Sexual maturation and seasonal changes in reproductive phenomena of male Japanese monkeys (Macaca fuscata) at Takasakiyama

The authors examined testis tissues and blood which were collected from free-ranging Japanese monkeys of the Takasakiyama troop during four periods in 1971 (mating season: late January-early February; early birth season: June; late birth season: August; and intermediate season between birth season and mating season: October), and studied their sexual maturation and seasonal changes in reproductive phenomena. Results of observations on the testis and plasma testosterone concentration were in agreement with each other. Except in a few cases, the testis was infantile until October at 4 years old and developed rapidly during the following two months, and spermatogenesis started in the mating season at 4 years old (in exceptional cases, it started one year earlier). After the following two-year process of sexual maturation, monkeys attained full maturation in the mating season at 6 years old. For seasonal changes in reproductive phenomena also, results of observations on the testis and the plasma testosterone were in agreement with each other. Activity of the testis repeated an annual cycle of being maximal in the mating season, regressing in the birth season, and redeveloping toward the following mating season. Such seasonal changes were noticeably observed with 4- to 6-year-old animals, which are in the process of sexual maturation.