Q. Why do some domestic cats have vertical pupils and other domestic cats have round pupils?
A. All domestic cats have vertical pupil slits. You may be seeing these cats in different contexts and lighting sources which causes the variation in their pupil appearance. In bright light, all domestic cats will have thin vertical pupil slits, unless they are fearful, stressed, anxious, etc. or there is an optic problem. Sympathetic nervous system input causes the pupils to dilate when a cat is experiencing certain emotional states.