A new study may explain why animals have the pupils they have. There are several different shapes of pupils in the animal kingdom. Predatory animals such as cats, for example, the vertical splits for pupils while prey have horizontal, rectangular shaped pupils. Humans on the other hand have round pupils.
Martin Banks from University of California, Berkley and his colleagues investigated eye shape in predatory and prey animals to try and find out what advantage these eyes have for different animals. The team examined the pupil shape and feeding habits of 214 land based animals. They found a strong correlation between predatory animals and vertical pupil shape.
This shape likely evolved in predators so they could better gauge depth. The depth of an object can be assessed by the brain in a process of stereopsis. The brain can estimate the depth of an object by comparing the distance between the image as it meets each eye. A vertical pupil is actually better for estimating depth using stereopsis so this may explain why predators have vertical pupils.
This explains why cats have vertical pupils. While most household cats are domesticated and don’t regularity hunt, their ancestors were certainly predators so this pupil shape would have been left over as cats became pets.
Animals of prey have a completely different eye shape though. This is because they use their eyes for scanning the horizon for predators says Banks. 36 or 42 prey animals had these rectangular shaped pupils.
Using a computer simulation or an eye based off that of a sheep, Banks and his team found that this shape minimizes light input from above and below. This creates a panoramic type of view and stops light from above that could distract the animal.
Another fascinating find was that some animals of prey such as sheep and goats can rotate their eyes individually so one can be focused on the horizon at all times monitoring it for predators.
Finally there are human eyes, which are circular. This may be due to fact that humans are active foragers and hunters during the night and day but it is still unclear.
Evolution has created all of these eye shapes to fit specific lifestyles of various animals. Banks and his colleagues also investigated whether these eye shapes evolved independently or share a common origin. For example, the slit eyes in cats. Does this shape date back to one origin or did it arise several times? By creating evolutionary trees the researchers found that specific shapes arose several times in each family.