That's the title of a 1999 book by anthropologist Sarah Blaffer Hrdy. (No, that's not a typo.) The phrase has at least a double meaning, referring both to maternal instincts and behavior in nature and to the nature of motherhood.
Some animals practice semelparity, putting all their literal eggs in one metaphorical basket by breeding only once in a lifetime. Examples include the salmon who swims upstream to spawn and die or the spider whose newborn young eat her body. More commonly, "higher" animals practice iteroparity (what a cool-sounding word), like us and our primate kin—reproducing multiple times. A female in an iteroparous species has to balance the welfare of the newest infant against her prospects for maximizing the number of offspring who survive over the long term. "Nurturing" is only one trait of the ideal mother in nature; she may also compete against other females for the status and resources that give her children the best chance to thrive or even make hard choices about cutting her losses with one baby for the sake of future babies who will have better prospects for survival.
A culture of sapient aliens in which the dominant female's pheromones suppress ovulation in the other females in the group, as among some social mammals on Earth, would have a very different family structure from ours. Among sapient aliens with biology like that of the above-mentioned spiders, a female who devised a way to survive the birth of her children might be condemned as scandalously immoral.
Female primates during their fertile periods often mate with numerous males so that those males will protect the resulting offspring rather than threatening them. It's not uncommon for males of many social species (lions, for instance) to kill infants sired by other males in order to bring the females into estrus immediately. In some human hunter-forager cultures, people believe a fetus is built up gradually by repeated infusions of semen from multiple acts of intercourse. Women deliberately consort with several men during pregnancy, and everyone who's had sexual relations with her during that time is deemed a father to the baby. Suppose an alien species existed in which this belief reflected biological reality, so that a baby really did have multiple fathers? In their society, polyandry would probably be the norm.
Among the vast majority of primates (including Homo sapiens in most cultures), males take little part in caring for infants. A satirical novel about a women-ruled planet I've read, however, takes the logical position that because women bear the burden of pregnancy and nursing, the father should do all the rest of the work of child-rearing. Men in that society stay home to care for the house and the babies, while after giving birth women don't do much with infants besides breast-feed them.
In hard times, some pregnant animals can re-absorb or spontaneously abort embryos at an early stage. Some species even have the power to alter the sex ratios of their offspring by selectively miscarrying embryos of one sex, according to which sex has the best opportunity for reproductive success depending on the availability of resources in a particular breeding season. Think what an advantage would belong to an intelligent species that could consciously perform this kind of "natural" birth control.
Maximizing the number of surviving offspring to carry her genes doesn't mean a mother necessarily nurtures every infant she bears. In the case of a too-large litter, females of some species may abandon the weakest, maybe even eating them to "recycle" their substance as nourishment for the mother herself and her favored young.
We might find it difficult to accept as "civilized" a planet where mothers have a duty to cull sickly newborns and where eating the culls is considered perfectly reasonable. Or a society that has institutionalized and ritualized the practice of dominant males killing the children of their predecessors, as the mating duel to the death is ritualized in the Vulcan Pon Farr ceremony.
Imagine encountering a species of advanced aliens who practice one or more of these pragmatic "nature red in fang and claw" customs. Think of the Martians in Heinlein's STRANGER IN A STRANGE LAND, whose culture shocks the human characters for several reasons, not only that the Martians practice ritual cannibalism (among other things) but that they cast their young out into the desert to fend for themselves and prove their worthiness to survive. As Mike, the human "Martian," explains, among his adoptive people competition for fitness happens in infancy and childhood rather than adulthood. (We get a glimpse of this process in the earlier novel RED PLANET, which appears to be set on the same version of Mars.) Another kind of struggle for fitness among children occurs in Suzy McKee Charnas's MOTHERLINES. Upon weaning, children leave their mothers and join the "child pack." They grow up wild, learning to provide for themselves and form rivalries and alliances among their age-mates. Only at adolescence are they reclaimed by their mothers and readmitted to adult society.
Adjusting to intelligent aliens with customs like these might be more shocking to our sensibilities than the three genders and male pregnancies of the TV series ALIEN NATION.
Margaret L. CarterCarter's Crypt