Wednesday, July 18, 2007

What if the reason you were attracted to someone wasn’t because of good looks or intelligent conversation, but smell?

What if the reason you were attracted to someone wasn’t because of good looks or intelligent conversation, but smell? We’re not talking a good perfume here – just natural chemical signals that you sniff out entirely subconsciously. They draw you in, excite you, make your heart twitterpate.

It might sound a little mad, but scientists believe that subconscious behavioral responses to these communicative scents, called pheromomes, happen on a regular basis, in a variety of situations, to a multitude of species. And now, after years of debate over whether we have the right olfactory equipment to process pheromonal cues, evidence is mounting that these scents are at work in people.

Pheromones do all kinds of things: for example bees release pheromones when attacked by a predator, triggering aggression in neighboring bees. Ants mark trails with pheromones to lead them from food to nest. Sex pheromones are, of course, particularly interesting. The first was described in 1956 in the silkworm; just one sniff of a particular protein excreted by the female worms (or butterflies, actually), sent the males’ wings a flappin.

Even today, the best examples of sex pheromones come from insects, because their behavior is generally quite stereotypical – even if their courtship itself is quite bizarre.

Take the common fruit fly, Drosophila melanogaster. For an insect with only four pairs of chromosomes (compared to our own 23 pair), fruit flies have a courtship ritual that outshines even the most elaborately planned dinner date. One of the most in-depth studies of fruit-fly courtship and mating comes from Jeffrey Hall of Brandeis University. In 1994, in the journal Science, Hall described how the male fruit fly notices the female and begins to “tap” her abdomen and follow her around, showing his interest. Often, the male extends and vibrates his wing, producing the “love song” that launches a thousand maggots, so to speak.

If the female is receptive (i.e., she doesn’t skedaddle), the male engages in a kind of foreplay (touching her genitalia) before attempting to copulate. If the male fruit fly does not vibrate his wing fast enough, if he is too slow overall, or if he’s an unusual color, the female will almost certainly not mate with him.

The complex nature of the dance raises many questions; how do the males know which female to go for? And how do the females fend off the advances of other males once they’ve already mated?

It seems that females emit a pheromone which, combined with the right pheromone receptor in males, keeps things organized during these courtship displays. The pheromone, 11-cis-vaccenyl acetate (or ‘cVA’ for short), is unique. It’s the equivalent of an “I’m taken” sign, or a wedding ring. Olfactory receptor neurons (ORNs) in the males detect the pheromone prompting them to steer clear of the taken ladies and move on to the single ones.

This simple yet effective dating system was described by Aki Ejima and his colleagues at the University of Lausanne in Switzerland in the April 2007 issue of Current Biology.

But the tidiness of these results all but disappears when the pathway short-circuits, suggesting we haven’t put together all the pieces of the Drosophila love match. If male fruit flies lack ORNs, they start chasing OTHER males around and attempt to copulate with them. The normal males run for their lives.

If we haven’t got a handle on simple, well-studied creatures like flies, what of more complex organisms, such as humans? For starters, many physiologists doubt whether we even possess the necessary nose to detect pheromones. In all other mammals, pheromone detection takes place in a deep portion of the nasal cavity called the vomeronasal organ. Studies from the 1940s found that most adult humans did not have such an organ deep in their nose and in those that did (25% or thereabouts), it was just a useless remnant of evolutionary history.

But there is overwhelming evidence that some smells affect our behavior profoundly. The most famous example is probably the research of Martha McClintock of the University of Chicago. In her 1971 paper in the journal Nature, she argued that women’s menstrual cycles become synchronized when they live in proximity, and that this is due to unconscious odor cues, or pheromones.

Several other studies also suggest that pheromones are at work on a daily basis in people. For example women tend to prefer the smell of sweat-drenched T-shirts from men who differ from them in an important set of immunity genes. The theory is that such opposites-attract pairs would have kids with particularly diverse, and hence well-armed, immune systems.

Likewise, women tend to prefer the scent of manly, dominant men during their fertile phase. Men prefer the smell of women’s sweaty T-shirts worn during their more fertile menstrual phase. Heterosexual and homosexual men and women show strikingly different patterns of body odor preferences, and even the presence of biological fathers in a young girl’s life can affect the age at which she sexually matures.

However encouraging these behavioral studies are, researchers still come up dry in the hunt for the chemical pathways involved.

But there is new hope. A 2006 study published in Nature by Stephen Liberles and (nosy Nobel Prize Winner) Linda Buck of the Howard Hughes Medical Institute seemingly found pheromone receptors in the normal part of mice noses - no vomeronasal organ required. These receptors, called trace amine-associated receptors, or TAARs respond to compounds in mouse urine. Two such chemicals are present in different concentrations in male and female urine and one has been linked to the maturation rate of females. The genes that encode TAARs are also found in the human genome.

Though it is early days, this recent study shows that humans may yet show evidence of pheromones, perhaps even sex pheromones, and we may finally be able to keep up with the fruit fly when it comes to understanding sex.
Posted by Ajay :: 9:40 AM :: 0 comments

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