Gorilla females have a menstrual cycle length of about 30 days and ovulation is indicated by a rise in the main progesteron metabolite pregnanediol-3-glucuronide, which can be measured in the feces. Here, the feces from one individual female gorilla was collected for one year in Volcanoes National Park in Rwanda (modified from Habumuremyi et al., 2016, Physiol. Behav. 157, 185-195). The stippled horizontal lines represent the baseline and the threshold for ovulation-related rises in pregnanediol-3-glucuronide, respectively, showing that ovulation in this individual female occurred always around the full moon. Mating times are indicated by asterisks. Gorillas have no natural enemies and need therefore not to hide in full moon nights. The picture of the gorillas with baby was taken by Derek Keats at the Volcanoes National Park in Rwanda.