Why do snakes have slits for pupils? Vertical pupils are an adaptation for ambush hunting. A 2010 study at sydney university found that of 127 Australian snake species, the ones that ambushed their prey by night had vertical pupils, while those that actively chased down prey in the day had round pupils. the constriction of round pupils helps distant prey stay in focus, but it also lets in less light. Vertical pupils improve vision across a wider range of light levels, and enable the animal to detect horizontal movement more effectively. it’s also possible that vertical pupils are more camouflaged, since they break up the round outline of the eye