In Nature, Plenty of Animals Pass the Paternity Test

But a growing body of research indicates that paternal involvement in raising offspring is in many cases an excellent evolutionary strategy after all. It shows up in some species more than in others, depending on various biological and social factors. But the more that scientists look -- from insects to fish to small mammals and even humans -- the more they find examples of exemplary fatherhood. There is nothing, it turns out, inherently natural about being an MIA dad.

"There are more and more examples popping up of males stepping up and doing as much or even more than females in parental care," said Jeffrey A. French, a professor of psychology and biology at the University of Nebraska at Omaha. "We see a wide variety of species showing biparental or even exclusively male parental care."

Some creatures have evolved adaptive quirks that ensure the male is raising his own offspring. Female giant water bugs, for example, cement their eggs onto the dad's back immediately after they mate. For weeks he cares for his brood of bugs-to-be, stroking the eggs with his hind legs and making frequent trips to the surface to oxygenate them until they hatch.

Male sea horses go further, gestating their young inside swollen bellies and enduring a day or two of labor before giving birth to a mini-herd of a dozen or so sea ponies.

Then there is the antarctically heartwarming emperor penguin, whose eggceptional egalitarianism was made famous in last year's Oscar-winning documentary "March of the Penguins."

Many male birds, in fact, are Stepford husbands, in part because their bird brains are drenched in female hormones. Compared with females, male spotted sandpipers have much higher blood levels of prolactin, a hormone linked to maternal behavior (and crucial to milk production in mammals). Drunk with tender feelings, the sandpiper sire sits on the pair's eggs for the entire three-week incubation period and cares for the hatchlings for weeks afterward -- even as Mom, in many cases, is out philandering.

About 20 percent of fish families feature males that attend to eggs or newborns. A male stickleback will make a river-bottom nest from bits of aquatic plants and, after fertilizing the eggs his mate has laid, will hover above and aerate the gelatinous mass with his fins. Day after day he picks at the eggs, removing mold and debris. And when a newly hatched fry wanders too far, Dad collects it in his mouth and gently delivers it back home.

Behavior like that can warm a female fish's coldblooded heart -- as male sand gobies apparently know. Although these European fish fan their developing eggs as sticklebacks do, they also have a nasty habit of occasionally snacking on them. But a 2004 study found that male gobies cut way back on the egg salad -- and fanned even harder -- when mate-seeking females were around, in what appeared to be an effort to impress the ladies.

"It's certainly the case that in many species females prefer males who appear to have parental abilities," said Sigal Balshine, a behavioral ecologist at McMaster University in Ontario. "Females think they are so sexy."

Male parenting is less common among mammals, which by definition raise their young on mother's milk. Yet examples abound, each with its own evolutionary twist.

Recent studies of the bat-eared fox, for instance, found that males are the major nocturnal caregivers of newborns in the den. The animals, it turns out, eat termites, which are hard for Dad to bring home for his hungry, lactating mate. To stay nourished and keep her milk flowing, Mom is better off staying out all night foraging.

Paternal care also makes evolutionary sense for mammalian species in which the birthing process takes an especially big toll on the mother, and newborns are especially helpless. That is the case with marmosets, tamarins and titi monkeys -- species in which the males do most of the lugging around of offspring and other forms of care. Again, hormones help. Testosterone concentrations drop dramatically in a marmoset dad after the mom gives birth, turning the usually frisky primates into doting homebodies.