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There's a few other general differences between estrus cycles and menstrual cycles. At ovulation, animals with an estrus cycle go into estrus, also known as "heat". During estrus, the animal is sexually receptive, and it usually isn't outside of estrus. Estrus cycles also frequently include a dormant phase, where the animal doesn't have a hormone cycle at all. And finally, estrus cycles tend to happen less frequently than menstruation, sometimes as infrequently as once per year.