For obvious reasons, most of our knowledge about the biological bases of maternal behaviour has come from studies of animals. For example, it would be unethical to artificially manipulate hormone levels in women in order to investigate the role of hormones in their behaviour. Nevertheless it is possible that the principles governing such an important behaviour that emerge from studies of animals may be echoed in the behaviour of our own species.
The observation that blood transfusions from maternal rats induced maternal behaviour in nonpregnant females, suggested that hormones may play an important role in switching on (initiating) maternal behavior. The diagrams on this page illustrate the dramatic changes in hormone levels around parturition in rats. Similar changes occur during human pregnancy.
Is this cluster of hormonal changes responsible for switching on maternal behavior ? It is difficult to study the role of hormones in natural pregnancy because if you inject or remove hormones during pregnancy there are multiple effects on lactation and behavior that are difficult to disentangle. Jay Rosenblatt examined the hypothesis that the decline in progesterone level seen towards the end of pregnancy triggers the onset of maternal behaviour.