Biologist Katherine Wynne-Edwards, who conducted the research with Storey, explains that while cortisol is seen as the "fight or flight" hormone, it might more accurately be described as the "heads-up-eyes-forward-something-really-important-is-happening" hormone. It may help prepare parents for approaching birth. Cortisol levels normally increase in women as pregnancy advances; indeed, a cumulative rise in stress-hormone levels sets off labor and delivery.
The third hormone, testosterone, is abundant in male animals during mating but decreases during nurturing. If bird fathers are given testosterone, they spend more time defending their territory and mating than taking care of existing offspring. Research has shown that human males experience a surge in testosterone when they win sporting events and other competitions.
In Storey's study, testosterone levels plunged 33 percent in fathers during the first three weeks after birth. Levels then returned to normal by the time the babies were four to seven weeks old. However brief the dip in testosterone, it may have effects that endure for the life of the child. According to University of California at Riverside psychologist Ross Parke, it may "let the nurturing side of men come to center stage." The dip may set in motion the more-cooperative, less-competitive enterprise of parenting. By encouraging fathers to interact with their kids, this brief hormonal change might actually induce the bonding process.
Wynne-Edwards and graduate student Sandra Berg designed another study to test Storey and Wynne-Edwards' earlier findings. They measured the hormone levels of the fathers over a longer period of time and incorporated into the study a control group of men who had never had children. The control group was matched by age, season and time of day tested—all of which can affect hormone levels. Finally, by using saliva samples instead of blood draws, they were able to test the fathers and the men in the control group much more frequently.
In addition to confirming the earlier findings for testosterone reduction and cortisol change, the researchers also found that the fathers had elevated levels of estrogen. The increase started 30 days before birth and continued during all 12 weeks of testing after birth. Although estrogen is best known as a female sex hormone, it exists in small quantities in men, too. Animal studies show that estrogen can induce nurturing behavior in males.
Acting in the brain as well as in other parts of the body, estrogen in men, and testosterone in women, makes humans extremely versatile behaviorally. "We spend an awful lot of time looking for differences between the sexes and trumpeting them when we find them," observes Wynne-Edwards, "but our brains are remarkably similar, built from the same DNA."
In fact, going into the study, Wynne-Edwards predicted that the "daddy brain" would use the same nerve circuits, triggered by many of the same hormones, as the "mommy brain." "If Mother Nature wanted to turn on parental behavior in a male," she reasoned, "the easiest thing would be to turn on pathways already there for maternal behavior."
The studies also found that a father's hormonal changes closely paralleled those of his pregnant partner.
The Intimacy Effect
The researchers believe that intimate contact and communication between partners may induce the hormonal changes that encourage a father to nurture his children. Storey explains, "My best guess is that women's hormone levels are timed to the birth—and men's hormone levels are tied to their partners."
Exactly how this occurs is unknown. There may be actual physiological signals exchanged between partners in close contact, such as the transmission of pheromones. Similar to odors, pheromones are volatile chemical substances that animals constantly give off through their skin or sweat but that are undetectable. Pheromones can stimulate specific reactions—especially mating—in other animals. Think of a female dog in heat attracting all those barking mate dogs in the neighborhood.
Classic studies show that menstruation is communicated, and synchronized, through pheromones among dorm mates in college. If women in dorms respond to one another's pheromones, then a man and a woman who share intimate space could certainly communicate chemical messages. These pheromones could biologically cue a man that his partner is pregnant and kick off the hormonal changes that prompt him to be a dad in deed as well as in seed. Pregnancy certainly could, in fact, be signaled.
The level of intimacy within a couple seems to be a factor in how a mother's body chemically signals approaching birth to a father. All of the men tested were living with their pregnant partners. Emotional closeness may also generate hormonal changes, although this possibility was not examined in detail. Still, couples reported feeling closer to their partner if they were taking about the baby and sharing details about the pregnancy.
Whether this is the cause or the result of hormonal changes remains unknown for now. But the intimacy effect and the subsequent hormonal shifts may also be the reason many men experience pregnancylike symptoms.
Honey, We're Pregnant
When he is not taking care of Seamus, Hudnut treats both men and women in his practice. He recalls several patients who came to him complaining of such typical pregnancy symptoms as weight gain and nausea—all of whom were men. He remembers one second-time father who knew that his wife was pregnant even before she told him. He started having morning sickness, just as he had during her first pregnancy.
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