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 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. [...]
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 as usual with topics about men:
And yet despite these findings, few scientists treat the physiology of fathers as a serious subject in its own right. [...] Between 2000 and 2006, the journal Hormones and Behavior published nearly three times as many studies of mothers as of fathers, and this year the count so far is 16 to three. A 2000 review framed research into physiological fatherhood as "an opportunity to better understand maternal behavior, by studying parental behavior in the absence of pregnancy and lactation." Interest in how men's bodies prepare themselves for fatherhood only seems to matter to the extent it sheds light on mothers. Meanwhile, the ways in which dads screw up their kids is a thriving area of research.