Mothers and Others
Page 27
What everyone needs in the [new] millennium is access to the Internet and a grandmother.
—Anonymous (cited in Farmer 2000)
If allomaternal assistance is so beneficial for maternal fitness, why don’t all mother apes solicit help? It can’t be lack of interest by prospective allomothers—most primates are fascinated by babies. As we just saw, the neural underpinnings for kindchenschema are in place, and apes are no exception. Rather, the main obstacle to shared care is the mother’s lack of trust in her surroundings and reluctance to allow anyone else to hold her infant. Mother apes in the wild are obsessively anxious about their babies. Frankly, if I were a mother chimpanzee, I would be too.
Primatewide, mothers have to worry about strange males, and in both chimpanzees and gorillas, infanticide by males is a major source of infant mortality. But because females typically leave their natal kin to breed in other communities, they have to worry about unrelated and potentially infanticidal females as well. This is especially true of highly omnivorous common chimpanzees, who eat baby gazelles and colobus monkeys when they can get them. Baby chimpanzees are a no less delectable source of proteins and lipids. Furthermore, because chimpanzees’ primary diet is ripe fruit, elimination of a rival mother’s infant means greater access to a finite local resource for the killer’s own line.1
Great Ape mothers are notoriously possessive. When Flo, one of Jane Goodall’s Gombe chimpanzees, gave birth to the infant Flint, his older sister Fifi (on the left) was not allowed to take him, although she was obviously interested and eager to do so. In this photograph, Fifi appears to be eying her brother with what looks to me like resigned yearning. (Hugo van Lawick/Jane Goodall Institute)
Provided they can be dispatched with impunity, baby chimpanzees are fair game. Fortunately for mothers, rival females are rarely any bigger than they are, and few are willing to take the risks males do. The stakes don’t warrant it. Elimination of a nursing infant provides a male one of his few chances to inseminate a fertile female, but infanticidal females merely gain a meal or eliminate a tiny rival who might never grow up anyway—scarcely worth the risk of being wounded by his protective mother. Not surprisingly, the first observations of infanticide by chimpanzee females targeted babies of mothers with some disability that hampered retaliation—illness, a paralyzed wrist, extreme subordination.2 Infants of strange females attempting to immigrate into the community are especially vulnerable because community females may gang up on her.
One February day in 2006 a team of primatologists studying chimpanzees in the Budongo forest of Uganda noticed an unfamiliar female moving in from another community. Young adult females usually migrate earlier in their reproductive careers, but this new arrival was already a mother, carrying a week-old infant. The pair was attacked by six resident females. Five had infants of their own, clinging tight to their mothers as they charged. Screaming and bleeding, the strange female was no match for this xenophobic consortium. As her attackers caught hold and pounded on her back, she crouched low to the ground, shielding her baby. Three community males approached, also screaming, noisily ricocheting off tree trunks, but none attacked. One old male actually looked as though he was trying to pry a resident female off, but to no avail. The alpha female wrested the baby away, only to lose it to another female who snatched it from her and delivered a lethal, neck-spanning bite.3
Given the company chimpanzees keep, it is understandable that a mother would be reluctant to allow even a well-intentioned older sibling to hold her baby. Caring and attentive as a sister would be, she might not be sufficiently experienced or imposing to ward off a more dominant adult. If ape mothers insist on carrying babies everywhere, it is not because they instinctively seek continuous tactile contact with babies; it is because the available alternatives are not safe enough.
I vividly recall the incident that first brought home this realization. I was watching a group of bonobos at a zoo in the Netherlands. The apes were in their winter quarters—several indoor cages connected by open doors. The only other person around was my host, a young scientist who had just provided sugar cane, a favorite food. The dominant female and the rest of the group were in a connecting cage, some distance away from a mother and baby. Apparently feeling quite safe, the mother actually set her baby down so she could use both hands to eat.
Something similar was observed among common chimpanzees in a zoo in Singapore. The mother even allowed cagemates to carry her 3-month-old infant.4 But I know of only one published account, described by its author as “an unusual incident,” in which a wild ape mother voluntarily permitted another female to not only hold but actually adopt her newborn infant. This involved a 13-year-old Gombe chimpanzee named Gaia who happened to be living in the same community as her own mother when she gave birth for the first time. As females from a more dominant matriline approached and tried to inspect the baby, the grandmother (Gremlin) took the baby from her inexperienced daughter and “turned her back toward them” in a protective mode. Thereafter, the grandmother retained the baby, nursing him along with her own two-year-old son right up until Gaia’s baby died at age five months.5
There are more than a dozen cases of wild chimpanzee orphans being adopted by another group member (usually a close relative). What was so unprecedented about this case was that the mother was still alive. All other reported lapses in maternal possessiveness involve new mothers under more protected conditions. Particularly telling is the case of the chimpanzee Ai who allowed Tetsuro Matsuzawa, her unusually reliable friend, access to her new baby, access such as no scientist had ever before been voluntarily permitted. In another instance, an inexperienced mother gorilla at the San Diego Zoo allowed her own mother to carry her baby.
