Menstruation – the regular renewal ofthe lining of the uterus – is a curiousphenomenon. Most mammals do notmenstruate, so why do humans, higherprimates, certain bats and the elephantshrew?Menstruation evolved relativelyrecently in each of these three mammalian groups. There have been many hypotheses to explain it. Cyclical regressionand renewal may be energetically lesscostly than maintaining the endometrium in a metabolically active stateneeded for implantation. Other theoriessuggested that menstruation evolved toprevent sperm-borne pathogens infectingthe endometrium. However, in this issueof BioEssays, Emera et al. [1] presentarguments that menstruation is aninevitable consequence of spontaneousdecidualisation.
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