Reproductive termination in female Japanese monkeys: A comparative life history perspective

This study explores the question of reproductive termination (loss of reproductive ability) in female Japanese macaques (Macaca fuscata) from the Arashiyama West (Texas) troop. We used a large sample of completed lives to identify reproductively terminated female Japanese macaques and to consider reproductive termination in Japanese macaques from a comparative life history perspective, which permits meaningful comparisons to be made with human female menopause. We classified a female as reproductively terminated if the time lag between last parturition and death exceeded two standard deviations of the female's own mean lifetime interbirth interval (Caro et al. [1995] Int. J. Primatol. 16:205-220). Seventy of the 95 females in the sample had at least 3 infants over their lifetime (the minimum required for the calculation of a mean and standard deviation), and thus were included in the analysis. Of these 70 females, 20 showed reproductive termination. Reproductively terminated females ranged in age from 14.5-32.7 years, although in females under age 25, reproductive termination was unlikely. The majority of females up to age 25 showed continued parturition. However, after age 25, reproductive termination was population-wide. Length of postreproductive life for reproductively terminated females varied from 0.07-4.4 years, with a mean of 2.08 years. Variation in length of postreproductive life was not related to the age at death of the female. While the occurrence of population-wide reproductive termination after 25 years does suggest similarities with human female menopause, the age at which this termination occurs is very late in the life span, and it was experienced by only 2.9% of the population. Female Japanese monkeys over age 25 are visibly aged and show outward signs of weakness and deterioration, quite unlike the healthy middle age of menopausal human females. Accordingly, as a life history characteristic, reproductive termination in Japanese macaques does not appear to coincide with menopause as experienced by human females.