It's plain to see that conceiving a baby alters a woman's body. The growing fetus forces her tummy outward, smoothing out the bellybutton's inward divot. Lactation engorges her breasts with antibody-rich milk, and over the nine months leading up to delivery, healthy moms will gain between 25 and 35 pounds (11 to 15 kilograms) of weight from amniotic fluid, increased blood supply, additional fat storage -- and, of course, the eagerly awaited newborn. And when that birthday comes, the hormone oxytocin surges through a new mother's bloodstream during childbirth, forging the initial maternal-infant bond.
But what about new dads? A 2009 British poll suggests that dads-to-be put on pregnancy weight of their own. The 5,000 respondents gained an average of 14 pounds (6 kilograms) during their partners' pregnancies, attributing the scale's uptick to eating out more often and stocking more snacks in kitchen cupboards to satisfy prepartum cravings [source: Belkin]. Otherwise, men's bodies remain superficially unaffected by their children's births, but that doesn't mean that they're untouched by the parenting process on a deeper level.
Scientific research, which has only recently begun to investigate how fatherhood affects men internally, has uncovered intriguing neurological and physiological transformations that underscore males' crucial role as caregivers. In November 2009, a study out of the University of Calgary determined that interaction with new offspring stimulates neurogenesis, or brain cell growth, in male rats [source: Mak and Weiss]. Mouse dads that nuzzled up to their newborn pups benefited from neurogenesis in their olfactory region and hippocampus, which regulate smell and long-term memory, respectively. In mammals, odor mediates parent-child recognition, and those same mouse dads were able to identify their young even after a sustained period of father-pup separation [source: Mossop].
A similar type of nose-brain bonding takes place in mouse moms' brains as well, but intriguingly, mouse dads that don't come in contact with their brood don't experience it. Moreover, the same hormone that cranks up milk production for breastfeeding, prolactin, also controls that new dad neurogenesis [source: Mossop]. The big difference is that prolactin secretion and breast milk production occur regardless of interaction between mother and child, whereas paternal investment must precede neurogenesis and other physiological shifts in fathers, both rodent and human.
Does daddy change when his testosterone drops?
Humans classify as a biparental species, since both parents are expected to take part in childcare. But more often than not, mothers tend to devote much more time to the kids than fathers. Whether dads are employed or not, in fact, average American moms still clock more hours cleaning, feeding, clothing and transporting children. Specifically, according to the 2008 National Survey of Families and Households out of the University of Wisconsin, the wife-to-husband ratio of childcare is five to one, and in general, dads don't spend more than two or three hours per week with their kids [source: Belkin].
However, that paternal contact, albeit less time-intensive than mothers' interactions, not only correlates to improved child development outcomes down the road, but is also linked to better long-term health in men [source: World Health Organization Europe]. Engaged fathers, for instance, tend to live longer and enjoy better mental health than their childless and single counterparts. And those paternity rewards may be the direct result of fatherhood diminishing testosterone production.
In September 2011, Northwestern University anthropologist and human biologist Lee T. Gettler published a study finding that sparked countless sensational headlines. Examining the testosterone levels of Filipino men across a five-year period revealed a drop in the stereotypical He-Man hormone after a particular shared milestone: fatherhood. Although separate studies had identified a testosterone-fatherhood correlation in other species, this was the first to document longitudinally the hormone drop in human dads [source: Gettler et al]. Compared to the men who remained single throughout the study term, those who got hitched and had babies had much less testosterone in their saliva [source: Gettler et al]. Not only that, the men who went on to become fathers often had higher than average amounts of testosterone at the start of the study, when they were single. So what gives?
Evolutionary biology maintains that males with more testosterone tend to be more sexually desirable, since the hormone selects for certain come-hither traits, such as facial symmetry and bulked-up musculature. Therefore, as potentially fitter mate prospects, it makes sense that those testosterone-rich fellows settled down [source: Lende]. Meanwhile, the fatherhood-prompted dip in testosterone production may represent an evolved mechanism for preening hunter-gatherer males into caregivers. In other words, by tamping down on the aggression-sparking and sex drive-revving chemical that is testosterone, the male body essentially settles itself down to help raise the kids, with heightened protective and care-giving instincts [source: Lende].
