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Scientists are still unraveling natures secret olfactory signals

Since pheromones were first defined in 1959, scientists have found many examples of pheromonal communication. Credit: Thinkstock

Strange as it may sound, some scientists suspect that the humble armpit could be sending all kinds of signals from casual flirtation to sounding the alarm. Thats because the bodys secretions, some stinky and others below the threshold your nose can detect, may be rife with chemical messages called pheromones. Yet despite half a century of research into these subtle cues, we have yet to find direct evidence of their existence in humans. What Are Pheromones? Humans and other animals have an olfactory system designed to detect and discriminate between thousands of chemical compounds. For more than 50 years, scientists have been aware of the fact that certain insects and animals can release chemical compoundsoften as oils or sweatand that other creatures can detect and respond to these compounds, which allows for a form of silent, purely chemical communication.

Although the exact definition has been debated and redefined several times, pheromones are generally recognized as single or small sets of compounds that transmit signals between organisms of the same species. They are typically just one part of the larger potpourri of odorants emitted from an insect or animal, and some pheromones do not have a discernable scent.

Since pheromones were first defined in 1959, scientists have found many examples of pheromonal communication. The most striking of these signals elicits an immediate behavioral response. For example, the female silk moth releases a trail of the molecule bombykol, which unerringly draws males from the moment they encounter it. Slower-acting pheromones can affect the recipients reproductive physiology, as when the alpha-farnesene molecule in male mouse urine accelerates puberty in young female mice.

Some researchers have proposed a third group of pheromones called signalers that simply transmit information such as an individuals social status or health. Mice can select appropriate mates based on odor cues, deriving information in part from unique proteins associated with a mouses genetics. The Trouble with Humans

So far, scientists have had some success in demonstrating that exposure to body odor can elicit responses in other humans. As in rodent research, human sweat and secretions can affect the reproductive readiness of other humans. Since the 1970s researchers have observed changes in a womans menstrual cycle when she is exposed to the sweat of other women. In 2011 a Florida State University group demonstrated that the scent of ovulating women could cause testosterone levels to increase in men.

But there is no evidence of a consistent and strong behavioral response to any human-produced chemical cue. Maybe once upon a time we could react more viscerally, says chemist George Preti of the Monell Chemical Senses Center. Today, however, our reactions seem to be much subtlerand harder to detectthan those of a silk moth. This subtlety has led researchers to propose another kind of chemical messenger, known as a modulator pheromone, that affects the mood or mental state of the recipient. In an example of this type, researchers at Stony Brook University found in 2009 that sniffing the sweat of first-time parachute jumpers could increase a persons ability to discriminate between ambiguous emotional expressions. The implication is that chemicals in the jumpers sweat might constitute an alarm signal, which puts the recipient on high alert and makes them more attentive to details.

Yet to demonstrate definitively that pheromones are at work, researchers need to point to the molecules responsible, which they have not yet done. To date, scientists have collected evidence for possible pheromone effects but have not definitively identified a single human pheromone. A Signature Scent

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Are Human Pheromones Real?