As they age, female primates behave differently from younger females. If they are still breeding, mothers spend more time in direct contact with infants and wean them later than younger mothers do. In general, older mothers are more committed to these last installments on their lifetime reproductive success. In addition to their aging ovaries, such heavier investment may be one reason why older mothers experience longer intervals between births.45 For female primates at or near the end of their reproductive careers, this tendency to “give of themselves” may also lead them to audaciously defend offspring born to female kin.
As with most Old World monkeys, the maximum lifespan for a langur is around 30 years. As they approach this age, females become less active in troop affairs, avoid competition with other animals, and—even in those matrilocal species with routine infant-sharing—rarely take infants just to carry them around. In an emergency, however, these same socially marginalized old matrons become the most active in defending the group’s feeding grounds from neighboring groups. They are also the most daring in defending infants attacked by infanticidal males. Among langurs, sooty mangabeys, and savanna baboons, it is these 20- to 30-year-olds who take bigger risks to defend an endangered infant than the victim’s own mother does.46
Such episodes are uncommon, but the heroics are unforgettable. Some time ago, scientists observing baboons at Moremi, Botswana, watched as a new male arrived in the troop. Shortly after, he attacked an infant sired by his predecessor. The newcomer chased the seven-month-old female, knocked her down, and dragged her along the ground, then threw her six feet into the air. Nearby group members vocalized and rushed forward but were rebuffed by the male, who resumed his attack on the stunned infant, “biting her in the head, the groin and below the navel.” At this point, several females rushed to intervene. The defenders included both the infant’s mother and her mother, who in spite of being older, smaller, and weighing much less than the muscular young male, was the most audacious. “The grandmother attacked . . . with particular persistence,” prompting retaliation from the male, who inflicted a deep cut on the crown of the old female’s head. Despite her wound, the grandmother continued to harass the male, managing to temporarily hold him off before he renewed his attack, once again dragging the infant and biting her in the stomach and ribs. Twenty-two minutes later, the baby was dead.47
Another instance of matronly heroics involved the langur monkeys I studied at Mount Abu in Rajasthan. A usurping male had been stalking a young mother and her infant for several days. After each abortive attempt, the male would give up for a time, then resume. To evade him, the mother retreated onto the flimsy outer branches of a jacaranda tree when suddenly the infant (but not his mother) fell. The male, who had been riveted to the pair, immediately bounded to the ground and was the first to reach the fallen infant. He was pursued seconds later by the two oldest females in the group. After a fierce struggle, they managed to retrieve the superficially injured infant and return him to his mother. Then the oldest of the two, a solitary female I called Sol because she spent so much time on the margins of the group, continued to harass the male.
This female had ceased to menstruate and no longer bore offspring. She spent her days foraging on the outskirts of the troop and never attempted to take and hold babies. Her main contact with other females was to give way when they displaced her from a feeding position. To all appearances Sol was just biding her time until she died. Yet the striking thing was how abruptly Sol could transform herself into a super-hero. I was a young woman myself, 26 years old and still childless when I watched, astounded, as again and again this worn-toothed old female fought with a male twice her weight and armed with dagger-sharp canine teeth.
In general, monkeys are cautious about escalating aggression, warily sizing up the opposition in advance so as to avoid being wounded. I marveled at Sol’s audacity. Based on what is known about the breeding structure of langur groups, she was almost certainly either a grandmother or great-aunt to the infant she defended.48 It was her extraordinary selflessness that first inspired my interest in the evolutionary importance of old females.
(Top) After retrieving the slightly wounded infant from the male, a grimacing Sol continued to punish the male, slapping at his face and pulling the hairs on his face with one hand while fending off his bites with the other. To me this postmenopausal old female was signaling: “Attack this baby one more time, and it is going to cost you.” (Bottom) Nevertheless, four days later, the male was able to grab the infant again (his body can be seen swinging from the male’s muzzle like a rag), and Sol together with another old female charged to the rescue. Although they succeeded in getting the baby back, he was horribly wounded, with cuts in his head and deep wounds in his groin. (S. B. Hrdy/AnthroPhoto)
Since then, systematic observations across species have shown that the presence of a mother makes her daughter more secure. Across the very hierarchical cercopithecine Old World monkeys, among vervets, baboons, and macaques where a female’s rank is inherited from her mother, having a grandmother nearby has a significant impact on the childrearing success of younger kin. This is so even if the grandmother is still fertile and preoccupied by her own infants. Just her quotidian presence results in modest improvements to her daughter’s or her grandchild’s security. In the case of vervet monkeys, a young mother foraging with her own mother nearby will allow an older infant to wander about more freely than at other times. The independence permitted a two-month-old vervet with his or her maternal grandmother present was comparable to that of a three-month-old who did not have a grandmother’s support.49
Modest differences add up, especially in the case of young and inexperienced mothers. When vervet matriarchs were experimentally removed, their absence was correlated with a marked decline in survival and fertility of daughters between the ages of four and six years. Vervet females were less likely to be threatened or attacked by competing females and were more effective at keeping their babies alive if their own mother was in the same group. Similarly, Japanese macaque females who have a postreproductive mother nearby give birth for the first time at earlier ages, and give birth again after a shorter interval than do females without a mother present.50 Since females of higher rank give birth at a younger age and produce more offspring who reach adulthood, over many generations the cumulative effects of mother-supported rank are potentially enormous.
