Cats have vertical pupil, horses have horizontal pupil.
In the animal kindom, s-shaped, w-shaped and "beads on a string" shaped pupils also occur, why?

Hi Peter,

the pupil allows light into the eye, therefore the shape plays an important role in how images are received for processing by the brain - imagine them as you would a camera aperture. Pupils that can open very wide are able to let in lots of light (useful in the dark), small pupils let in much less light, but offer greater depth perception (useful for animals that need to pinpoint the location of a target - be it food or a position). Round pupils allow reconstruction of an image with minimum distortion (useful when navigating through a complex 3 dimensional space), slitted pupils impose a movement or pattern filter (useful for spotting predators or prey).

All of the various pupil patterns seen in animals are a response to the way in which they need to perceive the world around them. The pupil shapes have given them an advantage over other pupil shapes that are possible. If you combine factors you start to see why pupils are the shape they are. Apes are an example of animals with round pupils that aren't that big. This is so they can accurately see the 3 dimensional space around them and assess the distances needed to be travelled within it. Cat's vertical eyeslits can open very wide to allow activity at night, can close small for daylight activity and they probably filter signals to give importance to those objects passing across their field of view. Horse pupils are horizontally slitted, which may allow better recognition of obstacles as they move into sight at speed (so effectively in the vertical plane).

Those are some of the theories explaining pupil shape - they give us a useful mental tool box for addressing these questions, but it is very difficult to be certain that there are not other issues, such as limitations on developmental pathways in the evolution of eyes, so this is a far from exhaustive answer!