Mechanisms Controlling Seasonal Breeding in Japanese Monkeys

Abstract

Mechanisms controlling seasonal breeding in Japanese monkeys (Macaca fuscata fuscata) were studied. 1) Analysis of a total of 310 births recorded at our Institute over 12 years revealed that births were limited to the period from March to August with a peak in May. No difference was found in the birth season between outdoor and indoor samples, but the time of peak of births was slightly earlier in indoor than in outdoor samples. 2) In both monkeys housed indoors and outdoors, clear seasonal differences were observed in the hormonal profiles: Monkeys exhibited cyclic endocrine patterns characteristic of ovulatory cycles during the breeding season (autumn and winter), while anovulation persisted throughout the nonbreeding season (spring and summer). 3) Estradiol-treated ovariectomized monkeys exhibited marked seasonal changes in response to the negative feedback action of estradiol on LH secretion: Serum LH remained high during the breeding season, but decreased to very low levels during the nonbreeding season. There was no difference in the positive feedback action of estradiol on LH secretion between the breeding and nonbreeding seasons. 4) Artificial manipulation of photoperiod alone had no effect on the manifestations of annual reproductive cyclicity of Japanese monkeys, whereas simultaneous manipulation of photoperiod and ambient temperature was at least to some extent effective in modifying the ovarian functions. 5) Serum melatonin exhibited diurnal changes with high values in nighttime and low values in daytime both in monkeys housed indoors and outdoors. These results suggest that seasonal breeding of the Japanese monkey is governed by biannual changes in the response of the hypothalamo-hypophysial axis to the negative feedback action of estradiol, and that multiple annually cyclic environmental factors appears to influence on the seasonal breeding of this animal.