When two people get knocked off their feet by physical chemistry, their friends may offer this standard glib explanation: "It's all about the pheromones." But in fact, 50 years after the term "pheromones" was coined by biologists to refer to the chemical messages passed within many insect and mammal species, researchers still haven't found proof that humans emit or detect such chemicals. In an essay in Nature [subscription required] marking the 50-year milestone, zoologist Tristram Wyatt sums up the state of the research, and reminds the gullible not to buy any love potions that boast of their pheromone content. The first studies took place in 1959, when German researchers discovered a chemical called bombykol that's secreted by female silk moths and that immediately sends males into a mating frenzy. Following that Nobel Prize-winning work, biologists

proceeded to find pheromones "across the animal kingdom, sending messages between courting lobsters, alarmed aphids, suckling rabbit pups, mound-building termites and trail-following ants. They are also used by algae, yeast, ciliates and bacteria" [Wired News]

, Wyatt writes. Pheromones have been found to play a part not just in mating rituals, but also battles for dominance, warnings about approaching danger, and cooperative behavior. Despite 50 years of searching researchers have found no definitive proof that human pheromones exist, but some of the studies have been quite suggestive. In Martha McClintock's famous study, she found that dabs of sweat from a menstruating woman caused other women's menstrual cycles to fall into sync. McClintock couldn't find a chemical to explain the phenomenon, but Wyatt says that researchers are still hunting.

"[T]his one I think is the most likely - and this could have a really good practical application.... If you could genuinely interfere with female hormones in this way and affect the menstrual cycle you could end up with an effective, sniffable contraceptive" [BBC News]

, says Wyatt. In another study, researchers tested the brain responses of volunteers who smelled the sweat of either someone preparing to skydive for the first time, or someone under no stress.

The skydivers' sweat did indeed produce the signals in the parts of the volunteers' brains associated with fear, although it is unclear whether they did in fact feel scared [BBC News].

Although such evidence seems compelling, researchers still haven't been able to determine whether chemical compounds are producing the effect.

"We produce a large number of compounds, and bacteria ferment our secretions," said Wyatt. "You're trying to find a few active compounds from a forest of thousands of compounds" — and to top it off, they might only work in particular combinations, making their isolation even harder [Wired News].

Some research has suggested that pheromones are simply a communication system that was discarded somewhere along the evolutionary line that led to humans.

Recent research showed that at about the same time our primate ancestors gained color vision, they also lost the genes for so-called vomeronasal organ (VNO) receptors, Wyatt said. Non-human animals use the organ to detect pheromones. (Turns out, mice use both their VNO and main smelling system to detect pheromones, so maybe humans don't need that specialized organ.) "It may be at that point that we moved from running things mostly by pheromones to doing things much more in the visual fashion," Wyatt said [LiveScience].

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Image: flickr / emmma peel