Pair bonds and parental behaviour are among the most variable social traits. To understand how and why these traits are so variable, we investigate three issues in this chapter. First, we present an overview of recent work on molecular and neural aspects of pair bonds and parental care using microtine rodents as model organisms. We focus on two neuropeptides, oxytocin and vasopressin, and show that although both molecules are found in both sexes, oxytocin plays a more prominent role in regulating parenting and pair bonding in females, whereas vasopressin serves this role in males. Variation in the expression of oxytocin and vasopressin receptors appears to contribute to species and individual differences in social behaviour. These studies also show that although oxytocin and vasopressin function in distinct brain regions, they act within the same neural circuit. Therefore, females and males appear to accomplish behavioural changes in pair bonding and parental care by altering the responsiveness of the same neural circuit. Second, studies of pair bonds and parental care in natural populations have revealed that these traits are often tied together. Cost–benefit analyses of both traits in a game-theoretic framework provide novel insights into how diverse pair bonding and parental care may have evolved. Recent work emphasises the role of social environment in influencing pair bonding and care. Finally, we point out that currently there is a schism between proximate and ultimate approaches to understanding pair bonding and parental care.
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