Why Most Animals Don't Have Periods

Periods are a necessary part of human reproduction, but most other animals don't have them. If they don't need them, why do we?

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Why Do Women Have Periods When Most Animals Don't? (BBC)
"Most women menstruate. But most other female animals don't bleed outwardly like us. Even among those that give birth to live young as we do, only a handful of species menstruate.
"So menstruation isn't just inconvenient and unpleasant: it's also a mystery. Why do we menstruate at all? And if it's such a good idea, why don't other animals do it?"

The Evolution of Human Reproduction: A Primatological Perspective (Wiley Online Library)
"Successful reconstruction of any aspect of human evolution ideally requires broad-based comparisons with other primates, as recognition of general principles provides a more reliable foundation for inference. Indeed, in many cases it is necessary to conduct comparisons with other placental mammals to test interpretations."

The Evolution of Endometrial Cycles and Menstruation (University of Chicago Press Journals)
"According to a recent hypothesis, menstruation evolved to protect the uterus oviducts from sperm-borne pathogens by dislodging infected endometrial tissue and delivering immune cells to the uterine cavity...I propose the alternative hypothesis that the uterine endometrium is shed/resorbed whenever implantation fails because cyclical regression and renewal is energetically less costly than maintaining the endometrium in the metabolically active state required for implantation."