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Fathering against type

Because of biology, mammalian male parents may act in ways that are less than admirable. But human dads can trump nature.

June 15, 2013|By David P. Barash
(David Gothard, For The Times )

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.

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