Mothers often say they would step in front of a bullet to save their child's life. Numerous animals act in similar ways, but the biological basis of the behavior, known as maternal aggression, is not well understood. New findings indicate that levels of a specific peptide in the brain may answer the question of what makes a mother willing to lay down her life for her offspring.

We've known for a long time that fear and anxiety decrease with lactation, Stephen Gammie of the University of Wisconsin-Madison explains. Maybe it's this decrease that allows mothers to attack during a situation that normally would evoke a fear response. Gammie and his colleagues studied mice a few days after they gave birth and tested how the animals reacted to differing levels of corticotropin-releasing hormone (CRH), which is released in the brain and helps control behavior. For four days, some animals received injections of CRH while others received a placebo. The team then placed unrelated males--who can be dangerous to young pups--in the cage with the new moms and tracked their responses. Control mice and those given very low doses of CRH displayed expected behavior, going on the offensive to protect their offspring from danger. Moms given higher levels of CRH, however, didn't attack at all. If anything they were skittish, Gammie says. They showed a fear response.

Differing CRH levels did not affect any other mothering instincts, such as nursing, however. A link between CRH and caring behavior in mothers could also help explain situations in which women don't safeguard their children. If CRH needs to be low to see maternal protection of offspring, as our work suggests, Gammie remarks, then it explains why moms with high postpartum depression and high CRH not only may neglect, but also may abuse, their children. The findings appear in the August issue of Behavioral Neuroscience.