What a pheromone is
There are alarm pheromones, food trail pheromones, sex pheromones and many others that affect behavior or physiology. Pheromones are used by many organisms, from basic unicellular prokaryotes to complex multicellular eukaryotes. Their use among insects has been particularly well documented. Some vertebrates, plants and ciliates also communicate by using pheromones. Establishing a pheromone relies on demonstration of an odour-mediated behavioural or physiological response, identification and synthesis of the bioactive molecule(s), followed by bioassay confirmation of activity.
In animals and plants
Some species release a volatile substance when attacked by a predator that can trigger flight in aphids or aggression in ants, bees, termites and wasps. Certain plants emit alarm pheromones when grazed upon, resulting in tannin production in neighboring plants. These tannins make the plants less appetizing to herbivores. An alarm pheromone has been documented in a mammalian species. Androstenone was the first mammalian pheromone to be identified. Androstenone is found in high concentrations in the saliva of male pigs, and when inhaled by a female pig that is in heat it results in the female assuming the mating stance.
Bombykol
Bombykol is a pheromone released by the female silkworm moth to attract mates. It is also the sex pheromone in the wild silk moth, Bombyx mandarina. The preferred IUPAC name of bombykol is (10E,12Z)-hexadeca-10,12-dien-1-ol. The female moth makes bombykol from hexadecanoic acid. In the last step, a reductase enzyme converts the carboxylic acid group into a primary alcohol. The pheromone is found in glands at the end of the female abdomen.
Unlike the wild silk moth, the domesticated silk moth cannot fly, but the male responds to the female's pheromone by getting excited and flapping his wings in what is known as a flutter dance. Bombykol molecules diffuse through open pores in the hairs on the moth's antennae and reach a receptor protein in the membrane. A pheromone binding protein binds the bombykol in a pocket and transports it to the receptor.
Human pheromones
As humans are mammals, it is possible, perhaps even probable, that we have pheromones. There is no robust bioassay-led evidence for the widely published claims that four steroid molecules are human pheromones: androstenone, androstenol, androstadienone and estratetraenol. Positive results in studies of these molecules need to be treated with scepticism, as they are highly likely to be false positives. Studies of the steroids claimed as human pheromones have suffered from small sample sizes, publication bias, false positives and poor methodology.
Experiments have focused on three classes of possible human pheromones: axillary steroids, vaginal aliphatic acids, and stimulators of the vomeronasal organ. Axillary steroids are produced by the testes, ovaries, apocrine glands and adrenal glands. Androstenol is present in the axillary sweat of humans. Androstenone is synthesized from androstadienone by 5α-reductase, and can be converted into 3α-androstenol or 3β-androstenol by 3-ketosteroid reductase.
In a 1978 study, people wearing surgical masks treated with androstenol rated the pictures they were shown as warmer and more friendly. Androstadienone has been reported to significantly affect the mood of heterosexual women and homosexual men, but it does not alter behavior overtly. Claus Wedekind and colleagues found that both men and women prefer the axillary odors of people whose MHC is different from their own.
One of the most promising human pheromone leads is a nipple secretion from the areola glands produced by all lactating mothers, which stimulates suckling by any baby not just their own. It may be that we will find that there are no pheromones in humans.
The vomeronasal organ
In reptiles, amphibia and non-primate mammals pheromones are detected by regular olfactory membranes, and also by the vomeronasal organ (VNO), or Jacobson's organ. The organ is present and functional in all snakes and lizards and in many mammals, including cats, dogs, cattle, pigs and primates.
An active role for the human VNO in the detection of pheromones is disputed; while it is clearly present in the fetus it appears to be atrophied, shrunk or completely absent in adults. The genes that encode the VNO receptors are nonfunctional pseudogenes in humans. Although there are sensory neurons in the human VNO, there seem to be no connections between the VNO and the central nervous system. The associated olfactory bulb is present in the fetus, but regresses and vanishes in the adult brain.
Pheromone receptor genes have also been found in olfactory mucosa. It is disputed whether the chemicals reach the brain through the VNO or other tissues. A review of the scientific literature concluded that on current evidence most in the field are skeptical about the likelihood of a functional VNO in adult human beings.
