Access of large and nonvolatile molecules to the vomeronasal organ of mammals during social and feeding behaviors

The vomeronasal organ, a chemosensory structure in the nasal cavity, is important in the detection of mammalian chemosignals, many of which are thought to be large molecules having low volatility. We conducted a series of experiments to determine whether nonvolatile molecules enter the vomeronasal organ during a variety of behavioral contexts in five species. We found that a nonvolatile dye entered the vomeronasal organ during investigation of urine from conspecific donors (experiment 1), during investigation of urine from heterospecific donors (experiment 2), during self-grooming (experiment 3), and during social grooming (experiment 4). In other experiments, we determined that nonvolatile molecules entered the vomeronasal organ during consumatory behaviors. Animals that ate a dye-adulterated familiar food had the nonvolatile marker in their vomeronasal organs (experiment 5). Animals that drank either familiar or novel dye-adulterated solutions also had the nonvolatile marker in their vomeronasal organs (experiment 6). In Experiment 7, large (66,000-dalton) fluorescent molecules were mixed with female urine which was then presented to male animals. We observed that the large molecules were transported to the vomeronasal organ. In the final experiment, we determined that mere contact between the snout of a dead animal and the stimulus resulted in transport of nonvolatile substances to the vomeronasal organ. We conclude that the vomeronasal organ, like the olfactory epithelium, is in continuous contact with the environment, but unlike the olfactory epithelium, the sensory receptors of the vomeronasal organ interact with molecules of low volatility, in addition to more volatile odorants.