Olfactory System

Vomeronasal - Flehmen
(Jacobson's Organ)

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Vomeronasal Olfactory System. Immediately caudal to the incisor teeth is a papilla onto which open two nasopalatine canals. These canals allow slow passage of odors from the mouth to the vomeronasal organ located within the hard palate. The vomeronasal organ (organ of Jacobson) is lined with olfactory cells, and has central pathways different from those of olfactory epithelium. Impulses first travel to the accessory olfactory bulb and then to areas of the hypothalamus associated with sexual behavior, feeding behavior, and, possibly, social interactions.

Flehmen is the behavior associated with the inhalation of odors into the nasopalatine canals. Beginning as early as 6 weeks, a cat will sniff a particular odor source, such as urine, often touching it with its nose and perhaps its tongue.   The head is then raised with the lips drawn back, nose wrinkled, and mouth partially open for inhalation . This behavior is similar to that seen in ruminants and horses; however, the philtrum of the feline upper lip prevents its complete elevation. Flehmen, also called lip curl or gape, is most frequently displayed by tomcats.

Photos: Graham Meadows


The Vomeronasal, or Jacobson's Organ

John Bradshaw
Anthrozoology Institute, School of Biological Sciences, University of Southampton

The vomeronasal, or Jacobson's, organ occurs in many mammalian species, but not in higher primates, including man. Their structure suggests that they are used only intermittently, as accessory olfactory organs. They are connected to both the oral and nasal cavities by the nasopalatine canal, which runs through the incisival foramen; the lower opening can be seen as a slit immediately behind the upper incisor teeth. The paired vomeronasal organs are blind sacs running backwards from the canal, to which they are connected by very fine ducts, only 30-40 microm wide. Thus the penetration of odors to the chemoreceptors in the organs themselves is unlikely to occur passively, in the way that odor molecules can reach the olfactory epithelium every time that the cat breathes. It has been suggested that a vasometer pumping mechanism expels some of the fluid that fills the sacs out into the canal, and then sucks it back again, drawing in odorants from the mouth and nasal cavity.

The external sign that a cat is using its vomeronasal organ is the gape or 'Flehmen' response, a 'grimace' in which the upper lip is raised and the mouth is held slightly open for a few seconds. This is performed by both mates and females, in heterosexual encounters mostly by males, following actual naso-oral contact with urine scent marks or females. Females will respond in the same way to urine marks, if there is no male present. The need for actual contact with the olfactory material implies that this is a sense more akin to taste than to smell, because the stimuli may be fluid borne throughout. The only stage at which they might be in the vapor phase is in the transfer from the nose and lips to the opening of the nasopalatine canal. The precise role of the gape in sexual behavior has not been fully investigated, but in other species it has been found that a fully functional vomeronasal organ is essential for successful completion of the first courtship sequence, but that sexually experienced animals can rely on the olfactory sense alone to identify estrous females.

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