Provisioned food was severely restricted for 5 years, from 1972 until June 1977, in the insular Koshima troop of Japanese macaques, The monkeys experienced very poor nutritional conditions during this period. The feeding policy was changed in 1977 and food conditions improved, but only in summer. A remarkable consequence of intensive provisioning in summer was the occurrence of perineal swelling in many young females outside of the mating season, Females that matured during the period of pool food conditions showed an extraordinary delay in primiparity and exhibited conspicuous perineal swelling in the summer of 1977, Researchers studied the development of perineal swelling under standard conditions of constant provisioning from 1990 to 1995. First perineal swelling appeared at 5 years of age under standard conditions but appeared at 8 years of age under poor food conditions. The extraordinary delay in primiparity did not occur among females that were less than or equal to 6 years old when food conditions were improved in summer. We deduced that female Japanese macaques have a critical age of 5 to 6 years for sexual maturation. If the critical age is exceeded under poor food conditions, sexual maturation is hard to achieve even with improvements in food conditions. Seasonal changes in the incidence of perineal swelling were related to the period of high body weight, Rapid improvements in food conditions probably caused perineal swelling and activated sex hormones in female Koshima Japanese macaques.