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Both smells and pheromones may arouse instinctive
behaviors in mammals
July 16, 2001
BLOOMINGTON, Ind. -- Ever notice how male dogs come from the other side of
the neighborhood when a female dog is in heat?
All it takes is a few molecules of a certain chemical to enable mammals to
smell their own species up to a half-mile away, said Milos Novotny,
Distinguished Professor of Chemistry and director of the Institute for Pheromone
Research at Indiana University.
The chemicals, called pheromones, are detected by the vomeronasal organ (VNO)
in the animal's nose. Unlike the part of the nose that detects ordinary smells,
this super-sensitive organ is connected directly to the mid-brain.
"This is the shortest organ-to-brain distance in mammalian biology," Novotny
said. "A cascade of biochemical processes can be triggered quite selectively by
specific olfactants such as pheromones at incredibly small quantities. Studies
of mammalian pheromones can have a significant effect on pest control, promoting
endangered species, and, perhaps above all, for understanding our own sense of
smell and associated behaviors."
Signals from a mammal's nose caused by normal smells called odorants go to
various places in the cortex, in the upper part of the brain, which is why
humans are conscious of smells. But pheromone signals go directly to the
mid-brain, without being processed by the conscious brain. What happens after
that is not completely clear, but there is a lot of evidence that the animal's
behavior and hormonal levels are influenced.
In a paper published July 12 in the journal Nature, Novotny and
co-workers at Harvard Medical School in Boston headed by Linda Buck reported
that the vomeronasal organ can actually detect both odorants and pheromones. The
VNO detected odorants classified as animalic, camphoraceous, citrus, floral,
fruity, green/minty, musky, sweet or woody. Like pheromones, these odorants were
detected at extremely small concentrations.
"This suggests that in mammals, as in insects, odorous compounds released
from plants or other animal species may act as 'semiochemicals' -- signaling
molecules that elicit behaviors that are advantageous to the sender or the
receiver," Novotny said.
"The house mouse provides a classic example of an elaborate pheromone
communication system: to signal inter-male aggression and dominance, to show
readiness for mating, to slow down or accelerate the onset of puberty as needed,
or to signal stress to the other members of a colony," he said. "Other mammals,
including possibly humans, use structurally diverse substances for pheromone
signaling."
The established view is that mammals detect odorants in the olfactory
epithelium (OE) of the nose and detect pheromones in the vomeronasal organ. OE
signals are relayed to various areas in the cortex of the brain, while VNO
signals are targeted to areas of the mid-brain that control instinctive drives,
neuroendocrine responses and innate behaviors. The findings by Novotny and his
collaborators demonstrate that the VNO and OE do not, in fact, detect mutually
exclusive sets of chemicals.
Novotny's laboratory identified the first definitive mammalian pheromones in
the house mouse in the late 1980s, including their chemical structure, synthesis
and biological effects. Before then, the term "pheromones" was largely confined
to the world of insects. Since then, he has identified pheromones in rats and
hamsters as well.
His current emphasis is on the neurochemistry of neurons in the VNO and OE.
He is the leader of interdisciplinary studies that bridge the physical sciences,
life sciences and social sciences, including chemistry, neurobiology,
psychobiology, biochemistry, wildlife ecology, medical sciences, and animal
physiology and behavior.
The Institute for Pheromone Research at IU is a center of excellence in the
rapidly developing areas of chemical communication (semiochemistry) and
biochemical aspects of olfactory perception. It promotes interdisciplinary
collaborations between IU scientists and a worldwide network of researchers in
chemical communication.
Novotny can be reached at 812-855-4532 or
novotny@indiana.edu
(Hal Kibbey, 812-855-0074, hkibbey@indiana.edu)