Fathering against type

Evolution is a terrific thing for humans to learn about but a terrible thing to learn from. Case in point: fathers.

Biology generates behavior that renders mammalian fathers something less than admirable, at least by ethical perspectives. Two factors are especially responsible: the consequence of being sperm-makers, as compared with the egg-making specialty that defines females, and the divergent effects of internal fertilization on male and female reproductive tactics — in brief, “Mommy’s babies, Daddy’s maybes.”

The upshot is that it’s nearly impossible to identify any mammal species in which males do as much fathering as females do mothering. Both sexes are primed by evolution to maximize their fitness, but by different routes. For females, reproductive success is largely a matter of getting pregnant, giving birth and then nursing the offspring, whereas sperm-makers are selected to seek additional reproductive partners: that is, to sleep around.

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When it comes to direct care of offspring, that second hard fact of mammalian biology comes into play: internal fertilization. Here again, mammalian males are woefully inclined to go AWOL, and for perfectly good reasons (not morally good, maybe, but deriving from understandable evolutionary pressures). Among, say, fish, where fertilization is external and fathers can have confidence that a fishlet carries its genes, males are often doting parents. But the great majority of male mammals leave the post-birth stage of parental investment — nourishing the infants via lactation — to the mothers.

Among all mammals, reproducing females are uniquely specialized to provide milk for their young (hence the origin of “mammal,” coined by Linnaeus from the Latin mammalis, “of the breast”). Less widely appreciated is the underlying evolutionary reason why.

After all, by the time lactation is required, females have already undergone the rigors of giving birth, not to mention having carried their offspring successfully through the demands of pregnancy. So it would seem only fair for the fathers to pitch in at this point. But they don’t. (It is no explanation, incidentally, to attribute this to the fact that they are outfitted with milk-producing breasts; the question is why.) The reason, almost certainly, is because these males enjoy nothing like the confidence of females when it comes to “knowing” that their offspring are in fact theirs.

This doesn’t mean that all male mammals are necessarily dead-beat dads. There are species in which males hunt for food that they share with the up-and-coming generation, once the latter are old enough to consume solids. In others, males may defend a territory, which can provide females and offspring with protection from both competitors and predators. But these are hardly equal to the work put in by mammalian mothers in bearing and raising their young.

Moreover, a horrifying pattern has become clear in recent decades, one that is pretty much a male province and is the opposite of devoted parenting: infanticide. Many, although assuredly not all, mammals are polygynous — that is, their reproductive social system is a variant on harem keeping, in which a dominant male enforces (with varying degrees of success) breeding hegemony over a coterie of females. Periodically, however, one male is overthrown by another.

Not uncommonly, a newly ascendant male proceeds not only to mate with the available females but to systematically stalk and kill any nursing infants. From a strictly evolutionary perspective, these actions are perfectly appropriate — indeed, fitness-enhancing — because lactation tends to inhibit ovulation, so nursing mothers aren’t available to aid in the reproductive quest of the recently jumped-up male.

When anthropologist Sarah Hrdy first described this phenomenon among langur monkeys in India, her colleagues were incredulous. It was “unnatural,” attributable to an unrepresentative sample perhaps, or protein deprivation, or overcrowding. But natural it is. Male-generated infanticide has now been documented for so many mammalian species that when biologists encounter a previously unknown case of “male takeover,” we are surprised if infanticide is not subsequently reported.

The picture of benevolent paternity is not altogether bleak for mammals, however. Among several species of nonhuman primates, for example, even newly “promoted” males tolerate infants born to females with whom they have previously copulated, which has led to the hypothesis that such forbearance (itself adaptive), may have generated the evolution of concealed ovulation among females.

By keeping males in the dark as to when they ovulate, primate females could be taking out a kind of “infanticide insurance” in the event of a subsequent male takeover, by raising the prospect that a male who copulated with a given female on the sly and then became harem master might have fathered some or all of the infants. (“Isn’t that my old flame from last month? And her cute kid looks just like me!”)

There are, as well, a few cases of devoted paternity, even given the consequences of sperm-making and the inability to precisely identify dads. Marmoset males carry their infant offspring and even reportedly sometimes assist in the birth process. Good fathers are found among the Malagasy giant jumping rats, fat-tailed lemurs, even California mice, species that appear to be sexually monogamous and in which males — enjoying a high probability of genetic fatherhood — are behavioral fathers as well, helping to maintain and defend offspring that are almost certainly theirs.

And then there’s that particular species known as Homo sapiens. They can decide to rise above the urgings of natural selection, put aside any inherent questions about paternity and act paternally. Then again, the human species is unusual, and not just in the potential helpfulness of fathers but (and probably not coincidentally) in the helplessness of its infants, which adds a biological payoff to devotedness among dads.

In a biological world governed by evolutionary imperatives, all too few of which constitute ethical touchstones, these are things we might well celebrate — and not only on Father’s Day.

David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington. His latest book is “Homo Mysterious: Evolutionary Puzzles of Human Nature.”