Men may not go on a hormonal rollercoaster with their pregnant partners, but once the baby shows up, their bodies biologically transition into "daddy mode," suggests a new study finding that levels of testosterone, the "macho" sex hormone, drop in new fathers.
"Men are, to a certain degree, hardwired to take care of their kids," study researcher Lee Gettler, of Northwestern University in Chicago, Illinois, told LiveScience. "This is important because traditional models of human evolution have portrayed women as the gatherers that take care of the kids and stay behind."
The hormone drop makes sense, the researchers say, since high testosterone tends to boost behaviors linked to competing for a mate, risky activities that may conflict with the responsibilities of fatherhood. [History's 12 Most Doting Dads]
In fact, the biggest testosterone drops were observed in fathers of newborns and those highly invested in child care.
The finding that fathers are hardwired to care for children adds to previous cultural models of human evolution, which traditionally depict the mother as being hardwired for hands-on child care.
The study followed 465 men participating in the Cebu Longitudinal Health and Nutrition Survey, started in the Philippines in 1983, when the participants were 1 year old. At age 21.5 (in 2005), the researchers tested the single male participants' testosterone levels when they woke and when they went to sleep. The measurements were repeated at age 26 (in 2009), when about half of the participants had become fathers.
Men who stayed single showed a small age-related decline of about 12 to 15 percent in the male sex hormone, while the testosterone levels of new fathers — those with a baby between 1 month and 1 year — on average dropped about 30 percent. Hormone levels in fathers of newborns (1 month and younger) dropped four to five times lower than levels in single men levels and twice as much as fathers of older children.
"Newborn babies come with really intense physical, emotional and psychological changes," Gettler said. "We kind of see men's biology responding to that, in line with what we would expect in men trying to transition into this new role of being a father to a newborn."
As to the effect of the lowered testosterone, the researchers can't be sure. There could be effects on libido and muscle mass, though they are probably mild, since the participants' levels are still within the normal range.
"The reason sex life changes is a lot more complicated than that a father's testosterone levels go down," Gettler said. "The reality is there's not a terribly strong relationship between testosterone levels and libido."
Lower testosterone could influence the amount of time a man spends with his family, essentially by tempering his urge to go out and reproduce. Higher testosterone has been associated with increased risk-taking and competition with other males. This could be why testosterone levels are even lower with increased child care investment.
"When fathers make this choice, this active decision to be involved, testosterone reacts to that by going down even further," Gettler said. "Their bodies respond by saying, 'This is where we are focused now, we are focused on the kids.'"
The finding may also explain why having a partner and becoming a father are good for a man's health and longevity. This could be somewhat mediated by the changes in testosterone levels. Some researchers believe testosterone lowers immune function: Higher testosterone levels may interfere with the immune system's ability to fight off infection. If this is true, lowering testosterone could be an investment in men's health.
The researchers plan on following up with these men at around age 30.
The study was published today (Sept. 12) in the journal Proceedings of the National Academies of Sciences.