Chapter 3 describes the hormones that promote the immediate onset of maternal behavior at parturition, drawing on research from rats, rabbits, sheep, mice, and nonhuman primates. The critical hormones include rising levels of estradiol, prolactin, and placental lactogens that occur near the end of pregnancy on a background of progesterone withdrawal. In contrast to the onset of maternal behavior, due to maternal experience, its maintenance does not require hormones. Laboratory strains of female mice, produced by inbreeding and selective breeding, are anomalous in that they do not require pregnancy hormones to show prompt maternal behavior when presented with conspecific infants under low-stress conditions. However, the physiological events of late pregnancy boost maternal motivation in these mice to allow for effective maternal behavior in challenging environments. The same processes operate in species that exhibit naturally occurring alloparental behavior, such as marmosets, where such behavior has evolved by natural selection.
