You may have noticed that some animals like goats and horses have horizontal pupils, while humans have round pupils. Why is this? Just a fluke of evolution? Not so – there is an important survival strategy behind this type of pupil development in prey animals. Read on to discover why.
Pupils
First, let’s cover what a pupil is and what it does. The pupil of the eye is actually a hole in the eye tissue known as the iris, which is the colored part of the eye (blue, hazel, and so on).
The pupil allows light to enter the inner eye and can shrink or expand to control the amount of light that enters. This visual data that enters through the pupil is then sent to the brain for analysis which creates our sense of sight.
Predators and Prey
Now that we’re clear on the function of the pupil, we’re halfway to the answer. Prey animals, such as horses, have pupils that are shaped horizontally. Predator animals, such as lions, have pupils that are either vertical or round.
Now, think about where exactly on the heads of these two animals the eyes are placed. On predators like lions, the eyes are placed forward-facing on the head because this produces what is known as binocular vision and depth perception, handy things for judging distances (see: catching your next meal).
This causes a smaller field of view for the predator, though. Humans, predators with forward-set eyes, have about 180 degrees of vision in our field of view. On prey animals, namely hoofed mammals, the eyes are placed towards the sides of the head. This gives poor depth perception, but when you eat only plants, that doesn’t tend to matter much.
What it does, however, is give the prey animal excellent field of view around its body, allowing it to easily scan its environment for danger. The horizontal pupils only add to this effect, creating a panoramic vista around the animal. Horses and their relatives, like zebras, have as much as 350 degrees of vision in their field of view!
To imagine this, think of watching a widescreen movie on a bigscreen tv. Now take that range of vision and mentally wrap it around your body to where the image goes almost all the way around you. This incredible scope is one of the things that makes for a successful prey species. Just look at the diversity of hoofed mammals in Africa alone and you start to understand the success and value of the modern prey animal’s eyes.