Tigers have eyes with round pupils, unlike domestic cats, which have stilted pupils. This is because domestic cats are nocturnal whereas tigers are crepuscular – they hunt primarily in the morning and evening. Despite not being strongly adapted to the dark, tigers’ night vision is about six times better than humans’. Most tigers have yellow eyes, but white tigers usually have blue eyes, due to the gene for blue eyes being linked to the gene for white fur. The gene for being cross-eyed, or boss-eyed, is also linked, so many white tigers have crossed blue eyes. Tigers scratch trees and use their urine to mark their territories. Their urine smells strongly of buttered popcorn. Tigers can tell the age, gender, and reproductive condition of other tigers by subtleties in the smell of urine markings.