system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why female animals don't have periods?
gpt
Most female animals don't have menstrual cycles. Commonly, most female mammals have an estrogeous cycle. That means that they absorb the entire endometrium if there is no conception. Whereas humans (and other primates, elephant shrews and certain kinds of bat) shed this layer.