Ordinarily, a wild gorilla would have left her natal group long before she gave birth for the first time, and as in the Gombe case, it was unusual for this 11-year-old female to be living with her mother. Having lost her first infant, this new mother was still inexperienced and, like many first-time primate mothers, seemed unsure what to do. She left the neonate on the floor. As Masayuki Nakamichi, a Japanese primatologist then working in San Diego, apprehensively watched, the grandmother came over, picked up the baby, and held it out near her daughter’s face, as if demonstrating what needed doing. The grandmother then handed the baby to its mother, who eventually learned to care for it.6
In this rare photo, the grandmother held the newborn under the face of her daughter for 15 seconds (shown here) before gently pushing the baby gorilla toward its young and inexperienced mother, who finally took it. (Masayuki Nakamichi from Nakamichi et al. 2004:76)
With the exception of the Gombe case, which was more nearly a concerned grandmother taking protective custody of a baby from an inexperienced and inept daughter, all the other exceptions to exclusive maternal care of newborns in Great Apes occurred in captivity without predators or potentially infanticidal conspecifics lurking nearby. Even more importantly, I suspect, in all cases the mothers were in unusual company, alone with a familiar, competent, trusted adult. Such admittedly rare incidents nevertheless reveal something important. Sufficiently confident of their physical and social surroundings, even gorilla, chimpanzee, or bonobo mothers will share care.
So why do hunter-gatherers differ from other apes in this respect, by routinely sharing care? Might there be neurochemical differences that encourage postpartum women to be more trusting than other apes? Different monkey species in the genus Macaca range from the highly aggressive and rigidly hierarchical rhesus and pigtail macaques, among whom mothers never voluntarily allow access to their infants for fear that the dominant female would kidnap the infant and not give it back, to the unusually tolerant and far less competitive Barbary and Tonkean macaques, who freely share care. These species-specific behavioral differences are correlated with neurophysiological differences. Highly possessive rhesus and pigtailed macaque mothers are reported to have reduced serotonin activity compared with the more tolerant, infant-sharing Barbary and Tonkean macaqu
es.7
Since both birth and lactation coincide with higher-than-usual levels of the tolerance-and-trust-promoting neurotransmitter oxytocin, it would be particularly interesting to know if there are physiological differences between human mothers and other apes in how they respond to neuropeptides like oxytocin in the postpartum period. Might such differences affect receptors to such “affiliative” and trust-promoting hormones?8 Complete sequences of all ape genomes may one day make possible the kind of comparisons already being done with closely related species of social insects. At present so little is known about the comparative neurobiology of maternal behavior in apes that we can neither confirm nor rule out physiological differences postpartum. No one knows, for example, why nonhuman ape mothers often feel compelled to eat the placenta right after birth, while women (even more omnivorous than other apes) eschew this potent dose of extra hormones. Might women already be more responsive to oxytocin, less anxious about social contacts postpartum, and therefore less in need of a nurture-promoting cocktail?
Research from other mammals reveals that adults dosed with oxytocin do indeed become more trusting and affiliative.9 Voles exposed to higher levels of oxytocin in early infancy exhibit greater propensity to bond with others and behave in more nurturing ways later in life.10 Furthermore, a species such as prairie voles in which parents rely on others to help care for their young has more oxytocin receptors in certain brain regions than does a closely related species that does not form pair bonds or tolerate alloparental care. Whereas prairie vole mothers readily accept help, other vole mothers attack anyone who approaches their young.11
It is entirely possible that neurological differences between humans and other apes affect how these mothers behave after giving birth. But I know of no studies showing this. Furthermore, even if differences could be documented, we would still need to explain how infant sharing became common enough for natural selection to act on and favor a subset of mother apes whose underlying physiologies inclined them to be more tolerant of others postpartum. Rather than dwell on what we do not yet know about the comparative physiology of apes and humans, let’s turn to what the ethological evidence tells us to expect from mothers under various circumstances.
In order for a hyperprotective Pleistocene hominin to voluntarily allow access to her infant, she would have to be in the company of others who were competent and trusted, perhaps her own mother—a full-grown and experienced caregiver familiar from birth. Until recently, most evolutionists took for granted that hominin females had no such candidate around. Now new evidence forces us to reassess this assumption and to consider whether it was not only possible but likely that early hominin mothers gave birth in the vicinity of matrilineal kin. This new way of thinking about the company kept by Pleistocene mothers completely alters the theoretical feasibility of shared care in an ape.
In this chapter, I describe new findings about both Great Ape and hunter-gatherer residence patterns, showing that these creatures were more flexible than previously assumed and that it was not impossible for ancestral apes to give birth near matrilineal kin. In the case of Pleistocene foragers, there is reason to believe that they were not only living under circumstances where alloparental care was feasible but also where food sharing was increasingly important for survival and successful childrearing. I conclude by discussing the different qualifications and variable availability of different kinds of caretakers—older siblings, cousins, co-mothers, fathers, possible fathers, and especially grandmothers, whose impact on child survival has only recently begun to be studied but is already yielding some surprising twists.
ON THE IMPORTANCE OF GIVING BIRTH NEAR KIN
Primate social organization is famously variable. But across species, two generalizations hold up remarkably well. First, females who live among kin are better able to defend their interests than those who leave their natal groups to forage and breed among nonkin. Second, mothers are most prone to share infants when they feel confident that they can readily get them back unharmed.
