Scientists are still unraveling nature’s secret olfactory signals
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. That’s because the body’s 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.
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 compounds—often as oils or sweat—and 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 recipient’s 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 individual’s social status or health. Mice can select appropriate mates based on odor cues, deriving information in part from unique proteins associated with a mouse’s genetics.
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 woman’s 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 subtler—and harder to detect—than 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 person’s ability to discriminate between ambiguous emotional expressions. The implication is that chemicals in the jumper’s 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.
As the hunt for human pheromones continues, scientists have also investigated other potential explanations for the subtle effects of smells. Consider, for example, the finding that human infants will crawl toward the odor of their mother’s breast. Baby rabbits are known to begin nursing when exposed to a specific pheromone from a lactating mother rabbit. Yet the human infants might simply be attracted to a mother’s so-called odor print, or unique personal scent. Odor prints are influenced by diet, environment, health and genetics. They consist of far too many compounds to be described as pheromones themselves.
The failure to identify human pheromones has not stopped some enterprising individuals from trying to make a profit from love potions purporting to contain pheromones. In reality, these products often use pig pheromones. “They don’t have any history in the biomedical literature—they just fell out of the sky,” says olfactory neuroscientist Charles Wysocki, also of Monell. For now, the idea of perfumes and potions based on human pheromonal communication just doesn’t pass the sniff test.