Effect of maternal and grandmaternal care on population dynamics and human life-history evolution: A matrix projection model

We present a matrix population model for a single-sex human population comprising non-orphan daughters (whose mothers are alive) and orphan daughters (whose mothers are dead). Orphans suffer higher mortality than non-orphans, which simulates the need for daughters to receive maternal care in order to survive. The way that maternal care affects population dynamics and life-history evolution is then analysed for demographic regimes that encompass large ranges of daughter survival, mother survival and fertility. We provide stable age-distributions of orphans and non-orphans for each regime and perform sensitivity analyses on daughter survival, adult survival and fertility. The results show that natural selection will favour (i) faster daughter independence from maternal care, (ii) higher adult survival at all ages, and (iii) early reproduction to the detriment of late reproduction. We then build scenarios concerning the coevolution of daughter survival and maternal care with adult survival and fertility. We also incorporate grandmaternal care into the model. We show that (i) the acute altriciality of human babies, (ii) the increased maternal care resulting from emergence of complex sociality and (iii) the role played by grandmothers in caring for granddaughters may have led to the emergence of specific human life-history traits: a short reproductive period characterised by a reproductive senescence and menopause, as well as an extended lifespan characterised by a post-reproductive life.