Why don’t any animals have menstrual cycles?


Introduction: The Absence of Menstruation in the Animal Kingdom

Menstruation, the shedding of the uterine lining in females of certain mammalian species, is a unique biological phenomenon. However, despite its prevalence and importance in human reproduction, it is absent in the animal kingdom beyond primates. This raises the question: Why don’t any animals have menstrual cycles? This article will explore the evolutionary and adaptive reasons for the lack of menstruation in non-primate species and the possible role of hormonal mechanisms, embryonic implantation, and environmental factors in its evolution.

The Difference between Menstruation and Estrous Cycles

Before exploring the evolutionary factors behind the absence of menstruation in non-primate animals, it is essential to differentiate between menstruation and estrous cycles. In menstruating species, the uterine lining is shed regardless of whether fertilization occurs. This results in the discharge of blood and other cellular debris from the vagina. In contrast, estrous cycles result in the release of a fertile egg and the thickening of the uterine lining only if fertilization occurs. If fertilization does not occur, the lining is reabsorbed or expelled. This fundamental difference between menstruation and estrous cycles has important implications for understanding why menstruation is absent in non-primate animals.

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