Pheromones
Powerful messages can be delivered by smell, a fact known long before the means by which they did so was discovered. The ancient Greeks knew that secretions from a female dog in heat were attractive to male dogs. In his beekeepers’ manual The Feminine Monarchie, published in 1623, Charles Butler warned that an injured bee’s “ranke smell” would attract other angry bees to sting. In the late 19th century, New York’s first state entomologist, Joseph Lintner, noted the crowds gathered on the sidewalk outside, looking at the spectacle of 50 wild silk moth males attracted to a virgin female moth in his office window. Lintner correctly observed that the attraction was probably a chemical, detected by the large, elaborate antennae of the males. He predicted that such an irresistible and farreaching force could be used to control agricultural pest moths, if only chemists could identify and synthesize these powerful molecules, now known as pheromones.
Lintner’s prediction has been generously fulfilled. But because the quantity of pheromone produced by each animal is so small, it was almost 70 years before the first pheromone could be chemically identified, in 1959, by the Nobel Prize-winning German chemist Adolf Butenandt and his large team.
In the half century since then, as the technology for isolating and identifying trace amounts of compounds has become more refined, pheromones have been found in almost every kind of animal, in squid, lobsters, ants, fish, salamanders, and mice, to name just a few. Although pheromones are important in many species for finding mates for sex, they can have a wide variety of other functions, such as one produced by mother rabbits that prompts suckling by their pups. In many social insects, such as ants, bees, and wasps, almost every part of colony behavior is mediated by pheromones, from queen signals affecting worker reproduction within the colony to Butler’s “ranke smell” alarm pheromone that activates colony defense against enemies.
Pheromones are chemical signals that have evolved for communication between members of the same species. A pheromone signal elicits a specific reaction in the receiver, for example, a stereotyped behavior (releaser effect) or a developmental process (primer effect). Some pheromones can have both effects.
Most pheromones are detected by the sense of smell. However, not all smells are pheromones. Mammals, including humans, also give off a cloud of molecules that represent our unique individual “smell” or chemical profile.
To definitively demonstrate that a pheromone exists, one must design a repeatable experiment, a bioassay, that shows that a smell molecule (odorant) causes a particular effect on the receiver, for example inducing a behavior.
It is highly likely that humans, like other mammals, have pheromones. However, despite the many sites on the Internet offering to sell “pheromones” to make one sexually irresistible, no human pheromones have been chemically identified to date. Sadly, there is no evidence for the widely published claims that the molecules androstadienone and estratetraenol are human pheromones of any kind.
Another misuse of the term pheromone is in so-called pheromone parties, a trend started by an artist in Brooklyn in 2010. At these events, singles sniff numbered T-shirts that have been worn by others at the party to see if they like the smell and photograph themselves with the shirt in a numbered bag to facilitate meeting up later. Although these get-togethers might sound like fun, they are misnamed, because the smells on the shirts are unique to each individual, not species-wide pheromones that trigger the same behavior in every person. The idea behind these parties is based on the surprising observation that mice preferred to mate with mice that were immunologically different from themselves in the major histocompatibility complex (MHC), a difference that can be detected by smell. When a similar experiment was tried with Swiss students sniffing T-shirts, it was discovered that women similarly found the smell of men immunologically different from themselves more attractive. However, subsequent replications of the experiment have not shown the effect consistently and, disappointingly, studies of large-scale human genome data have been equally inconclusive. Clearly, the idea of choosing a potential partner by smell has popular appeal even though the evidence is not there yet.
The biggest challenge for work on human pheromones will be the identification of behaviors and physiological responses that will make robust bioassays. Because of our cultural complexity and diversity, influences on human behaviors are notoriously difficult to study.
American Scientist