When an older female has more than one daughter in the troop, she spends more time near the youngest or least experienced daughter, the one who most benefits from her support.51 In the langur case, aging females with little potential for directly contributing to the next generation’s gene pool (that is, females whose reproductive value was low) were much more willing to defend the offspring of their kin, with whom they shared some genes.52 The objection might be raised that valiant old females like Sol or the Moremi baboon grandmother were just defending a group member the way any adult female would.53 But no other adult females present, not even the infants’ own mothers, took anything like the risks their elders did.
The strongest evidence for the generalization that primate mothers breed more successfully with social support comes from Amboseli baboons. In five different groups, the mothers with the highest rates of infant survival were the most socially integrated. The most successful females all had a half-dozen or so close female associates.54 Taking into account the fact that “giving impulses” go up with age, it is easy to see why young females would benefit from remaining in their natal groups to take advantage of such selfless allies.55
Yet only a minority of mother apes have matrilineal kin nearby. Most social mammals, and the majority of monkeys, are matrilocal, but not Great Apes—even though their residence patterns are somewhat more flexible than previously assumed. However, if we accept Alvarez’s corrective, hunter-gatherer mothers (who among other things have different dietary needs than other apes) depart from the Great Ape pattern, and in this respect more nearly resemble Old World monkeys.
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sp; So what changed in the line leading to Homo sapiens to make it more advantageous and more possible for daughters to be near their mothers when they breed? What tipped the cost/benefit balance among early hominins in favor of young females remaining near kin? Or, alternatively, what made it possible for old females to relocate to be near female relatives who needed them? And what so increased the benefits of having aging kin nearby that natural selection began to favor longer postmenopausal lifespans? For these things to happen, three conditions had to be met.
First, great-aunts and grandmothers needed sufficient freedom of movement so they could live near kin or move to be where they were needed—that is, they had to have the opportunity to help. Second, old primate females needed some motive for their increasing helpfulness or altruism on behalf of kin. Finally, these old matriarchs had to find some means to help—something useful they could do to enhance the reproductive success of younger kin, something so chronically useful that it outweighed the extra pressure that females past breeding age put on local resources.
IT’S TIME TO TALK ABOUT FOOD
What little we know about australopithecines suggests that although they walked on two legs, these tiny-brained 80-pound apes were built a lot like chimpanzees. By 2.5 million years ago, Homo habilis was starting to look more human, walking upright and using tools. No one knows for sure what led some of these creatures to evolve into heavier, larger-bodied, longer-legged, longer-faced, and larger-brained Homo erectus.56 Various factors were involved, as we will see in the next chapter, but one thing seems clear. Whatever else was going on, Homo erectus had found new ways to find, process, and digest the food needed to support both their larger bodies and, especially, their energetically more expensive larger brains.57 To date, the most plausible scenario is one set forth by anthropologists James O’Connell, Kristen Hawkes, and Nicholas Blurton Jones. According to their version of the grandmother hypothesis, new opportunities to help kin generated selection pressures favoring longer lifespans among postmenopausal women. But what were the new opportunities?
O’Connell and colleagues propose that long-term trends toward a cooler, drier climate at the end of the Pliocene pressured the precursors of Homo erectus to seek new ways to supplement their primary diet of fruit. By around two million years ago, game was increasingly important, but its availability was unpredictable. A division of labor between men who hunted and women who gathered also became more critical. O’Connell and others suggest that when neither meat nor more nutritious plant foods like nuts were available, our ancestors fell back on large underground tubers that plants in dry areas use to stockpile carbohydrates.
