Amazing Nature

Incredible Animals' Eyes

Originally published in Green Country, Issue 132 (Jun 2018)
Author: Green Power
Visual angle of different animals
Visual angle of different animals

Humans have front-facing eyes that allow us to see forward and to the sides, but not backwards unless we turn our head and body. Things in the animal world are more complex and interesting. Some animals even have 360-degree vision. The different visual fields are not just for fun; they may sometime hold the key to life and death for the animals!

There are two concepts concerning vision we should first clarify: the visual angle and stereoscopic vision. Visual angle indicates the scope of area to be seen. The larger the visual angle, the wider area can be perceived. Stereoscopic vision, on the other hand, helps to detect the size, depth and distance of the objects to be seen. Humans, for instance, have two eyes in front with a visual angle of 210 degrees. The two eyes, being slightly apart, receive similar but different images on the retina as light enters the eyes. When the image is transferred through the optic nerve to the brain, stereoscopic vision is formed, allowing us to accurately judge the location and size of the object. Humans have stereoscopic vision spanning about 120 degrees.

Side view of Eurasian Woodcock Scolopax rusticola
"Eurasian Woodcock" by Jason Thompson is licensed under CC BY 2.0

Front view of Eurasian Woodcock Scolopax rusticola
Eurasian Woodcock (Scolopax rusticola) has eyes on the rear sides of its head, and has a wider field of view than other birds."Eurasian Woodcock" byJason Thompson is licensed under CC BY 2.0

Dynamics between predators and prey

Some animals (mostly prey animals) have eyes on both sides of the head. The wide visual field allows them to spot approaching predators and flee life threatening situations. Horses, for example, have 350 degrees of visual angle, while goats and sheep have visual angles covering 320 to 340 degrees. Birds in the Woodcock (Scolopax spp.) family have almost panoramic field of vision, with their eyes growing closer to the rear than other birds. They can easily see behind them! Nonetheless, the animals like horses have limited stereoscopic vision, spanning about 20 to 65 degrees. Woodcocks even have less than 10 degrees of stereoscopic vision.

Close-up of a primate
Primates also  have extraordinary stereoscopic vision that enables them to perform delicate  actions.

For other animals (mostly raptors and carnivorous mammals), their frontal eye placement creates a narrow visual angle. Yet it also helps the formation of stereoscopic vision with overlapping visual fields. The improved accuracy in judging distance and location provides advantages in hunting. Cats have some 200 degrees of visual angle and 130 degrees of stereoscopic vision. Primates also have extraordinary stereoscopic vision, which enables them to perform delicate actions such as picking fruits and picking up twigs.

Close-up of an owl
Owls are perfect  hunters as they excel in both stereoscopic vision and visual field, with the  ability to turn their heads to a large degree.

Rectangular pupils

Some animals can turn their heads or eyes for a broader field of vision. Owls and tarsiers, for example, have narrow angles of vision but their heads can turn 270 degrees and 180 degrees, respectively, which allows them a wide view. Chameleons have protruding eyes which can rotate independently, so that they have a full 360-degree field of vision.

Close-up of a sheep's pupil and its pupil is rectangular
Sheep have rectangular pupils for a horizontal peripheral field of vision that helps with escaping predators.
Photo from Pixabay

A study in 2015 showed that grazing animals such as cows, goats and horses have rectangular pupils that are narrow and horizontally orientated. This gives them a spectacular field of view to keep alert to predators as they graze out in the open. They can also easily view a larger area and increase their chance of survival as they run away from predators. Interestingly, even when grazing, as they lower their head their eyes will adjust and keep their pupils in the horizontal orientation!

A horse's pupil remains level as it moves its head
As grazing  animals lower their heads, they can keep their pupils in a horizontal  orientation.