Faced with a threat to their young, mothers often act as if they feel no fear. A new study shows why. Neurobiologist Stephen Gammie of the University of Wisconsin at Madison notes that levels of corticotropin-releasing hormone, a chemical that causes nervousness, are suppressed in mothers of newborn children. “During normal lactation, corticotropin-releasing hormone decreases,” Gammie says. “We hypothesized that if they had low fear and anxiety, that might increase the likelihood that they would defend their offspring.”
To test this idea, he and his colleagues experimented on several groups of mice that had recently given birth, injecting them with different doses of the hormone. When faced with male intruders who menaced their brood, mothers with low levels of corticotropin-releasing hormone confronted the threat with ferocious displays of hostility. In contrast, those who received high doses of the hormone quavered in their cages.
Abnormal levels of the hormone have been linked to mood disorders in humans; Gammie hopes his research might help explain why some mothers suffer postpartum depression, and in rare cases even neglect their infants after giving birth.