Menstrual Cycle Lengths and the Estimated Time of Ovulation in the Cynomolgus Monkey (Macaca fascicularis)

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Abstract
Establishing of an efficient system for the judgement of an optimal mating time is an important prerequisite for the successful development of a large scale breeding of macaques as laboratory animals. In general, an optimal mating time was judged on the basis of individual records of menstruation. The length of the previous menstrual cycle has usually been used for the prediction of the next cycle length. The lengths of 240 menstrual cycles of 60 cynomolgus monkeys of the breeding colony were statistically analyzed. The mean and the SD of those cycle-lengths were 29.4 and 4.3 days, respectively. Correlation between the length of a menstrual cycle and the length of the previous cycle was statistically significant (P < 0.05). Practically, the correlation was not meaningful because the correlation coefficient (r) was very small even if the length of the second one of 2 successive cycles (r = 0.26) or the average length of 3 successive cycles (r = 0.36) is selected as a reference length. A cycle length apparently can not be predicted on the basis of the length of the previous menstrual cycle. However, a marked increase of serum FSH concentration was observed from 8 to 15 days after menstruation. In 60% of the animals observed, FSH increased 10 to 11 days after menstruation. Ovulation was observed 1 of 2 days later than the day of FSH increase. Therefore, the optimal mating time is judged to be between 11 and 14 days after menstruation regardless of the menstrual cycle length.

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