Hormonal Mechanisms of Grandmothering : The Coevolution of Physiology, Life History and Behavior

The evolution of menopause, grandmothering and long lifespan represent key events in the evolution of human life history. Demographic studies have amply demonstated inclusive fitness benefits from grandmaternal care, but the hormonal bases of such care, and how it evolved in relation to other reproductive and demographic traits, have yet to be addressed in detail. We propose and evaluate a novel hypothesis for the coevolution and adaptive covariation of life history, physiology, and behavior among women in this context. The hypothesis centers on relatively low testosterone, which promotes: (1) earlier, higher fertility and fecundity, (2) earlier cessation of ovarian activity (leading to earlier grandmothering), and (3) enhanced alloparental care. The hypothesis can help to explain among-female variation in grandmaternal care, and potential trajectories for the concerted evolution of grandmothering, prolonged human lifespan, and associated life history traits. A suite of convergent evidence supports the hypothesis, and it makes new predictions that are straightforward to test.