Menstruation (mammal)
Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.
Menstrual synchrony
This idea was extended to humans in a study by Stern and McClintock. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily.