The belief that mothers instinctively and automatically nurture their babies is increasingly controversial among those who study human behavior. In her new book, "Mother Nature: A History of Mothers, Infants, and Natural Selection" (Pantheon) Sarah Hrdy draws upon her background as a primatologist, scientist and mother to examine the belief. A UC Davis professor emerita and National Academy of Sciences member, Hrdy concludes that women have innate maternal responses, yet they do not necessarily nurture each baby born -- and neither do other mammals. "There is probably no mammal in which maternal commitment does not emerge piecemeal and chronically sensitive to external cues. Nurturing has to be teased out, reinforced, maintained. Nurturing itself needs to be nurtured," Hrdy writes. "One step at a time, something as simple and discrete as a build-up of hormone receptors in the brain can determine whether a particular signal from a pup fails to register, or becomes magnified into a network of responses. Small nudges favoring distance by the mother, or affiliation, can make an enormous difference in whether a bond between mother and infant is forged." In the 700-page book, Hrdy uses her observations from her years of study to examine historical social pressures and customs surrounding infants and mothers, even probing the concept of "working mothers" and the ensuing tradeoffs. For most of human existence, and for millions of years before that, Hrdy says primate mothers have combined productive lives with reproduction. Yet the factories, laboratories and offices where women today go to work are "even less compatible with childcare than jaguar-infested forests ..." Hrdy will give a public talk about her book at 4 p.m., Monday, Nov. 29, in 166 Young Hall on the UC Davis campus.