How fatherhood affects the body and brain

By Emily Anthes - Slate Magazine:

But in the last handful of years, scientists have shown that normal, healthy, non-pregnancy-envying men often undergo real bodily changes when they're expecting children. Research shows that male marmosets and cotton-top tamarins—primates that, like humans, split child-rearing duties between the mother and father—gain as much as 20 percent of their body weight while waiting for the birth of their offspring. The finding suggests that couvade is biologically adaptive rather than psychologically neurotic: The hypothesis about the marmosets and tamarins is that the pregnancy paunch prepares a dad for the extra energy he'll expend in helping to rear his baby.

In addition, dads-to-be have elevated levels of cortisol and prolactin, hormones that are also present in high levels among mothers who are attached and responsive to their children. A father's testosterone level also drops by about a third, on average, in the first three weeks after his child is born. These hormonal shifts, which are likely sparked by exposure to the pregnant woman's hormones (there is correlational evidence that dads who spend time with moms experience the changes), mirror those experienced by mothers and may similarly prepare men for parenthood. Men who have relatively little testosterone have been shown, for instance, to hold baby dolls longer than men who are flooded with the sex hormone. High levels of testosterone, on the other hand, are associated with "incompatible non-nurturing behaviors," as one researcher put it. If dads roared along on their usual levels of the hormone, the theory goes, they'd be too busy fighting other men and seducing other women to do much diaper-changing.

There's also preliminary but tantalizing evidence that fatherhood can change the brain. A 2006 study found enhancements in the prefrontal cortex of the father marmoset. After childbirth, the neurons in this region showed greater connectivity, suggesting that having young children could boost the part of the brain responsible for planning and memory, skills parents need when having kids gives them more to keep track of. The neurons also had more receptors for vasopressin, a hormone that has been shown to prompt animal fathers to bond with offspring. (Receiving an injection of vasopressin, for instance, prompts a male prairie vole to cuddle and groom a youngster.)

And yet despite these findings, few scientists treat the physiology of fathers as a serious subject in its own right. Researchers have been investigating some of the hormonal swings in humans for almost a decade, and longer in other species; still, most of this work remains on the fringe.