How’s That for Female Bonding?
Gal pals know that hanging out together produces some uncanny shared behaviors: voice inflections, inexplicable urges to get the same haircut, waistline-threatening binges and--perhaps strangest of all--synchronized menstrual cycles.
This menstrual “coupling” was first documented 24 years ago by biopsychologist Martha McClintock, now a professor at the University of Chicago. She logged the menstrual onset of 135 dormitory residents at the beginning of Wellesley College’s school year. Within four months, the cycles of close friends and roommates came within four days of each other.
The causes of menstrual synchrony became clearer in 1986, when organic chemist George Preti at the University of Pennsylvania’s Monell Chemical Senses Center and his colleagues daubed an extract of underarm sweat from women on the upper lips of 10 women. Within four months, the subjects’ periods had adjusted themselves to begin within days of the donors’ date of menstrual onset. Nine women who received a placebo swab of ethanol on their upper lips experienced no change in their cycles.
The study produced the first human evidence of pheromones, aromatic chemical compounds emitted by individuals that affect the sexual physiology of others. Scientists have always known that animals secrete pheromones, which are airborne hubba-hubba signals that function as a sex attractant.
McClintock said menstrual coupling is an example of nature’s “mutual entrainment,” the tendency for biological entities to hook up and move together. Fireflies signal together; schools of fish move their fins synchronously.
The effect is most powerful when two women spend a lot of time together. Israeli studies of mothers and daughters suggest that the effect endures over time, McClintock said.
But the effects of one woman’s pheromones on another woman’s glands appear to require more than shared space. Cynthia Graham, a researcher at the Kinsey Institute, found that women who are simply roommates are less prone to synchronous cycles than roommates who are also close friends.
“It has been suggested that when women spend time together, they are kind of emotionally aroused and that it creates some kind of heightened pheromonal activity,” Graham said.
Male sweat also seems to help set women’s reproductive clocks. In another study, women with irregular periods moved toward regular cycles after their upper lips were regularly daubed with men’s underarm sweat. (These research subjects must have been paid.)
But McClintock, who is currently studying how female rats orchestrate their reproductive cycles to give birth at the same time, said human pheromones may affect ovulation, pregnancy and lactation.
McClintock’s research on female Norway rats shows that their pheromonal signals increase or suppress their neighbors’ fertility. They give birth together, benefiting from the rat equivalent of a mother’s support group with a wet-nurse “my-milk-is-your-milk” mentality.
(This is fabulous when you’ve got 12 mouths to feed. Drained rat moms can dump their lot on lactating girlfriends, which can nurse up to 32 pups, and dive into a pile of shredded paper for a snooze. Two-thirds more pups per litter survive because of communal mothering.)
As good as shared breast-feeding sounds, communal mothering in the ‘90s is not likely. But, hey, menstrual synchrony and pheromonal influences could come in handy. The unorganized woman who never knows when to expect her next period can just ask her bud.
And women with irregular cycles who need to get on track for fertility purposes: Pump it up on a StairMaster next to a buffed, sweating he-man, and ask if he’d mind if you stuck your nose in his pit.
