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The answer I get depends on who I talk to. One group says cats and snakes have oval pupils to reduce the light that gets in during the day. Another group says that there's no definite reason.
"Both snakes and cats are primarily night hunters," says Professor Neils Pedersen of the School of Veterinary Medicine at the University of California, Davis. Consequently, they have sensitive retinas. According to Pedersen, the oval pupil shape protects the retina during the daytime. A cat can close a slit more completely than a round pupil and that is why the shape is oval.
"Because that is the way God made them," counters Robert J. Munger, DVM, DACVO at the Animal Ophthalmology Clinic in Dallas. Tigers have round pupils and domestic cats, oval ones. Both "function just fine so there is no obvious reason. Everything else is just speculation..."
Back in 1916, R.I. Pocock of the London Zoo took a look at cats. He found that tigers, lions, jaguars, and leopards have round pupils. Whereas, domestic cats and most little cats have oval ones. In fact, he divided the cat population into two groups: based, not on pupil shape, but rather on some small bones (the hyoid apparatus) at the base of the cat's tongue. It turns out that all cats with a well-developed hyoid can purr but do not roar and they have oval pupils. Those cats, however, whose hyoid fails to develop properly--and this includes all the big cats--can not purr, do roar, and their pupils are round.
The big cats have a hyoid in which one 'bone' fails to develop into a bone. It remains a thin thread of ligament; hence, the tongue and the larynx are loosely attached to the base of the skull.
Like cats, some reptiles have slit pupils, others round. Crocodiles, pythons, vipers, tuataras, and geckos are active at night but spend time basking in the sun. Their pupils are usually vertical slits. But not always.
I imagine slit pupils do help cats and snakes hunt in the bright glare of day. However, I suspect the advantage is small--too small for natural selection to account for an oval shape.