards, I began to work on the details of the cat’s face. I found a picture of a cat as a reference and I noticed something — the pupils of the cat’s eyes were vertical slits. I thought this was peculiar because I knew that lions, tigers, cheetahs, and many other wild cats had circular eyeballs, similar to humans. But the house cat was different. Why?


Research published by Martin Banks and his coworkers on August 7th, 2015, reveals why some animals have different shaped pupils. House cats are not the only example. Goats have horizontal, rectangular pupils and some animals barely have pupils at all. But what governs these differences? Why are goat pupils so different from cat pupils?

In order to answer these questions, Banks chose 214 different land species and analyzed their pupil shapes and activities (e.g. their eating habits, hunting techniques, sleeping patterns, etc). After analysis, Banks and his team separated the animals into different categories according to their eye shape: vertical, horizontal, and circular.

“We restricted ourselves to just pupils that are elongated or not. […] So they’re either vertical, horizontal, or round.” Banks says.


Vertical Pupils:

Throughout the animals that had vertical pupils, it was clear that they were predators that were active during the day and night. Vertical slit pupil animals were mostly ambush predators, such as cats and snakes.

Since almost all animals have two eyes, the brain needs to merge the two images created by each eye into one clear, rendered picture. Stereopsis is the method in which the brain utilizes the distance between these two pictures to judge depth. If you place an object in front of your head, close one eye, and afterwards, open the same eye while closing the other eye, you will notice a change in distance. The farther away the object is from your head, the less the object moves.

In the following diagram, you can see that object 1 is closer to the the two eyes than object 2; however, the distance between the two viewpoints of object 1 is greater than the distance in object 2.

Stereopsis Diagram

Because gauging distance is so important for ambush predators when they pounce, animals like cats have vertical pupils, for they are the best shape for stereopsis.


Horizontal Pupils:

On the contrary, animals who are grazing prey, such as sheep, goats, and zebra, have horizontal pupils. Banks saw that 86% of the herbivores he observed had horizontal pupils. He explained that grazing animals needed to expand their field of vision in order to watch out for predators. Horizontal pupils help many prey animals form a panoramic view. Keep in mind that these animals also have their eyes on the sides of their heads, so their field of vision is extremely elongated.

Further proving Bank’s point, Banks and his team saw that the eyes of most grazing animals rotate independently from the head of the animal.

For example, take a look at this sheep’s eye in the following image. Notice how the pupil, as explained before, is a horizontal, rectangular shape.

Sheep Eye rotation 2
Source: Live Science

Now, look at the pupil on this sheep’s eye in the next photo. We can see that when the sheep leans down to feed, the pupil remains parallel to the ground — it does not rotate with the head.

Sheep Eye rotation
Source: Live Science

This phenomenon allows the sheep to still have the same horizontally stretched view in order to look out for predators when grazing.


Circular Pupils:

Finally, there are the animals with circular eyes. As it turns out, the main difference between vertical pupil animals and circular pupil animals is that animals with vertical pupils are closer to the ground. A house cat is shorter compared to a tiger; consequently, the house cat has vertical pupils and the tiger has circular ones. A fox has vertical pupils, and a wolf does not. A snake has vertical pupils and a tyrannosaurus rex probably had circular pupils. See the pattern? Animals with circular pupils tend to chase down their prey instead of pouncing on them, and they are normally active during the day.


With your new knowledge of animal eyeballs, try to figure out which of these pupils belongs to which animal. Please comment with your answers!

The answers will be posted here: http://www.flyingpigblog.com/posts/eyeball-evolution-game-answers/

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