Do the old aged females experience a long post-reproductive life span?: The cases of Japanese macaques and chimpanzees

The average time lag between the last parturition and the disappearance/death was 6.0 years for the Japanese macaque females that survived at least to the age of 20 years in the Arashiyama B troop. Since it may take 1.5 years for the last offspring to become able to survive without the mother, 4.5 years may correspond to the “post-reproductive life span (PRLS),” which occupied about 16% of the average span of their lives (27.3 years). During the PRLSs, the females continued to become estrous and mate with the males for at least several years. On the other hand, one of the oldest females showed neither estrus nor copulatory behaviors. There may be several causes for the lack of reproductive success among old aged females: (1) in spite of ovulation, other physiological functions related to conception may have declined; (2) the old aged females may not have been able to maintain pregnancy, and may have aborted; or (3) the old aged females may have exhibited estrus without ovulation, and may be considered to have been in menopause. In the Arashiyama B troop, PRLS may not be an unusual phenomenon, because, out of the 32 females born from 1954 to 1963, 13 females (41%) survived to the age of 20 yrs and most of them exhibited conspicuous PRLSs. Similar PRLSs were found in a non-provisioned population of the Japanese macaque of Yakushima Island, and in a wild population of the chimpanzee of the Mahale Mountains National Park, Tanzania.