Availability and adaptive value of reproductive and postreproductive Japanese macaque mothers and grandmothers

Prior tests of the grandmother hypothesis have suggested that postreproductive female Japanese macaques, Macaca fuscata, do not significantly improve the survivorship of their descendents. However, not all postreproductive females are grandmothers, and not all grandmothers are postreproductive. In this study we looked at the daughters and grandchildren of 70 female Japanese macaques to assess the availability and adaptive value of reproductive and postreproductive mothers and grandmothers. We found that postreproductive Japanese macaque grandmothers are very rare. Only 2.8% of the sample of daughters had a postreproductive grandmother available when they began to reproduce, and only 4.2% of the reproductive life span of the daughter (less than 6 months for the average female) was spent with a postreproductive grandmother available to help the daughter. Grandchildren who survived to age 5 had a postreproductive grandmother available to them for only 4% of the first 5 years of life (i.e. a couple of months on average). The presence of a living mother, irrespective of her reproductive status, was associated with improved reproduction in her adult daughters, and the presence of a postreproductive grandmother was associated with significantly improved survivorship of grandchildren to age 1. While improved maternal investment does not appear to be the primary explanation for reproductive termination in Japanese monkeys, the few postreproductive females that have unweaned grandchildren available appear to have a positive influence on their survival.