For Father’s Day, Slate.com had an article on how fatherhood alters the male body and brain. One of the studies showed 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 [“sympathetic pregnancy”] 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.
More directly applicable to the human father,
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.