As might be expected this is all about evolution and survival. The fundamental reason why the domestic cat’s iris constricts the pupil to a slit as opposed to an oval – as occurs for the tiger – is that it makes survival easier as a small, crepuscular, ambush predator. The infographic explains this more succinctly.
Note: Some older videos on this page were hosted on Vimeo. That account has now been retired, so a few video blocks may appear blank. Thanks for understanding — there’s still plenty of cat content to enjoy!