Learned Recognition of Maternal Signature Odors Mediates the First Suckling Episode in Mice

BACKGROUND Soon after birth, all mammals must initiate milk suckling to survive. In rodents, this innate behavior is critically dependent on uncharacterized maternally derived chemosensory ligands. Recently, the first pheromone sufficient to initiate suckling was isolated from the rabbit. Identification of the olfactory cues that trigger first suckling in the mouse would provide the means to determine the neural mechanisms that generate innate behavior. RESULTS Here we use behavioral analysis, metabolomics, and calcium imaging of primary sensory neurons and find no evidence of ligands with intrinsic bioactivity, such as pheromones, acting to promote first suckling in the mouse. Instead, we find that the initiation of suckling is dependent on variable blends of maternal "signature odors" that are learned and recognized prior to first suckling. CONCLUSIONS As observed with pheromone-mediated behavior, the response to signature odors releases innate behavior. However, this mechanism tolerates variability in both the signaling ligands and sensory neurons, which may maximize the probability that this first essential behavior is successfully initiated. These results suggest that mammalian species have evolved multiple strategies to ensure the onset of this critical behavior.