Male marmosets in South America not only carry, feed, and groom their twin babies (pictured, a baby black-tailed marmoset with its mother in a Tokyo zoo), they may even act as "midwives" during birth, grooming and licking the newborns.

Marmoset dads may be so involved because of the high cost of birth for the mother, whose unborn babies eventually make up 25 percent of her body weight—equal to a 120-pound (55-kilogram) woman giving birth to a 30-pound (14-kilogram) infant.

Though nature's default is female-only care, animal fathers such as marmosets will often pitch in when "kids are expensive," for instance, when raising offspring requires two full-time parents that are mated for life, the University of Winnipeg's Forbes said.