People ask “Why do domestic cats have slitted eyes?” Here are the reasons.
The reasons are (a) they can use the closing of the eyelid to reduce even further the light entering the eye beyond simply closing the aperture of the pupil and (b) to maximise blur behind and in front of the object focused upon to improve horizontal distances and make hunting more efficient and to improve stereoscopic vision.
A – If you envisage the cat’s eyelid coming down over the closed slit of the cat’s pupil you can see how the amount of light entering the eye and impinging on the retina is reduced even further than by simply closing the pupil. The cat needs this facility as their eyes are super sensitive to see at night. The domestic cat’s wild cat ancestor, the N.African wild cat, exclusively hunts at night and the domestic cat has inherited the eyes of this wild cat although the domestic cat has evolved to hunt at any time but primarily at dawn and dusk. Dr Morris says that lion’s circular pupil is evidence of the fact that the lion is primarily a daytime hunter and therefore their eyes are less sensitive to light. Although the lion does also hunt at night (Sunquist 2002).
B – Martin Banks at the University of California (on livescience.com) did some research on this. He believes that the slitted pupil allows better judgement of horizontal distances because it maximises the blur behind and in front of the target object upon which the eye is focused. The vertical slit also maximises stereoscopic vision he says. Both these aspects of the cat’s vision assists in catching prey by permitting better depth judgement and accurate targeting. This is another reason why the domestic cat is such a fabulous hunter. An animal designed to hunt and survive. Everything to do with the domestic cat is built around that.