Furthermore, a 2002 study examining the hormonal changes associated with marriage and parenting also revealed that husbands with lower testosterone spend more time with their wives [source: Gettler et al]. On the flip side of these family-oriented fellows, a 2002 study from the University of Toronto found that male participants with higher baseline testosterone levels were less sympathetic and responsive toward the sound of crying babies [source: Gettler et al].
However, just like the mouse dads that sprouted brain cells in response to offspring interaction, lowered testosterone isn't a standalone turnkey for paternal care-giving. In these cases, behavior seems to presuppose biology, first requiring quality interaction with offspring to signal the male body to dial down testosterone production. Single dads who didn't see their children experienced little hormonal change in the Northwestern University study, for instance [source: Gettler et al]. So, while it might seem like less testosterone makes a new father into less of a man, it does the exact opposite. It quite possibly primes that man to become more of a dad.
Author's Note: Do new fathers have lower testosterone than single men?
Lee T. Gettler's study finding that fathers appear to have lower average testosterone levels than single men was one of the most headline-making scientific discoveries of 2011. International news services and publications interpreted the data at will, typically drawing conclusions that lower testosterone correlated to lackluster sex drives and effeminate fathers without merit. Which is a major reason I was motivated to write this article. Taking time to read the entire study as well as interviews with the lead researcher Gettler quickly revealed massive scientific misinterpretation among the media. The notion that men might possess evolved mechanisms for care-giving, like women, doesn't make them any less male. Rather, it helps explain why fathering has such a salient impact on children.
- Belkin, Lisa. "When Mom and Dad Share It All." The New York Times. June 15, 2008. (March 15, 2012) http://www.nytimes.com/2008/06/15/magazine/15parenting-t.html?_r=2&pagewanted=all
- Gettler, Lee T. et al. "Fatherhood, Childcare, and Testosterone: Study Authors Discuss the Details." Scientific American. Oct. 05, 2011. (March 15, 2012) http://blogs.scientificamerican.com/guest-blog/2011/10/05/fatherhood-childcare-and-testosterone-study-authors-discuss-the-details/
- Gettler, Lee T. "Longitudinal evidence that fatherhood decreases testosterone in human males." PNAS. Sept. 27, 2011. (March 15, 2012) http://groups.anthropology.northwestern.edu/lhbr/kuzawa_web_files/pdfs/Gettler%20et%20al%20PNAS%202011.pdf
- Gray, Peter B. and Anderson, Kermyt G. "Fatherhood: Evolution and Human Paternal Behavior." Harvard University Press. May 01, 2010. (March 15, 2012) http://books.google.com/books?id=Yz_JkWGbMaQC&dq=fatherhood+estrogen&source=gbs_navlinks_s
- Lende, Daniel. "On Testosterone and Real Men: An Interview with Lee Gettler." Neuroanthropology. PLoS Blogs. Dec. 14, 2011. (March 15, 2012) http://blogs.plos.org/neuroanthropology/2011/12/14/on-testosterone-and-real-men-an-interview-with-lee-gettler/
- Mak, Gloria K. and Weiss, Samuel. "Paternal recognition of adult offspring mediated by newly generated CNS neurons." Nature Neuroscience. Nov. 17, 2009. (March 15, 2012) http://www.nature.com/neuro/journal/v13/n6/abs/nn.2550.html
- Mossop, Brian. "The Brain of Our Fathers: Does Parenting Rewire Dads?" Scientific American. Aug. 17, 2010. (March 15, 2012) http://www.scientificamerican.com/article.cfm?id=the-brains-of-our-fathers
- World Health Organization Europe. "Fatherhood and Health Outcomes in Europe: A Summary Report." 2007. (March 15, 2012) http://www.euro.who.int/__data/assets/pdf_file/0019/69013/E91129sum.pdf