Why do placental mammals dismantle their endometrium in cyclic pattern? And why the amount of blood that is lost relative to the reabsorbed amount differs between species Some species as in humans shed notable amount of blood in association with menstruation that occurs by the end of the menstrual cycle, whereas, other species barely discharge any blood in what is known as the estrous cycle. This essay discusses such evolutionary processes by presenting previous hypotheses and presents a new hypothesis in an attempt to give more insight into the evolution of menstruation in mammals and in humans in particular.

Key words: Menstruation, mammals, biochemical samples, accelerating evolution, self energy loss bait.