These South American monkeys form lifelong partnerships with their mates. The father transports and grooms the babies, while the mother takes care of nursing, according to research by Eduardo Fernandez-Duque, a biological anthropologist at the University of Pennsylvania.
According to Mark Pagel, an evolutionary biologist at the University of Reading in the U.K., "These strong pair bonds—what we often call monogamy—normally only evolve where the ecological conditions mean that both parents are needed to acquire enough food and provide enough protection to ensure offspring survival.
"So 'best fathers' often seem to arise from ecological necessity!"
Photograph by Luis Benavides, AP
The seahorse is a type of fish in which the males actually get "pregnant."
After a female seahorse deposits her eggs in the male's specialized pouch, the male carries up to 2,000 babies during its 10- to 25-day pregnancy. Above, a newborn hitches a ride on dad's tail.
"They're fascinating—males have more or less become females, [almost] transgendered," Pagel said. "They're devoted fathers."
First a male finds a waterhole and douses his belly feathers, which are adapted to retain more water than his other feathers. Laden with up to 40 milliliters of liquid, dad returns to the nest and summons his young with a call, spreading out the wet plumage. The chicks then "drink" water from his belly, sometimes for up to ten minutes.
Talk about back-breaking work—the male giant water bug, pictured above in California, literally totes around his brood of about 150 eggs until they hatch.
After a courtship of sparring and grasping, these ferocious insects mate, and the females cement their fertilized eggs to the males' backs with a natural glue.
Over the next three weeks, the male becomes a "very effective dad," Scott Forbes, a University of Winnipeg biologist and author of A Natural History of Families, said in 2009. The daddy water bug fiercely protects his eggs and periodically exposes them to air to prevent them from growing mold.
Photograph by Gerald and Buff Corsi, Visuals Unlimited
A male black-necked swan carries a cygnet—a baby swan—on its back while the rest of the brood follows.
Cygnets stay close to their parents for almost a year, and ride on their parents' backs for warmth and protection during the first weeks of life, according to the Wildlife Conservation Society.
The monogamous parents both carry their babies, a behavior especially pronounced in black-necked swans and mute swans.
Baby marmosets seem to pose next to their parents at a German zoo in 2010. Male marmosets in South America not only carry, feed, and groom their twin babies, but also may act as "midwives" during birth, grooming and licking the newborns.
Marmoset dads may be so involved in parenting because of the high cost of birth for the mother: While in the womb, the 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 will often pitch in instances like these, when "kids are expensive," the University of Winnipeg's Forbes said in 2009.
Photograph by Holger Hollemann, European Pressphoto Agency
For most birds, females are stuck with child care. But not so for South America's greater rhea (pictured, an adult and baby in Argentina).
Females mate with several males during the breeding season, and many birds will lay their eggs in one nest created by a male. The male then incubates up to 50 eggs for six weeks and cares for the newly hatched young. The dads aggressively guards the babies, charging at any animal—even a female rhea—that approaches.
Because females put such a "heavy investment" of energy and resources into producing large eggs, it makes sense for males to pick up the responsibility of caring for the offspring, Forbes said in 2009.
Photograph by Andres Morya, Visuals Unlimited/Getty Images
A barking frog—named for its throaty, dog-like call—sits at a natural spring in Ulvade County, Texas.
A protective father, the male frog hangs out by his brood for several weeks, wetting the eggs with his urine if they dry out.
In other frog species, males carry their larvae on their backs or swallow their newly hatched tadpoles to shelter them in special mouth sacs, giving the offspring a safe haven in which to develop, Forbes said in 2009.
Photograph by Bates Littlehales, National Geographic
Emperor penguin fathers endure below-freezing temperatures and forgo food to incubate their eggs.
After the female lays a single egg, her mate rests it on his feet and covers it with a flap of skin. When the egg hatches, the chick is kept warm under the skin flap, as seen above.
"You could think of that pouch as almost a male womb, except they're birds," the University of Reading's Pagel said. "They're very much like seahorses in that respect: [They've] taken over a role, rightly or wrongly, we traditionally associate with females."
For four months the males huddle together, not moving much, while the females fill up on seafood in the ocean. The females eventually return to help feed the newly hatched chicks.
Both Pagel and Forbes say that animal parents give us insight into how human families work. "We can see ourselves in other animals," Pagel said.
Photograph by Thorsten Milse, Picture Press/Getty Images