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Few people have spent time gazing into a sheep’s eyes, but if you have, you may have noticed something very strange about their pupils. Instead of being round, as is the case with humans, they are rectangular. You get a very clear view of this in the above video. The sheep have a dark, horizontal rectangle in the middle of their eyes and this is their pupil. Here we will examine why sheep have rectangular pupils and how this impacts their vision.
Sheep Habitats and Lifestyle

Domestic sheep are descendants of mouflon.
©Jesus Noguera photography/Shutterstock.com
To understand why sheep have rectangular pupils we first need to consider the environments in which they live and the challenges they face. Sheep are now one of the most economically important domestic animals, and most live in man-made enclosures of some kind where they have a degree of protection from the elements and predators. This has not always been the case.
The animals that we now call sheep were domesticated around 10,500 years ago from wild Asiatic mouflon. These wild sheep grazed an area called the Fertile Crescent, which is also called the “the cradle of civilization.” It is a crescent-shaped area of land in Western Asia and North Africa. Here, they had to navigate their way across rough, open terrain and were at the mercy of predators. So, any adaptation that allowed them to safely move around this landscape and see predators coming from a long way off would have been an advantage. Their eyesight was key to their survival.
Pupil Purpose and Shape

Pupils are different shapes to suit the needs of the animal.
©Kevin Wells Photography/Shutterstock.com
The pupils of the sheep in this clip appear as dark, horizontal rectangles. They are an opening in the center of the iris (the colored part of the eye). The role of the pupil is to let light in and onto the retina at the back of the eye where the light-sensitive cells are located.
Humans, other primates, and dogs have round (spherical) pupils, which are useful for general all-round vision. Cats, snakes, crocodiles, and alligators have vertical slits which are useful for night vision.
Sheep, goats, toads, octopuses, and cuttlefish have horizontal rectangular pupils.
How Rectangular Pupils Impact Sheep’s Eyesight

Sheep need to scan the environment for predators.
©raymond orton/Shutterstock.com
As you can see in the above clip, the sheep move away from someone approaching them from the side. Their rectangular pupils together with the position of their eyes on the side of their skull, give them excellent peripheral fields of monocular vision. This means that they can see two objects at once – approaching them from different directions. Their field of vision is between 270 to 320 degrees – their only blindspot is directly behind their head.
Furthermore, the narrower the pupils in relation to the horizon, the better is the depth perception. This is important when you need to figure out how far away a predator is and calculate if you need to run away immediately or if you have time to grab one last mouthful of grass!
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