Seasonal menstrual cycles in a macaque living outdoors oscillate in synchrony with the lunar month. Mid-cycle surges in gonadotropins, which induce ovulation, recur during the days preceding the new moon (LH, Luteinizing Hormone; FSH, Follicle Stimulating Hormone; P,
progesterone; E, estradiol) (modified from Walker et al., 1984, Endocrinol. 114, 1074-1081). Macaque photos taken by Paul Asman and Jill Lenoble in Rajasthan, India, on November 28, 2007.