Effects of infant loss on the interbirth interval of Japanese monkeys

This report analyzes the intervals between births which might be affected by interrupted lactation due to the loss of young in a group of semi-wild Japanese monkeys. Of a total of 168 births whose days of interbirth interval are countable, births with an approximate one-year's interval numbered 53 cases (31.5%) and those with a roughly two-year's interval 107 cases (63.7%). Among the 168 cases of birth, those resulting in loss of infant within 185 days, a figure obtained by subtracting the 180 days of the gestation period from 365 days, the mean interbirth interval, numbered 40, in which 38 females (95%) subsequently gave birth in the following year. On the other hand, only 11 (8.6%) out of the remaining 128 females who did not lose their infants gave subsequent birth in the following year. These data indicate that parturition in successive years is much more prevalent among females who suffer loss of infant than among those who do not. From the results, it is suggested that separation of infants from their mothers increases the reproductive ability of the mothers and this separation should be made at the latest, before or during the following mating season, for the purpose of increasing reproduction.