Fig. 1.

Menstrual cycle starting time determination in example macaque BB0499. Menstrual cycle start and duration were determined retrospectively using daily observation of bleeding (open circles), sex skin swelling (recorded on a scale of 1-4, left axis, grey bars), and plasma progesterone (right axis, filled circles). In this example, onset of menstrual bleeding defined day one of the cycle. Biopsies (cross symbol) were collected on days 8 and 29 of the cycle for this animal.