There are two possible answers to this question. Firstly, the game operates a system that monitors your stay on the island during this particular save. As the day progresses, the game implements different landmarks based on the days survived. These landmarks continue until day 40. At this point, the player will experience more cannibal attacks and visits from unique mutants. Day 40, particularly, adds in the mutant named John, a worm-type mutant who can spawn daily at random past day 40. Secondly, the game also monitors how much the player changes or messes with the default structure of the island. The more you build and cut down trees, the more cannibals will be attracted to the player's location.