June 05, 2017

Sex, Babies, and Dogs: Evolutionary and Endocrine Aspects of Changing Human Families

Peter B. Gray, University of Nevada, Las Vegas

Compared to great apes, several key features of human hunter-gatherer family life stand out: having more children at shorter inter-birth intervals, forming long-term reproductive partnerships, and investment by fathers and female relatives beyond their reproductive years. Behavioral reconstructions suggest derived features of human hunter-gatherer families evolved during the last several million years of Homo. As humans spread around the globe, a marvelous variety of human families has emerged and continues to change. Life history theory provides a functional approach, complemented by hormonal mechanisms, to structure how family members interact among themselves and with others. I discuss several key patterns intertwining human family change and hormones that differ from hunter-gatherer families. Almost all human children today are involved in formal education and many too in sports and school-based competitions organized by adults. How do adrenal hormones DHEA and androstenedione relate to children’s learning and social competition? In great apes and hunter-gatherers, the bulk of female reproductive years are spent pregnant or in lactational amenorrhea. Yet fertility has plummeted in much of the world and inter-birth intervals differ from ancestral conditions. How do hormonal shifts across pregnancy and postpartum impact maternal care and partnership dynamics, including sex lives, of parents? Fathers play variable roles across human families. Male involvement in family life is often linked to lower testosterone, and also involves oxytocin and prolactin. In some societies, dogs are increasingly viewed as family members, with several studies indicating that interactions with a dog can increase human oxytocin levels. Lastly, I speculate about patterns of female and male reproductive senescence and underlying hormonal mechanisms in relation to partnership maintenance and grandparenting.