Seasonal changes in the reproductive physiology of female rhesus macaques (Macaca mulatta).

Seasonal changes in menstrual cycle patterns and internal reproductive organs were studied in female rhesus macaques (n = 16) in indoor-outdoor housing in Chongqing, China. Uterine size and shape and endometrial thickening were evaluated during the early-secretory phase by using ultrasonography and MRI. From October to February, the macaques' menstrual cycles were short and regular, and the endometrial lining were easily visible by ultrasonography and MRI. However, from March through to September, menstrual cycles became irregular, endometrial lining were unclear, and the endometrium did not change markedly during the early-secretory phase. We conclude that the reproductive season for a female rhesus macaque in Chongqing, China is from October through February, whereas the nonreproductive season is from March through September. The menstrual cycle patterns and reproductive organs of the macaques showed marked seasonal variation throughout the 12-mo observation period. In addition, uterine size, volume, and imaging characteristics varied dramatically between reproductive and nonreproductive seasons.