Impact of brain evolution on hormones and social behaviour

In mammals, the social behaviour of males and females reflects their different lifetime reproductive strategies. Reproductive success in males is determined by the outcome of competition with other males, the dominant males mating with as many females as possible. Hence, males rarely form strong social relationships and male coalitions are typically hierarchical, with emphasis upon aggressive rather than affiliative behaviour. Females have a different strategy. They invest in the production of relatively few offspring, with reproductive success being determined by the quality of care and the ability to enable infant survival beyond the weaning age. Females, therefore, form strong social bonds with their infants and their female-female relationships are affiliative, especially among matrilineal kin who often assist with infant care. In a minority of mammalian populations (less than 5%), a promiscuous male strategy is not an option, owing to the low population density of females. In this situation, males and females form a partner preference (bond), defending their partners against intruders and both parents participating in infant care (Kleiman 1997). The questions addressed in this chapter concern the hormonal mechanisms that underpin these female-bonded social relationships and how the evolutionary development of the neocortex in large-brained primates has impacted on the role of "bonding" as being an integral adjunct of physiological homeostasis.