Puberty - The arrival of the monthly “friend”. The principle sign that a girl has become sexually mature and is capable of reproduction is the onset of menstruation, a period of cyclic bleeding from the vagina. (Smith 1997, pg 217) This is one of the key physiological functions of the female body. An ovarian, or estrus, cycle is characteristic of all female mammals, but menstruation, periodic uterine bleeding that accompanies the ovarian cycle, occurs only in women, female apes, and some monkeys. The average length of the menstrual cycle ....