Patterns of ovulation, conception and pre-implantation embryo development during the breeding season in rhesus monkeys kept under semi-natural conditions

Abstract

The occurrence of ovulation and conception in the female rhesus monkey (Macaca mulatta) under natural conditions is seasonal and maximum during the months of October to March. Such seasonality in reproduction suggests dependence on environmental factors like photoperiod, temperature and rainfall. There occur significant fluctuations in these environmental factors also during the breeding season itself. Whether there occur any discernible changes in the occurrence of ovulation and conception, and in the incidence of abnormality in pre-implantation stage embryo development in the rhesus monkey, associated with changes in environmental factors within the breeding season has not been known and was investigated in the present study using a total of 304 menstrual cycles over a period of five years. No preferential sidedness for any ovary to ovulate more was evident, the frequency ratio of ovulations in left ovary to right ovary being 1.02. Despite the absence of any pattern in the side selection of ovulations in successive cycles, the chance of ovulations in alternate ovaries for successive cycles appeared more likely (67% cases). An average of 21% of menstrual cycles were found to be non-ovulatory with significantly higher menstrual cycle lengths (p<0.03) compared with that of ovulatory cycles. An additional 28% of the mated, conception cycles yielded degenerated or desynchronized pre-implantation stage embryos. Furthermore, the number of non-ovulatory cycles (p<0.05) and the incidence of abnormality in pre-implantation stage embryos (p<0.02) were higher during the months of transition in (October) and out (March) of the breeding season. It has been suggested that an integrated influence of environmental factors, mainly photoperiod and temperature, induces adjustments in the function of the hypothalamus-pituitary and the ovarian axis and during transitional months of the breeding season it becomes more vulnerable to be desynchronized, thus resulting in disturbances in the follicular maturation, ovulation and oocyte presentation.