These storage organs occur throughout the savanna but are protected by a deep layer of sun-baked earth and are hard to extract. Savanna-dwelling baboons access shallower rhizomes and corms, and chimpanzees in the only population ever to be studied in a savanna habitat use pieces of wood to dig out shallower tubers, suggesting that australopithecines may have done so as well.58 But it takes special equipment to dig out the larger, deeply buried tubers. This is why, except for a few burrowing mammals like mole rats equipped with shovel-shaped incisors, humans are the only primates who exploit this widely available but difficult-to-access food source.59
Tubers are not only hard to extract, they can be fibrous and difficult to digest, hardly ideal food for children. Like nuts, they need to be premasticated or processed in some other way. To eat them, weaned youngsters would have to depend on older providers. Nevertheless, evidence is increasing that starchy tubers were an important fallback food among our ancestors. A 2007 report in Nature Genetics revealed that people like the Hadza who rely on roots and tubers have accumulated extra copies of a gene positively correlated with salivary amylase enzymes useful for the digestion of starch. Such copies are absent in Siberian Yakut herders and others with little starch in their diets. Tellingly, three times more copies of these genes are found in foragers who rely on starchy tubers than among chimpanzees, who, except for rare savanna populations, do not eat them.60
Not only do savanna-dwelling foragers have salivary juices specifically adapted for digesting starch, but African Homo erectus possessed the right teeth for the job. Isotopic analysis of their flat, thickly enameled molars yields results consistent with a diet containing underground roots.61 Once Homo erectus developed the use of fire, perhaps as early as 800,000 years ago, roasting tough, fibrous tubers would have rendered them more digestible and more useful still.62
Even before cooking, the addition of tubers to the other plant foods gathered by women would have provided new incentives for food sharing between hunters and gatherers, as well as new opportunities for postreproductive women willing to enhance the survival of kin. For women who knew where to look and who were willing to walk long distances, dig into hard earth, and carry their bounty back to camp, tubers provided a widely available if not particularly palatable source of calories when other foods were in short supply.63
The experience and diligence of old women would have been useful in other contexts as well. In many parts of Africa today, tree nuts provide a protein-rich staple for chimpanzees and humans alike. But perfecting the art of cracking their hard outer shells can take years.64 Furthermore, if every gatherer is a botanist—expert at identifying which plants are edible versus poisonous and predicting their availability—older women are the PhDs. Paula Ivey Henry describes one old Efe woman’s uncanny ability to locate medicinal plants and vegetable foods rarely used except during famines. Her own children had all died, yet this wizened old woman spent hours in the forest collecting fish, shellfish, nuts, fruits, and roots too scarce or hard to locate at that time of year for other women to bother with or even remember.65
The significance of ethnobotanical knowledge for the well-being of children in parts of the world where most people are perpetually undernourished is only beginning to be studied. In 2007 a team of American and Spanish anthropologists working among Tsimane forager-horticulturalists in Amazonian Bolivia reported a significant correlation between how much mothers knew about the diversity and uses of local plants and the nutritional status and health of their children. The effects were independent of other measures like household income or years of schooling.66 Although the researchers did not report how mothers acquired their special knowledge, most likely it was transmitted woman-to-woman.
Other forms of traditional knowledge—about environmental hazards, diseases, or people in distant communities—are less likely to be gender-specific. Across primates, aged females and, when they are still around, aged males provide vital reservoirs for intergenerationally transmitted knowledge. Whether Hamadryas baboons or foragers, it is the oldest group members who remember where to find water in drought years when all the usual sources have dried up. But with the exception of humans, information and skills are primarily transmitted through demonstration rather than through teaching or the intentional sharing of knowledge. When suffering from diarrhea, for example, chimpanzees seek out a particular plant that hinders intestinal parasites. But as far as I know, chimpanzees only medicate themselves. It was the skilled utilization of new food sources and technologies in the genus Homo, combined with the increased importance of sharing and teaching, that opened up new possibilities for kin-directed assistance between generations and for altering the cost/benefit ratio of keeping older group members around.67
THE MORPHING OF GRANDMOTHERS
We have come a long way since the days when evolutionists and anthropologists alike ignored females past reproductive age. Today, the presence or absence of postmenopausal women, their longevity, their efficiency, along with their dedication to kin have become legitimate research topics. The new significance accorded postreproductive females was very much in evidence in 2002, when I attended the first-of-its-kind international symposium on “the psychological, social, and reproductive significance of the second half of life” at the Hanse Institute for Advanced Study in Delmenhorst, Germany.
Throughout the last quarter of th
e twentieth century, sociobiologists, many of us women, had worked hard to expand evolutionary theory to include selection pressures on both sexes. Along with other fieldworkers, I had also been studying the contributions to infants’ well-being of both mothers and allomothers, old females included.68 But this meeting was the first time that researchers from around the world convened specifically to discuss the impact of grandmothers.69 Well past menopause myself, yearning for grandchildren, I was anything but a disinterested participant.
Kristen Hawkes was there, along with Ruth Mace, who two years previously had reported that the presence of a maternal grandmother halved child mortality among the Mandinka. The German primatologist Andreas Paul summarized accumulating evidence that menopause can no longer be considered uniquely human and that other primates, if they live long enough, may also cease to menstruate before they die, and also exhibit strong impulses to help younger kin. What is unusual about humans, Paul stressed, is not that follicles in a woman’s ovaries peter out around age 40 but how long women go on living afterward.70 Just why this might be useful was explained by the anthropologist Donna Leonetti as she described what she and colleagues were learning about Khasi tribal peoples from Meghalaya in northeast India.
The Khasi are among the few matrilineal peoples to retain their traditional way of life. Daughters, especially the youngest daughter, continue living with their mothers after they begin to bear children, and this residence pattern pays off in higher child survival. Twelve percent of Khasi mothers had lost one or more children before the age of ten, but the chances of a child dying were 74 percent greater if no grandmother lived with them.71
A young woman who does not happen to reside matrilocally may nevertheless return to her mother around the time of first birth. The Bavarian medical anthropologist Wulf Schiefenhövel stressed the value of such customs. Not only is support at hand during childbirth, but if the mother dies before her children are independent, or chooses not to rear her children, matrilineal kin are available to help. Among the Trobriand Islanders Schiefenhövel studied, 27 percent of children, especially firstborns, end up being fostered out shortly after weaning and are reared by allomothers. In about a third of these cases, the adopter was their grandmother.72
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