Some aspects related to conception of the Japanese monkey (Macaca fuscata)

The season of birth, age of the first parturition, gestation period, and vaginal bleeding and mating after conception were surveyed with Japanese monkeys (Macaca fuscata). The analyses of the former two items were dependent on the birth records in the Ohirayama troop collected from 1957 to 1973, and the analyses of the latter two items were dependent on data obtained by a 48-hour mating in a laboratory. Birth in the Ohirayama troop converged into the months from March to July, especially from April to June. The age of the first parturition was three years and 11 months at the earliest, and nine years and two months at the latest. The monkeys giving their first birth at the age of five or thereabouts were most frequently observed (68.6%), and most of the monkeys had their first parturition from about the age of four years to about the age of six years. The gestation period calculated from 17 cases, which was defined as the period from the first day of a 48-hour mating to the day before parturition, was 173 ± 6.9 days ranging from 161 to 188 days. In 25 out of 28 cases, the vaginal bleeding was observed after conception. It began slightly later (between 16 and 24 days after mating) than the forecasted time of the next menstrual hemorrhage, and usually lasted longer than bleeding of the usual menses. Each of three female monkeys caged together with a male monkey 30 days after conception was observed to have copulated, and the male was observed to have ejaculated.