[Excerpt] The Evolution of Abortion

See Also: Cooperative Breeding Hypothesis
Excerpt From: Hrdy (2009) Mothers and Others. Page 70-72, 99-100
Content Summary: 1300 words, 13 minute read

Child Abandonment in Nonhuman Primates

Many mammalian mothers can be surprisingly selective about babies they care for. A mother mouse or prairie dog may cull her litter, shoving aside a runt; a lioness whose cubs are too weak to walk may abandon the entire litter “with no attempt to nudge them to their feet, carry them or otherwise help. Some mammals (and this includes humans) even discriminate against healthy babies, if they happen to be born the “wrong” sex. But not Great Ape or most primate mothers. No matter how deformed, scrawny, odd, or burdensome, there is no baby that a wild ape mother won’t keep. Babies born blind, limbless, or afflicted with cerebral palsy – newborns that a hunter-gatherer mother would likely abandon at birth – are picked up and held close. If her baby is too incapacitated to hold on, the mother may walk tripedally so as to support the baby with one hand. 

Mother and ape mothers rarely discriminate based on a baby’s particular attributes, as some human mothers do. Except perhaps those born very prematurely, babies are cared for (and carried) almost no matter what. Even if her baby dies, the mother will continue to carry the desiccated corpse around for days.

Child Abandonment in Humans

Maternal devotion in the human case is more complicated. A woman undergoes the same endocrinological transformations during pregnancy as other apes. At birth, her cortisol levels and heartbeat reflect just how sensitive to infant cues she has become. But whereas the nonhuman ape mother undiscriminatingly accepts any infant born to her without taking into account physical attributes, the human mother’s devotion is more conditional. A newborn perceived as defective may be drowned, buried alive, or simply wrapped in leaves and left in the bush within a few hours of birth. “Defective” may mean anything from having too few toes or too few. It may mean being born with a deformed limb or at a very low birthweight, coming too soon after the birth of an older sibling, or having some culturally arbitrary “affliction” such as having too much or too little hair, or being born the wrong sex. 

Unlike any other ape, a mother in a hunter-gatherer society examines her baby right after birth and, depending on its specific attributes and her own social circumstances (especially how much social support she is likely to have) makes a conscious decision to either keep the baby or let it die. In most traditional hunter-gatherer societies, abandonment is rare, and almost always undertaken with regret. It is an act no woman wants to recall, a topic ethnographers must tiptoe around gingerly. Typically, interviewers will broach the subject indirectly, asking other women rather than the mother herself. Back when the !Kung still lives as nomadic foragers, the rate of abandonment was about one in one hundred live births. Higher rates were reported among people with strong sex preferences, as among the pre-missionized Eipo horticulturalists of highland New Guinea. Forty-one percent of live births in this group resulted in abandonment, and in the vast majority of cases the abandoned babies were newborn daughters whose mothers hoped to reduce the time until a song might be born. 

Once a baby has nursed at his mother’s breast and lactation is under way, a woman’s hormonal and neurological responses to this stimulation, combined with visual, auditory, tactile, and olfactory cues, produce a powerful emotional attachment to her baby. Once she passes this tipping point, a mother’s passionate desire to keep her baby safe usually overrides other (including conscious) considerations. This is why, if a mother is going to abandon her infant, she usually does so immediately, before her milk comes in and before mother-infant bonding is past the point of no return.

Two Kinds of Parenting Style

There are two kinds of primate parenting styles:

  • Continuous care and contact, where the mother’s hyper-possessive instincts rebuff offers of otherwise-interested babysitters
  • Cooperative breeding, where relatives (“allomothers”) take turns carrying the young, and sometimes provisioning them with food.

About half of all primate species use cooperative breeding models. However, only 20% of primate species do alloparents provision the young, and for the most part this provisioning does not amount to much. Let us call robust cooperative breeding those species that generously provision their young. So far the only full alloparents belong to the family callitrichidae– mostly marmosets and tamarins. Callitrichidae are famous for breeding fast and for their rapid colonization of new habitats. 

More than 30 million years have passed since humans last shared a common ancestor with these tiny (rarely more than four pounds), clawed, squirrel-like arboreal creatures. New World monkeys literally inhabit a different world from that of their primate cousins who evolved in Africa. Theirs is a sensory world dominated by smell rather than sight. Yet in many respects callitrichids may provide better insight into early hominin family lives than do far more closely related species like chimpanzees or cercopithecine monkeys.

What humans have in common with the Callitrichidae is worth itemizing. In both types of primates, group members are unusually sensitive to the needs of others and are characterized by potent impulses to give. In both groups, a mother produces closely spaced offspring whose needs exceed her capacity to provide for them. Thus the mother must rely on others to help care for and provision her young. When prospects for support seem poor, mothers in both groups are more likely to bail out than other primates are. Human and callitrichid mothers stand out for their pronounced ambivalence toward newborns and their extremely contingent maternal commitment. Infants have adapted, as we will see later, with special traits for attracting the attention of potential caregivers. And finally, humans have a marmoset-like ability to colonize and thrive in novel environments. 

What happens when you take a clever ape with incipient social intelligence, tool manufacturing, robust mindreading,then introduce cooperative breeding? This, we submit, is the recipe to produce a uniquely human cognitive system. Prosocial motivations transformed the mindreading system into a mindsharing system, which ultimately led to the development of norms, language, and cumulative culture.

This is the cooperative breeding hypothesis. 

The Dark Side of Cooperative Breeding

As noted above, By far, the most common exceptions to this general primate pattern are found in the family Callitrichidae. Like all cooperative breeders, tamarin and marmoset mothers depend on others to help rear their young. Shared care and provisioning clearly enhances maternal reproductive success, but there is also a dark side to such dependence. Tamarin mothers short on help may abandon their young, bailing out at birth by failing to pick up neonates when they fall to the ground or forcing clinging newborns off their bodies. Although infanticide is a hazard across the Primate order, observations almost always implicate either strange males or females other than the mother, not the mother herself. 

The high rates of maternal abandonment seen among callitrichids and humans are almost unheard of elsewhere among primates. Cooperative breeding systems endowed humans with a deep felt sense of cooperation and altruism… but increased rates of child abandonment are a corollary.

The Evolution of Abortion

Note: this section is my own; these are not Hrdy’s words.

It is possible to interpret modern debates about abortion to this ancient primate instinct documented above. As humans became increasingly culturally sophisticated, the motivation to abandon a child could be acted upon prenatally.

This is not to make an appeal to nature, “X is good because it is natural”. Indeed, our normative systems (mindsharing writ large) allow us to push against human nature when we so choose. And I won’t speak towards a moral appraisal of abortion here.

But let’s imagine human parenting systems were instead inherited from the continuous care and contact model of the other great apes. In such a system, I submit the topic of abortion would be as foreign as meat-eating might be to a talking gorilla.

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