Public Release: 

Both smells and pheromones may arouse instinctive behaviors in mammals

Indiana University

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.

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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

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