Humans are almost unique in the animal kingdom in that females stop being reproductively active midway through their life. In all other primates, females often have babies at the same time as their sons and daughters. Ceasing reproduction is seen in some species of toothed whale, suggesting that there will be an ecological explanation. The grandmother hypothesis suggests that females may stop reproducing so they can invest in grandoffspring. However, measures of the fitness gains in hunter-gatherers suggest that the gains are insufficient to outweigh the cost of the missed opportunity. However, these calculations ignore the importance of competition for resources between children, and how relatedness between family members is influenced by who moves between family groups.
This study used a mathematical model of inclusive fitness to work out how competition for resources should influence grandmother’s reproductive decisions. Inclusive fitness calculations take into account not only the evolutionary fitness from having offspring, but also the fitness from relatives having offspring. A mathematical model assumes that group productivity decreases the more both invest in competition (x and y), but the share of the reproduction increases with this investment. In humans and some toothed whales, it is females that move between family groups when they mate, so new mothers will be unrelated to their mate’s mothers. These grandmothers share one quarter of their grandchildren’s genes via their son (r), so it is in their interest not to compete too much with their daughters-in-law over resources for their children. This asymmetry in genetic interests leads to grandmothers (and daughters-in-law) investing nothing in competition over reproduction (circles in left figure), because this gives higher fitness than the resulting competition if they invest anything (right figure).
Further study of this mathematical model shows that the order of the decisions matters. Either the grandmother or the daughter-in-law could make their investment in competition clear before the other, and the other will respond in the best way for themselves. The calculations show that both females do best if the grandmother makes her decision first and irreversibly commits to it: menopause can be seen as such a commitment that the daughter-in-law can observe. This suggests that menopause evolved because it avoids competition within extended family groups so enhances the life chances of grandchildren, especially when combined with extended lifespan of grandmothers.
Posted by: AndrewDHigginson Posted Fri Oct 27 2017