Menstrual synchrony
Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.
Babirusa
Female babirusa cycle lengths are between 28 and 42 days and estrus last 2–3 days. The litter size for a babirusa is usually one or two piglets.