By Beth Azar
Monitor staff

The term pheromone has been bandied about in the press and the scientific literature lately, with recent claims that humans, like most other animals, secrete and detect these invisible, sometimes even odorless chemical messengers.

Pheromones are best known for their ability to attract members of the opposite sex?something that?s been proven in insects and rodents. But researchers find that such chemical signals also provide animals with information about each others? identity and trigger hormonal changes that stimulate ovulation and sexual maturation, squelch aggression and even precipitate abortion.

What hasn?t been clear is whether humans have pheromones that can elicit similar responses. Most mammals can detect pheromones through their olfactory system as well as another organ inside the nose, called the vomeronasal organ, or VNO. But scientists have been debating since the early 1800s whether the human nose can detect pheromones and whether it has a VNO. Now, one group of researchers says it has evidence that humans have an active VNO and that we secrete and detect pheromones. They?ve even gone as far as marketing ?human pheromones? in several perfumes.

Other researchers are more cautious. Humans appear to have the remnant of a VNO buried deep inside the nasal cavity, they admit. But studies have failed to find active receptors or links from this structure to the brain. While there seems to be some form of chemical communication between humans, they say, it?s unclear whether pheromones or the VNO is involved. Even in rodents, chemical communication is complex, involving more than the VNO: Chemical signals may include a mix of specialized pheromones and other odors that come from a variety of sources, including glands, urine and saliva. And animals may detect these signals through the olfactory system, the VNO or both.

Pheromones were originally discovered in insects. Researchers found specific chemicals that, when emitted by one member of species, affected another member of the species in a predictable way?attracting a male insect to a female insect or stalling sexual maturation in female bees who then became worker bees.

Most of the work on mammalian pheromones has been conducted in rodents, particularly rats, field mice and hamsters. Researchers have identified a few specific chemical signals in mammals that could be called pheromones. But they have also found that chemical mixtures contained in scent gland secretions, saliva, urine and sweat can provide equally strong signals.

Researchers believe that mammals have at least two types of pheromones:

? Primer pheromones cause a shift in the endocrine system of the receiving animal. For example, young female mice exposed to chemical signals of adult males mature faster. And a signal from a male mouse of one strain can induce a miscarriage in a female pregnant by a male of another strain, up to 17 days into a 21-day pregnancy.

? Releaser pheromones elicit specific behavior patterns. One of these is responsible for the ?sexual attraction? affect: Male hamsters, for example, are attracted by a secretion females release in greatest amounts just before they ovulate. Also, stressed rats produce a signal that alerts other rats to beware.

Other signals, which some researchers call ?information pheromones,? provide information about an animal?s identity. Animals can sniff another of their species and know what they ate last, how dominant they are, how healthy they are and whether they?re in heat, says Cornell University psychologist Robert Johnston, PhD. He finds that hamsters can use a mixture of olfactory and pheromonal secretions to identify individuals. It?s unclear whether this type of chemical communication is pheromonal because it only provides information and doesn?t elicit behavioral or chemical changes in other animals.

Pheromones themselves seem to be critical to sexual behavior in some mammals, including hamsters, rats and pigs. They allow males and females to find each other and they induce mating behavior in receptive animals. However, learning also appears to be critical to mating behavior, research shows. In fact, some pheromones may simply help lay down a behavioral scaffolding in young animals. Social learning then takes over as the animals age, says Charles Wysocki, PhD, of Monell Chemical Senses Center.

For example, Michael Meredith, PhD, professor of neuroscience at Florida State University, found that if he removed the VNOs from young male hamsters before they?d had an encounter with a female, they wouldn?t mate normally when they finally had the chance. But if he waited to remove their VNOs until after they?d had some experience with females, they behaved normally.

