Why do sheep and goats have irises that are elongated instead of round, like other animals?

Question

What is the evolutionary advantage for that?

Answer 1

I don't know the advantage but they aren't the only ones. lots of animals have varying shapes of pupils( the black part is the pupil not the iris)

cuttle fish have pupils that look like W's and have very good eye sight.

Answer 2

There are two different types of slit pupils - vertical and horizontal.  Sheep, goats, horses have horizontal.  The species that have vertical ones are predators like cats, foxes, and snakes.

A study was done in 1907 by a guy named William Abbott where he made artificial slit pupils for humans and it turned out that people had crisper vision by using a slit pupil that was perpendicular to their field of view plane.  

So, for predators that hunt small prey, they can potentially gain a sight advantage by having vertical slit pupils.  But, why do some herbivores have horizontal slit pupils?  Perhaps it gave them an advantage in avoiding predation by being able to see better in vertical rocky terrain (like bighorn sheep)?