A Secondary Olfactory System

In addition to having an extraordinary primary sense of smell, dogs and cats also have a second olfactory capability that we don’t have, made possible by the vomeronasal organ (VNO), also known as Jacobson’s organ. Located in the bottom of their nasal passage, the vomeronasal organ picks up pheromones, the chemical communicators unique to each animal species that advertise territorial markings, mating readiness, and other sex-related details.

The olfactory receptors in the nasal cavity are anatomically distinct from those in the vomeronasal organ. Each receptor neuron in the olfactory epithelium of the nasal cavity has a dendrite that ends in a knob with several thin cilia covered by mucus. Receptor neurons in the vomeronasal organ typically lack cilia but have microvilli on the cell surface.

The pheromone molecules that the organ detects—and their analysis by the brain—do not get mixed up with odor molecules or their analysis, because the organ has its own nerves leading to a part of the brain devoted entirely to interpreting its signals.

A cat exhibiting Flehmen will raise its head, draw its lips back, wrinkle his nose, and hold his mouth partially open. This allows the odors to enter the oral canals and reach the VNO. This can be initiated by a variety of circumstances such as smelling urine of an unfamiliar cat, being around a female cat in heat, or smelling where another cat has rubbed and left his oily secretions. A cat will identify familiar cats and greet non-hostile, unfamiliar cats this way. When a cat is greeting a person, the same behavior pattern can be observed. The cat will approach and smell the person, then turn and present her hind end. We should consider it an honor that cats respect us enough to greet us like they do other cats.