Mothers matter

Seeing as this is Mother's Day, I want to point to this working paper, Menopause and post-generative longevity: Testing the 'stopping-early' and 'grandmother' hypotheses. It is a 44 page review of a lot of literature that is out there. The short of it is that menopause is something of a mystery, our nearest relatives, the chimps, seem to be rather normal animals in that the female reproductive lifespan is coterminus with the whole of her life after sexual maturation. In contrast, human females engage in a proactive "shut down" of reproductive capacity. Not only that, after menopause they can live for many decades. One hypothesis is that because human infants are so helpless and fitness of offspring tends to decline with age as aneuploidy and other abnormalities increase as a frequency of fertilizations at a certain point it is more beneficial that the mother cease reproduction and focus on the upbringing of her offspring. Additionally, a supplementary/alternative hypothesis is that mothers focus on being grandmothers, and in particular, they focus on the well-being of their daughter's children (there is some empirical evidence from Ghana that maternal grandmother presence reduces infant mortality but paternal does not).

Hypotheses about human evolution are sketchy in some ways because, not to be homocentric, but we are a damn complex animal! Nevertheless, the raison d'etre of existence is reproduction from an evolutionary perspective, so menopausal shut down cannot simply be arandom feature of our species. One could hypothesize that it is a structural constraint/byproduct of our extreme longevity, but, my understanding of the physiology is that while male reproductive capacity tends to just "fall apart" over time, female shut down is swift and concerted as a cascade of physiological triggers initiate a transition over a few years. In other words, it seems a functional response to some sort of selective pressure. And so we have the emergence of these hypotheses....