This signaling system is particularly important to animals that are inexperienced sexually. Experiments by Michael Meredith, a neuroscientist at Florida State University in Tallahassee, Charles Wysocki, of the Monell Chemical Senses Center in Philadelphia, and others have shown that the VNOs play a key role in triggering sexual behavior in naive hamsters, mice, and rats.
A virgin male hamster or mouse whose vomeronasal organs are removed generally will not mate with a receptive female, they found, even if the male's main olfactory nerves are undamaged. Apparently, the VNOs are needed to start certain chains of behavior that are already programmed in the brain.
Losing the VNOs has a much less drastic effect on experienced animals, says Wysocki, who has been studying the VNOs for about 20 years. When male mice have begun to associate sexual activity with other cues from females, including smells, they become less dependent on the VNOs. Sexually experienced males whose VNOs are removed mate almost as frequently as intact males.
Do human beings have VNOs? In the early 1800s, L. Jacobson, a Danish physician, detected likely structures in a patient's nose, but he assumed they were non-sensory organs. Others thought that although VNOs exist in human embryos, they disappear during development or remain "vestigial"imperfectly developed.
Recently, researchers have come to a different conclusion. Both VNOs and vomeronasal pitstiny openings to the VNO in the nasal septumhave been found in nearly all patients examined by Bruce Jafek, an otolaryngologist at the University of Colorado at Denver and David Moran, who is now at the University of Pennsylvania's Smell and Taste Center in Philadelphia.
"This has opened up the possibility of a new sensory system in humans," says Rochelle Small. "We were often told that the VNO does not exist in adults, so we have taken a big step just to show that the structure is there."
She cautions that we still don't know whether this organ actually has connections to the brain, however. "The question now," she says, "is what its function might be."
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