The vomeronasal organ (VNO) is a specialized sensory apparatus located in the anterior portion of the palate, with ducts opening into the mouth just behind the front teeth. The organ is an elongated pouchlike structure that is lined with olfactory receptor cells. These cells are similar to those found in the olfactory mucosa except that they use microvilli instead of cilia. Scent information received by these receptor cells is projected via the accessory olfactory bulb (AOB) directly into the limbic system (amygdala and medial hypothalamus). Although there is some overlap between the olfactory system and the vomeronasal organ, the latter is particularly well suited for the detection of pheromone molecules of a higher weight than reliably detected by olfaction. This difference makes the vomeronasal organ more sensitive for the detection of nonvolatile chemical messages deposited in the urine and other bodily secretions. An important function of the vomeronasal organ is the detection and subcortical analysis of these sexual pheromones. Destruction of the vomeronasal organ results in the loss of normal sexual activities and several other vital functions (e.g., maternal care, aggressiveness, and secretion of sex hormones) in many mammals.
Although dogs do not exhibit the “lip curl” flehmen response observed in other mammals, many dogs do exhibit an analogous response called tonguing. When tonguing, the dog’s tongue is pushed rapidly against the roof of the mouth, with the teeth sometimes chattering and expressing profuse foam sometimes collecting on the upper lip. Tonguing is frequently observed after a dog licks a urine spot or “tastes the air,” following the exchange of mutual threat displays between two rival males. As the antagonists separate, one or the other may project his nose upward and initiate rhythmic sniffing and tonguing movements. The tonguing dog may actually extrude his tongue slightly in an effort to collect a sample. There is often a wide retraction of the lips together with a slight elevation of the muzzle. This action is accompanied by several brief sniffs and wide searching side-to-side movements of the head.
Eccles (1982) showed that the vomeronasal organ in cats is regulated by the autonomic nervous system. He studied the flehmen response in cats, discovering that the vomeronasal pouch or lumen suctions or expels depending on current sympathetic or parasympathetic stimulation. Under parasympathetic tone, the organ is constantly flushing and developing droplets around vomeronasal ducts. These droplets absorb airborne odorant or tastant samples that are then conducted via a sympathetic-induced pumping action into the lumen of the organ. After the odorant/tastant is sampled, it is expelled with a vigorous flushing action, thus clearing the organ and preparing it for another sample.
Whether dogs exhibit a true flehmen response remains controversial, with many authorities believing that dogs do not display the pattern, although some canid species (e.g., the coyote, side-striped jackal, and bushdog) do appear to exhibit a flehmen response. Overall (1997) suggested that the vomeronasal complex lacks functionality altogether, noting that the vomeronasal sacs are without chemoreceptors. This is clearly not the case, though, according to Adams and Wiekamp, who identified several types of receptors in the vomeronasal epithelium and concluded that the canine vomeronasal organ is “highly developed and unique amongst that of adult mammals” (1984:781). In addition, Salazar and coworkers (1992, 1994) described vomeronasal nerves and traced their destination to glomeruli in the accessory olfactory bulb. Although the vomeronasal organ system may be less well developed in dogs than in some other animals (e.g., rats and cats), it is a functional organ of some importance to dogs. Unfortunately, the significance of vomeronasal organ information for dogs is not known, but it likely serves some functional role in the exchange of pheromone information about social status and the animal’s reproductive state.
Some preliminary evidence supporting a sexual function for the vomeronasal organ system has been found in the study of the wolf’s response to methyl p-hydroxybenzoate, asexual pheromone. Klinghammer (unpublished data, personal communication) has discovered an intriguing phenomenon involving this pheromone among wolves. During the breeding season, captive wolf subordinates may court and mount an estrous female without interference from the alpha male, that is, until he detects the presence of this important sexual releasing hormone, at which point he actively defends his rights of exclusivity. The appearance of methyl p-hydroxybenzoate in a female wolf’s uterine secretions apparently coincides with ovulation and standing heat. The compound has also been found in the estrous secretions of female dogs and has been shown to elicit sexual arousal and mounting behavior in males when applied to the vulvas of spayed females.
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