In females, the first swellings of the sex skin occur between the ages of 7 ½ and 8 ½, and gradually increase in size until the female chimpanzee is about 10 or 11 and begins to be mated by males, which constitutes her first estrus. The changes in the sex skin are significant to the observer because they reflect hormone changes and the probable time of ovulation; the swellings are also a visual aid to the male chimpanzee, who can easily tell a great deal about the reproductive condition of the female. The female estrus cycle has a great influence on both male and female social behavior.
When a female chimpanzee comes into heat - or into estrus, as a scientist would say - the sex
skin of her genital area becomes swollen. Such swellings vary somewhat in size; some females develop a pale pink protuberance
that is fully as large as a three-pint bowl, whereas others have smaller protuberances. A swelling usually persists for about
ten days before becoming flabby and wrinkled and then shrinking away to nothing again. Normally it occurs at a point midway
between menstrual periods, which in the female chimp occur about every thirty-five days. It is during her period of swelling
that a female - whom I refer to frivolously as a "pink lady" -is courted and mated by the males.