The vomeronasal organ (VNO) is a small, paired tubular structure embedded in the nasal epithelium of amphibians, reptiles, and many mammals (although not humans). In mammals the VNO is located on the septum dividing the two nostrils (see Figure 52.3) and has a pore that opens into the nasal cavity. The VNO pulsates and draws a sample of nasal fluid over the chemoreceptors embedded in its walls. The information from these chemoreceptors goes to an accessory olfactory bulb in the brain, and from there to brain regions involved in sexual and other instinctive behaviors. The VNO of a rodent detects pheromones that signal the sex, strain, identity, and sexual receptivity of another individual.

966

In snakes the VNO opens into the roof of the mouth cavity. Each time the snake’s forked tongue darts in and out, the forks fit into the VNO openings and present the chemoreceptors located there with a sample of molecules from the surrounding air (see the chapter opening photo). Thus the snake uses its tongue to smell its environment, not to taste it. Why doesn’t the snake simply use the flow of air to and from its lungs, as we do, to smell the environment? In snakes, air flows to and from the lungs slowly (and can even stop entirely for long periods of time), but the tongue can dart in and out rapidly. It is a quick source of olfactory information used in social communication and hunting prey.