Small, volatile organic molecules are of extreme importance among many animals for the transmission of information on sexual availability to members of the opposite sex. Such molecules are called pheromones, after a Greek word meaning "to transfer excitement".
Pheromones among animals
Female butterflies of the genus Bombyx release a chemical called bombycol. As little as 100 molecules is sufficient to evoke a sexual response from a Bombyx male. This could be compared with the one million molecules of botulinual toxin A (the most toxic substance known) required to kill a mouse.
Some flowers fool insects by using pheromones. The orchid Ophrys insectifera releases a mixture of chemicals which attracts male hymenopteras (insects) of the genus Argogorytes. Because of the odour the males believe the orchid flowers are females of their own species, and they try to copulate. Naturally, they are unsuccessful, but pollen grains of the orchid attach to them. The next time they try to copulate with an orchid flower, the pollen grains are transferred and they succeed in pollinating the flower even if not in impregnating a female of their own species.
Even large animals can make use of pheromones. Two researchers from the Oregon Graduate Institute of Science in Portland, Oregon, USA, (as reported in Nature, 22 Feb. 1996) used 4000 litres of elephant urine, looking for a substance released by female elephants just before ovulation. Apparently, this substance lets bull elephants know that the time is right for romance. Surprisingly, it turns out that this elephant pheromone, cis-7-dodecenyl acetate is the same compound used by some insects.
Pheromones among humans?
The human body secrets several compounds with strong smells, as well as compounds which can be transformed by bacteria into chemicals with a strong odor. Volatile aliphatic acids occur in the normal vaginal secretions of many primates, including humans. Their strong odors (e.g., butyric acid with its smell of rancid butter) have been shown to stimulate male monkeys to increased sexual activity.
Many steroidal hormones and related chemicals have a noticeable odor, including chemicals called androstenones. In one experiment, some seats in a theater were sprayed with one androstenone. Women among the audience showed a statistically significant preference for these sprayed seats.
In another trial, subjects had to choose the most attractive women from a collection of photographs. It turned out that when a subject could smell an androstenone at the same time as he or she regarded a certain photo, it increased the probability that the lady on the photo would be selected.
Humans have glands at the base of the hair follicles, especially in the armpits and in the genital region, which produce yet unidentified chemicals, the odors of which might affect members of the opposite sex. The chemicals are spread over the hair surface and then very efficiently dissipated.
One interesting phenomena in this context is the "women's dormitory syndrome", a condition in which women living closely together after a while begin to synchronize their menstrual cycles. This has been attributed to the effect of a pheromone present in the underarm sweat of women.
The great commercial interests in human pheromones make it virtually impossible to obtain reliable information on this subject. Already two compounds isolated from female and male sweat respectively are being marketed as perfumes with real activity as sexual pheromones. The price tags are, however, almost prohibitive and their effects unproven.
An old American custom, quoted in "The Scent of Eros", was for the man to keep a handkerchief in his armpit while dancing. After the dance he would present it to his partner. Supposedly the anticipated effect was that of an aphrodisiac. Maybe the arrival of easily available soap has changed the perception of human pheromones?
Scents and perfumes
Man has probably always used various odorous preparations to increase his or her attractiveness to the opposite sex. Is it possible that this actually is an attempt to mimic "human pheromones" or is it just to create an atmosphere of positive associations? One of the most popular perfume smells, that of musk, has been shown to resemble closely the smell of testosterone, the male sex hormone.
The Romans used perfumes lavishly, including perfumes based on civet and ambergris. The former is derived from the secretion of the civet-cat, and the latter from the sperm whale. Ambergris is more a carrier of scents than a perfume of its own. It has been used to restore vital powers to those exhausted for various reasons.
Even the smell of food can act as an aphrodisiac. Chicago neurologist Alan Hirsch rated male response to various smells by measuring changes in penile blood flow and found that food outperformed perfumes. The food highest on the rating list included cinnamon buns, roast meat and cheese pizza (but also, and less surprisingly so, chocolate, vanilla, strawberry and peppermint). In some cases the average increase of penile blood flow was 40 %!