Menstruation as a defense against pathogens transported by sperm

Area of Science:

• Reproductive Biology
• Immunology
• Evolutionary Medicine

Background:

• Sperm can transport bacteria from genitalia to the uterus during mammalian insemination.
• Pathogen colonization of the uterus poses a risk to reproductive health and fertility.
• Existing knowledge lacks a comprehensive explanation for the evolutionary purpose of menstruation.

Purpose of the Study:

• To propose and evaluate the antipathogen hypothesis of menstruation.
• To investigate the adaptive significance of menstruation in mammalian reproductive health.
• To explore the role of uterine bleeding in preventing pathogen-induced infertility.

Main Methods:

• Comparative analysis of menstruation across mammalian species, including primates, bats, and insectivores.
• Examination of the physiological mechanisms of menstruation, such as spiral artery constriction/dilation and reduced clotting factors.
• Formulation of predictions based on the antipathogen hypothesis regarding the prevalence and characteristics of menstruation.

Main Results:

• Menstruation involves mechanical shedding of uterine tissue and delivery of immune cells, directly combating pathogens.
• Menstrual blood's low clotting factor content facilitates easy flow, enhancing its clearance function.
• Evidence suggests menstruation is widespread in mammals and its occurrence/intensity correlates with pregnancy probability and pathogen load.

Conclusions:

• Menstruation functions as a crucial antipathogen defense mechanism for the uterus and oviducts.
• The physiological adaptations of menstruation support its role in preventing uterine infections and infertility.
• Other forms of uterine bleeding (proestrous, postpartum) may also serve antipathogen functions, and their curtailment could be detrimental.