What arouses an animal or human from an inactive, nonresponsive state to a condition of activity and responsiveness? What are the biological mechanisms for this change? In this book Donald W. Pfaff focuses on a reproductive behavior typical of many female animals. Sensory stimuli from the male trigger responses in a well-defined circuit of nerve cells. At the top of the circuit, certain nerve cells receive and retain sex hormones such as estrogens and progesterone.