Observations and experiments on the reproductive cycle of one of the common Old World monkeys, Pithecus (Macacus) rhesus, which have been in. progress during the last six years, have given results which are in some ways quite unexpected and which may necessitate a revision of current ideas about menstruation. These experiments have been necessarily rather few and tedious, but the evidence is now sufficiently definite to be presented to the medical profession.1
Females of Macacus rhesus exhibit hemorrhages from the genital tract which resemble human menstruation, both in periodicity and in duration. In the animals of my colony the cycles are subject to greater irregularity than in women, but the modal interval is twenty-seven days, and the modal duration five days. It seems proper to call this phenomenon by the name used to describe the at least outwardly similar process in the human species. As used here, therefore, "menstruation,"
CORNER GW. THE RELATION BETWEEN MENSTRUATION AND OVULATION IN THE MONKEYITS POSSIBLE SIGNIFICANCE FOR MAN. JAMA. 1927;89(22):1838–1840. doi:10.1001/jama.1927.02690220014005