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Unlike humans (and other primates) whose ovulation is not readily apparent, nearly all placental mammals go into estrus cycle or "heat," displaying their readiness to breed (only) at specific (often annual) times, through pheromones and changes in behavior. This is in fact the key difference between menstrual and estral cycles-- in animals with estral cycles, fertility is displayed, and females will not be sexually receptive at other times. (Whereas it's concealed in animals with menstrual cycles, which are sexually receptive at all times.)
Female furries face a key choice when they are involved in sexual roleplay, between behaviors characteristic of animals that go through either menstrual or estral cycles. A female may roleplay being "in heat" to initiate a sexual roleplay that involves breeding and, in some cases, beginning a pregnancy. There are many female furs, however, that consciously roleplay a (humanlike) menstrual cycle. Others may make no specific note of it either way. Others still may prefer behavior intermediary between the two.