Fatherhood is a rarity in the animal kingdom -- a mere 5 percent of mammals have evolved invested paternal practices.

Health research has found a correlation between active fatherhood and improved long-term health. For instance, in 2011, a Stanford urologist calculated dads have a 17 percent lower chance of developing fatal heart disease than non-dads.

Men don't experience menopause, granted, but studies have found that male fertility certainly declines with age. For instance, semen samples from men over 35 years old contain more damaged sperm, and the rate of birth defects increases with both mothers' and fathers' ages. The cut-off might not be as dramatic for men, but their biological clocks tick-tock, nevertheless.

Although doctors haven't figured out why many men feel pregnant alongside their expecting partners, research has confirmed that it's a routine occurrence, and abdominal cramps are the most common psychosomatic side effect.

According to national survey data analyzed by a pair of University of California, Riverside, sociologists, dads' housework can translate to healthier peer relationships among their children and more frequent sex with their spouses. It didn't, however, find a correlation between cleaning and fewer parent-child squabbles.

New dads who interact with their infants display lower testosterone levels than men with no kids. Studies have found that even the act of holding a baby doll can diminish the amount of testosterone in a man's bloodstream.

A 19-year study published in November 2011 found that new fatherhood has the most potent effect on mens' criminal activity, and it's also linked with lower smoking, drinking and marijuana use.

About 40 percent of American kids grow up apart from their biological dads. But that doesn't mean they'll miss out on positive paternal parenting. A 2008 study from the University of Wisconsin-Madison found that stepfathers exhibited higher-quality parenting habits than biological fathers, more actively engaging children and assuming greater childcare responsibilities.

In 2010, New York University sociologist Judith Stacey published the results of extensive research on family dynamics and child development outcomes. Based on her findings, Stacey suggested that gay male couples might be the most invested parents, since the social and legal processes required for them to become parents demand the most planning and commitment.

About 25 percent of American mothers experience postpartum depression, and new dads aren't immune to the baby blues, either. A 2011 study from the University of Michigan found that 7 percent of new fathers report postpartum depressive symptoms and are three times more likely to spank their babies than non-depressed dads.