Meredith, who now studies the brain circuitry of hamsters? chemosensory system, finds that when the VNO is removed from an experienced male, there is still activity in the brain areas normally stimulated by signals coming from the VNO. This indicates that there is still chemosensory input to these brain areas, probably through the olfactory pathway, says Meredith. It may be that, with experience, hamsters learn to pick up on olfactory cues in place of pheromonal cues, says Meredith.

Human pheromones?

It seems clear that human mating is governed by much more than chemical signals, say researchers. But scientists have become intrigued by the possibility that some of our behavior may be subtly affected by chemicals secreted by our peers.

David Berliner, PhD, and his colleagues at the University of Utah have evidence that they say proves humans have a functional VNO. They?ve measured electrical activity in VNO tissue in response to chemical stimulation, which may indicate that the cells are transmitting signals. (However, cells that generate electrical activity are not necessarily transmitting signals to the brain, says Wysocki.)

And they?ve purified several potential pheromones found in sweat and other human secretions, says Berliner?s colleague Louis Monti-Bloch, MD, PhD. One secretion?purified from the skin of men?seems to affect mood in women. In one study, Monti-Bloch applied either the secretion or a placebo directly to the VNO of 40 women. The women exposed to the secretion displayed a statistically significant decrease in negative affect, says Monti-Bloch.

Other researchers find the Utah researchers? result intriguing, but not conclusive. ?Clearly there?s something present in humans but it is quite different than in animals,? says Meredith.

Wysocki says there must be chemical communication among humans. Infants respond to breast pads worn by their mothers but not those worn by other women, and a mother can identify the

T-shirt worn by her infant from a pile of T-shirts.

?We know it?s chemical, but is the information being transferred pheromonal?? asks Wysocki. ?No one knows.?

Effects on immunity

One intriguing finding in humans may extend findings in animals linking the pheromonal system and the immune system, says Johnston. Animal research shows that even among strongly inbred strains of mice?ones that share virtually all the same genes?individual mice will choose mates that differ from them only in terms of the major histocompatibility complex (MHC), which determines immune function. The more diverse an animal?s MHC, the better prepared it is to fight off disease. These results imply that animals somehow detect information about others? MHC?most likely through some type of odor signal or odor-pheromone mixture, says Johnston.

Geneticist Carole Ober, PhD, and her colleagues at the University of Chicago may have evidence for the same type of mate selection in humans. She studies the Hutterites, a religious group that has lived in virtual isolation in the United States and Canada since the 1870s. In a study recently published in the American Journal of Human Genetics (Vol. 61, p. 497?504), Ober and her colleagues examined the genetic makeup of 411 married couples.

They found that fewer than expected Hutterites selected a spouse who was similar to themselves in terms of MHC genes. Indeed, couples rarely shared a similar cluster of immune system genes, something that researchers would expect to occur quite often in such an inbred population.

In most cultures, it?s highly unlikely that two unrelated people will have matching immune system genes, says Ober. But it appears that, at least in a closed culture where the risk of inbreeding is high, people are able to discriminate individuals based on immune system genes.

It will be hard to determine whether the Hutterites are responding to some type of human pheromone. At the moment, the researchers with the most promise of finding human pheromones are those examining the menstrual ?synchrony? seen in women who live or work together, says Wysocki. University of Chicago psychologist Martha McClintock, PhD, received notice as an undergraduate student at Wellesley in the 1970s for an experiment proving that women who live together eventually develop synchronous menstrual cycles.

As McClintock turned to rats to search for a chemical signal that might be behind this phenomenon, she realized that synchrony is one special example of a more complex system. Her research finds that, at least in rats, two pheromones are at work: one shortens the female reproductive cycle, the other lengthens the cycle. Synchrony occurs when the two pheromones work in consort to ensure that all the females in a group ovulate at the same time.

But the same pheromone can act to inhibit ovulation altogether. In rats, for example, McClintock finds that a female rat that has stopped ovulating secretes a pheromone that suppresses ovulation in other female rats.

McClintock is currently testing whether changes in the human menstrual cycle are controlled by a similar set of pheromones.