Development and expression of maternal behavior in naïve female C57BL/6 mice.

Naïve female mice are usually described as spontaneously maternal. We investigated how many exposures to pups (15 min vs. 1 hr) were needed to induce full maternal behavior (FMB) in 20-22, 30-35, 60-65-days-old naïve female mice (C57BL/6), and how cohabitation with the parturient mother and newborn siblings facilitated juvenile maternal behavior (MB). Only 20% of the adults displayed FMB immediately after the first exposure to pups. Incomplete MB was present in 11%, 20%, and 30% of juveniles, adolescents and adults, respectively. Three-sixty minute exposures to pups induced FMB in all adult subjects. All naïve juveniles that were not exposed to their siblings and maternal fluids failed to show maternal behavior. In contrast, more than half of the juveniles present at their homecage during delivery of a second litter showed incomplete MB (34.5%) or FMB (21.5%) when tested individually housed in a novel cage. This study suggests that most adult female mice are not spontaneously maternal but gradually sensitized. Besides, naïve juveniles could be inhibited or not motivated to show MB, but display adult-like behavior toward pups if previously exposed to newborn siblings and maternal fluids.