April 13, 2009
Intra-species communication is ubiquitous and continuous among animals. While much of this communication can be witnessed, easily seen and heard, other forms are less overt and identifiable by the human eye or ear, for example. Pheromones are one example of such discrete forms of communication. Pheromones are chemical substances released by one animal that trigger hard-wired behaviors in other members of the same species. While the most abundant research has been done on the role of these subliminal signals among non-primate species, there have been some studies focusing on humans. Overall, such research has revealed that the potential exists for pheromones to strongly affect human behavior and desire specifically that involving sexual attraction. I, however, am most interested in the way that pheromones affect female-female interactions. I begin by examining female menstrual synchronization and go on to propose a possible link between fertility and leadership.
In most animal species, pheromone detection depends on the vomeronasal organ (VMO), a pair of mini-nostrils within the nasal cavity which perceive airborne pheromones and convey messages to the brain. Although the VMO is present in humans, it is unclear whether we posses the neuronal links necessary for pheromone information to be relayed to the brain. In other words, the debate continues as to whether in humans, the VMO is, like the appendix, a vestigial organ. Nonetheless, studies of human behavior indicate that pheromones are a real and pervasive form of communication between people. Apocrine glands, located in the underarm and pubic regions, are the main producers of pheromones. These glands develop in the embryo but become functional with the onset of puberty. (1, 2)
Martha McClintock’s 1971 study on female menstrual synchronization encouraged scientists to revisit the long-dismissed possibility of human pheromones. McClintock found that the menstrual cycles of women living together in a ventilated building, such as a college dormitory, gradually come to synchronize so that all women are on a single cycle, ovulating and menstruating in tandem. This initial study led McClintock to propose the possible existence of human pheromones. It was not until 1998 that she found evidence to support the idea that a chemical compound perceived through the olfactory system could account for the menstrual synchronization, now referred to as the “McClintock Effect,” that she saw twenty-seven years earlier. McClintock’s study involved placing odorless samples of other women’s sweat on the upper lip of female test subjects. She observed that the test subjects’ cycles either shortened or lengthened, depending on the donor’s cycle stage, in response to the exposure. Her study revealed that human axillary compounds regulate the biological rhythms of other humans. Still, scientists remained skeptical, criticizing McClintock for not isolating the specific chemical or pathway responsible for such an effect. (1)
Having spent the last four years in a female dormitory, I have personally witnessed the McClintock Effect. Still, it is unclear to me whether such synchrony is due to the averaging of disparate cycles (two cycles meeting in the middle), symmetric synchronization, or the conforming of one woman’s cycle to another, asymmetric synchronization. The latter situation requires a cycle-leader, i.e. a woman who sets the rhythm that the others will adopt. The question naturally arises as to what allows for such physiological leadership. Perhaps there is variation in pheromone strength and abundance, allowing some women to emit more powerful signals. (NB: I have not found any research definitively revealing whether pheromones are emitted continuously or in spurts and whether an individual’s pheromone production is constant throughout her post-pubescent life.) Maybe the brain only responds to the most salient pheromone and ignores the more subtle ones. Let’s say there’s a group of five women and that pheromone potency could be measured on a scale of one to five, five being the most intense. Odds are that at least one of these women is a five. If my hypothesis is correct, then the pheromones released by the women in the one to four range are ignored, becoming background odor not so different from white noise. As a result, the four other women would synchronize with number five’s cycle. On the other hand, I wonder if the cycle of a level four woman, though unable to influence the cycle of others, might be strong enough to avoid synchronization.
Perhaps the more interesting question is whether being a cycle-leader correlates with having other physical and even social attributes. For example, given that some women are more fertile than others, it seems reasonable to wonder whether these more fertile individuals are also the cycle-leaders. It hardly seems farfetched to think that fertility correlates with cycle strength which is governed by hormone production. Gynecologists often prescribe hormone supplements or oral contraceptives (pills containing a combination of estrogen and progesterone among other things) to women with hormone imbalances. Without these pills, such women menstruate extremely irregularly or not at all and are often unable to get pregnant, since amenorrhea typically correlates with a lack of ovulation, without which there is no egg to be fertilized. (It is rare but not impossible for a non-menstruating woman of fertile age to become pregnant.) If more regular and abundant hormone production causes a stronger cycle and increased fertility, then maybe cycle-leaders are the more fertile women.