Q: What is a pheromone? - Layperson

A: A pheromone is any chemical or set of chemicals produced by a living organism that transmits a message to other members of the same species. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Their use among insects has been particularly well documented, although many vertebrates and plants also communicate using pheromones. The existence of human pheromones is debated.

Women who work or live together tend to get their menstrual cycles in sync. That curious phenomenon known for years by scientists and many ordinary folk, has long been suspected as an indication that humans, like insects and some mammals, communicate subtly by sexual aromas known as pheromones. Last week Philadelphia researchers weighed in with two reports showing that scents, including under arm odors, do indeed affect menstrual cycles.

The reports came with a kicker: male scents play a role in maintaining the health of women, particularly the health of the female reproductive system. Researchers at the Monell Chemical Senses Center and the University of Pennsylvania School of Medicine have found that women who have sex with men at least once a week are more likely to have normal menstrual cycles, fewer infertility problems and a milder menopause than celibate women and women who have sex rarely or sporadically. So the researchers were hardly tentative about the meaning of it all.

"What we're saying here is that men are really important for women," said Winnifred Cutler, a biologist and specialist in behavioral endocrinology who conducted the study along with Organic Chemist George Preti. "If you look at all the data, the conclusion is compelling. A man or his essence seems essential for an optimally fertile system."

Insect pheromones of pest species, such as the Japanese beetle and the gypsy moth, can be used to trap them for monitoring purposes or for control by creating confusion, disrupting mating and preventing them from laying eggs. Bombykol is a chemically well-characterized pheromone released by the female silkworm to attract mates.

In mammals and reptiles, pheromones may be detected by the vomeronasal organ (VNO),( A small sac on either side of the nasal septum, containing receptor cells that pick up chemical signals (pheromones) from other organisms of the same species) or Jacobson's organ, which lies between the nose and mouth, although some are detected by regular olfactory membranes.

This signaling system is particularly important to animals that are inexperienced sexually. Experiments by Michael Meredith, a neuroscientist at Florida State University in Tallahassee, Charles Wysocki, of the Monell Chemical Senses Center in Philadelphia, and others have shown that the VNOs play a key role in triggering sexual behavior in naive hamsters, mice, and rats.

A virgin male hamster or mouse whose vomeronasal organs are removed generally will not mate with a receptive female, they found, even if the male's main olfactory nerves are undamaged. Apparently, the VNOs are needed to start certain chains of behavior that are already programmed in the brain.

Losing the VNOs has a much less drastic effect on experienced animals, says Wysocki, who has been studying the VNOs for about 20 years. When male mice have begun to associate sexual activity with other cues from females, including smells, they become less dependent on the VNOs. Sexually experienced males whose VNOs are removed mate almost as frequently as intact males.

Do human beings have VNOs? In the early 1800s, L. Jacobson, a Danish physician, detected likely structures in a patient's nose, but he assumed they were non-sensory organs. Others thought that although VNOs exist in human embryos, they disappear during development or remain "vestigial"—imperfectly developed.

Recently, researchers have come to a different conclusion. Both VNOs and vomeronasal pits—tiny openings to the VNO in the nasal septum—have been found in nearly all patients examined by Bruce Jafek, an otolaryngologist at the University of Colorado at Denver and David Moran, who is now at the University of Pennsylvania's Smell and Taste Center in Philadelphia.

"This has opened up the possibility of a new sensory system in humans," says Rochelle Small. "We were often told that the VNO does not exist in adults, so we have taken a big step just to show that the structure is there."

She cautions that we still don't know whether this organ actually has connections to the brain, however. "The question now," she says, "is what its function might be."

Terence McKenna proposed in his book "Food of the Gods" the controversial idea of exopheromones as chemical signals between, as opposed to among a single, species. He suggested that certain chemicals produced in abundance in various hallucinogenic plants and fungi, such as dimethyltriptamine and psilocybin may act as pheromones produced by one species (the vegetal) waiting for absorption by various others (for example, early primates or hominids). In this way a kind of ecological pheromonal system may be at work among species and ecosystems that have coevolved closely for long stretches of time

The term "pheromone" was introduced by Peter Karlson and Adolf Butenandt in 1959, based on the Greek pherein (to transport) and hormon (to stimulate). They proposed the term to describe chemical signals from conspecifics which elicit innate behaviours soon after Butenandt named it "bombykol" for the silkworm moth, "Bombyx mori" from which it was extracted. It signaled, "come to me!" from great distances. "It has been soberly calculated that if a single female moth were to release all the bombykol in her sac in a single spray, all at once, she could theoretically attract a trillion males in the instant," wrote Lewis Thomas in The Lives of a Cell.

Types of pheromones

Territorial pheromones

Laid down in the environment, these pheromones mark the boundaries of an organism's territory. In dogs, these hormones are present in the urine, which they deposit on landmarks serving to mark the perimeter of the claimed territory.

Trail pheromones

These pheromones are common in social insects. For example, ants mark their paths with these pheromones, which are non-volatile hydrocarbons.

Certain ants lay down an initial trail of pheromones as they return to the nest with food. This trail attracts other ants and serves as a guide. As long as the food source remains, the pheromone trail will be continually renewed. The pheromone must be continually renewed because it evaporates quickly. When the supply begins to dwindle, the trailmaking ceases. In at least one species of ant, trails that no longer lead to food are also marked with a repellent pheromone.

Alarm pheromones

Some species release a volatile substance when attacked by a predator that can trigger flight (in aphids) or aggression (in bees) in members of the same species. Pheromones also exist in plants; certain plants emit alarm pheromones when grazed upon, resulting in tannin production in neighboring plants. These tannins make the plants less appetizing for the herbivore.

Sex pheromones

In animals, sex pheromones indicate the availability of the female for breeding. Certain lepidopterans (butterflies and moths) can detect a potential mate from as far as 10 km. Pheromones can be used in gametes to trail the opposite sex's gametes for fertilization. Pheromones are also used in the detection of estrus in sows. Boar pheromones are sprayed into the sty, and those sows which exhibit sexual arousal are known to be currently available for breeding.

Male animals also emit pheromones that convey information about what species they are, and their genotype.

Epideictic pheromones

Recognized in insects, these pheromones are different than territory pheromones. According to Fabre (translated from French), "Females who lay their eggs in these fruits deposit these mysterious substances in the vicinity of their clutch to signal to other females of the same species so that they will clutch elsewhere."

Aggregation pheromones

Produced by one or the other sex, these pheromones attract individuals of both sexes.