Until recently, however, it was taken for granted that, like other apes, hominin females left their natal groups to give birth for the first time in another community, to rear young among unrelated, possibly rival, females who were unlikely to be supportive. Just suggesting that early humans lived in matrilocal settings was viewed by evolutionists as some heretical throwback to outmoded views about matriarchal stages in human evolution, bringing to mind advocates for Mother Right or Goddess Cults.12 There were two reasons for discounting such views. The first had to do with entrenched assumptions about the patrilocal tendencies of all hominid apes—Great Apes, australopithecines, and humans alike. The second reason was a tendency to project onto early Paleolithic ancestors patriarchal attributes from later time periods.
By “patriarchal” I mean a society with patrilocal residence patterns, patrilineal inheritance, and social institutions biased toward patrilineal interests. By this definition, few tropical gathering-and-hunting societies that have not yet adopted horticulture are patriarchal. Yet somehow, patrilocal living arrangements, and even patriarchal elaborations correlated with patrilocality, are routinely assumed to be human universals and are projected back in time onto our early Pleistocene ancestors.13 The supposed antiquity of patrilocality was further bolstered by comparisons with the African Great Apes, since both gorillas and chimpanzees exhibit marked patrilocal tendencies, with females migrating out to breed among nonkin. Given such entrenched assumptions, it was difficult to imagine hominins ever being sufficiently matrilocal to evolve shared care.
The preeminence of the man-the-hunter/sex-contract paradigm, with its accompanying stereotypes about nuclear families and maternal caregiving, was another obvious obstacle. So too was the habit among mammal researchers of assuming that cooperative breeding meant that a single dominant female would monopolize the group’s reproduction. Since marmosetlike reproductive suppression has never been reported for hunter-gatherers, this criterion if applied excluded early humans.14 But the most persistent barrier to thinking humans might have evolved with shared care had to do with residence patterns.
Three widely accepted lines of evidence pointed to male philopatry as a hominid universal. First, behavioral evidence from the African Great Apes initially suggested that females always left their natal group to breed. Second, apart from the special sex-induced alliances in bonobos, none of the Great Apes exhibited the strong female-female social bonds typical of matrilocal species.15 Third, and most impressive, George Peter Murdock’s classic cross-cultural compilations seemed to document a prevalence of patrilocality in humans as well. Murdock’s analyses of ethnographic information on 862 representative cultures from around the world, expressly coded for use in the Human Relations Area Files and in his Ethnographic Atlas, indicated that the vast majority of human cultures were patrilocal. This included hunter-gatherer societies, 62 percent of which, according to Murdock’s information, were patrilocal.16 Therefore, it seemed both logical and parsimonious to assume that the common ancestors of apes and humans also lived in male kin groups.17
Evolutionists had little incentive to challenge this received wisdom. Assumptions about the universality of patrilocal residence patterns were consistent with other widely accepted assumptions about naturally dominant males and “men in groups” who forged alliances with fathers and brothers to hunt and to protect their mates and natal territories. Furthermore, having men stay put while exchanging their daughters and sisters with other groups seemed like an adaptive way for fathers and brothers to avoid excessive inbreeding while simultaneously forging alliances with other groups—critical building blocks for early human social organization. At the time, sisters and daughters were viewed as essentially passive pawns in largely male-orchestrated transactions.18
Patrilocal residence became an integral feature of hominin family life as reconstructed by physical anthropologists in the twentieth century. The resulting assumptions were subsequently incorporated wholesale into early sociobiology a
nd evolutionary psychology. By the early 1980s a few anthropologists were pointing out that female interests and strategies were being overlooked.19 By the late 1980s, human behavioral ecologists studying foraging peoples were specifically asking why postreproductive women worked so hard. And by the 1990s, Kristen Hawkes and colleagues were arguing that assistance from maternal grandmothers had played a critical role in early hominin evolution—a hypothesis initially met with considerable skepticism.20 The main objection was that even if their older matrilineal kin survived long enough to be helpful (which many doubted), older women would not have lived near daughters (and new mothers) who needed their help.
A Hadza boy is shown sitting with his great-grandmother and her sister (on the right). Classically “hard-working” Hadza grandmothers energetically sharpen their digging sticks in preparation for a gathering expedition. When a mother has a new baby, as this boy’s mother did, Hawkes and company found that their older children’s nutritional status was correlated with how much time older kinswomen spent foraging. (James O’Connell)
Biologists and anthropologists alike—who in the early years were mostly male—had long taken for granted that the function of women was to bear and rear a man’s children. From this perspective, women past childbearing age were deemed irrelevant and of no theoretical interest. This prejudice surfaced occasionally in ethnographic descriptions of old women as “physically quite revolting” or “nuisances.” They were depicted as objects of ridicule—“old hags” whose behavior was obviously not worth studying.21 Such accounts took for granted that in our evolutionary past, postmenopausal females would have been too decrepit or shortlived to be of use. Demographic and archaeological evidence suggesting otherwise was discounted.22