OF MARMOSETS AND MEN
Earlier in this chapter we saw how Aka and Hadza men adjust levels of paternal investment in line with past experience and with the local availability of alloparents. These hunter-gatherers are anatomically and cognitively modern humans. They are fully endowed with language and have the foresight to consciously calculate the costs and consequences of their actions. By what logic, then, can I argue that smaller brained, prelinguistic hominins, with far more rudimentary technologies at their disposal and living as long as 2 million years ago, experienced similar emotions and were similarly capable of adjusting parental and alloparental effort and cooperating with one another this way? The answer, quite simply, is that all primates are social opportunists. Even those with nowhere near human levels of cognitive processing capacities, foresight, tool kits, or language are nevertheless adept at social compensation.
Consider some particularly well documented monkey examples. If a savanna baboon female loses her mother, she sets out to strengthen her relationships with her sisters; and if her sisters die, she cultivates tighter friendships with more distant kin; failing those, she turns to nonkin.88 Cooperatively breeding primates (as well as cooperatively breeding birds and mammals outside the Primate order) appear just as flexible, or perhaps even more so, when it comes to adjusting levels of care in line with variable family composition.
As in the case of men, male marmosets paired with pregnant females experience a cascade of hormonal changes that not only induce these callitrichid males to gain weight—the marmoset version of couvade—but also lower their threshold for responding to babies and becoming more nurturing.89 Just the scent of his infant is enough to produce a drop in the serum levels of circulating testosterone in a male marmoset.90
So far, surges in prolactin synonymous with male nurture have been reported only in New World monkeys such as marmosets and in men who hold and carry infants. However, declines in testosterone levels in males just spending time in close association with a pregnant or lactating female have also been reported for wild olive baboons, even though care in this species is largely confined to protectively remaining nearby and vigilant and does not usually include carrying infants.91 In the few cases where scientists have compared hormonal changes in closely related species with and without male care, the species without male care do not exhibit these pronounced hormonal responses around the time of birth.92
Much of what we know about callitrichids comes from captive studies. However, the psychobiologist Karen Bales and her coworkers have also studied sources of variation in parental care among rare and highly endangered golden lion tamarins in the forests of their native Brazil. Just as in the studies of human foragers, the more alloparents in a tamarin group, the less help the father provided. That is, the total amount of care the infant received remained roughly the same, even as the father’s contribution declined.93 But when alternative caretakers were not around, fathers helped more. The nonhuman primate mothers were similarly flexible and opportunistic, responding to the local caretaking situation and also to their own condition, investing more when they were in good physical shape (as measured by body weight) and when they labored under fewer energetic constraints. Bales summed up their findings this way: Mothers invest more than the absolute minimum either when “they have to or when they can.”94
Similarly opportunistic mothers and high- and low-investing fathers have also been described among the black tufted-ear marmosets at Jeff French’s Callitrichid Research Center in Omaha.95 With just 10 grams of gray matter—a hazelnut-size brain only 1/125th the size of an anatomically modern human one—tamarins and marmosets competently adjust parental and alloparental effort in line with their current physical condition, breeding prospects, and the availability, willingness, and competence of the assistance on hand. Cooperatively breeding nonprimate animals like meerkats are also opportunistic, helping or declining to help depending on a similar set of contingencies.96
Even before we begin to factor in the role of culture and conscious foresight in explaining the behaviors of parents and alloparents, we need to consider the wide range of situational, experiential, and endocrinological variables that affect individuals’ unconscious as well as conscious responses to infants. The importance of hormones initially came as quite a surprise when over a quarter century ago researchers first noticed that prolactin levels went up among male marmosets carrying babies. Although these research results were initially met with skepticism, they have since been replicated many times.97 Yet, even as evidence mounted that males experience hormonal changes when their mates give birth, it was not until 2000 that Wynne-Edwards and Storey looked for and found comparable hormonal changes in men.
There is no doubt in my mind that long-standing stereotypes about mothers who nurture and fathers who provide, stereotypes left over from the sex-contract era of anthropology, contributed to this delay. Even now when I mention hormonal changes in fathers, as I did recently to a nephew who was expecting his first child, the idea strikes many as too weird to possibly be true. “I thought prolactin was a woman’s hormone!” he exclaimed. But with new evidence and new ways of thinking about human childrearing, as the “real”—and very flexible—Pleistocene family steps forward, new questions along with new answers about what children need and how they got it during humankind’s long evolutionary past are emerging.
Only since the beginning of this century have scientists really begun to study the physiological underpinnings of male commitment in humans and to compare these effects with those in other animals. As this work gets under way, it is already clear that some remarkable, heretofore undreamed of similarities exist between marmosets and men. I am convinced that these parallels reveal important convergences in taxonomically quite distant primates, albeit primates who happen to share a deep evolutionary history of cooperative breeding. I also believe that this new physiological evidence underscores a conclusion arrived at some time ago by workers from social service agencies concerned about the global prevalence of deadbeat dads: There is a vast but all too often untapped potential for male nurturing out there.
6
MEET THE ALLOPARENTS
In the ants and other social insects, we are thus privileged to see not only how complex societies have evolved independently of those of humans . . . [but also] the forces of natural selection that formed and shaped them.
—E. O. Wilson and Bert Hölldobler (2005)
Self-conscious as we humans are, we cannot help crediting our mind-reading capacities and giving impulses to another distinctively human trait, a big brain and with it greater intelligence. Animals like chimpanzees and corvids show signs of anticipating future events and planning ahead, but not as routinely, inventively, or rationally as humans do. None combine “forethought” with our unusually well-developed impulses to share and cooperate. Nor does any other animal have anything like our species’ infinitely expandable language. It is taken for granted that big brains and language account for what is most special about humans.1
At first glance, the claim that Pleistocene apes in the line leading directly to Homo sapiens were also the only apes to share care of young appears to posit a connection between big brains and enlightened childcare. Yet shared care of young along with strategic maneuverings by alloparents are also found in primates with nothing like big brains. Tiny-brained marmosets and tamarins excel at sharing and coordinating care. Obviously brains are important. Human behavior cannot be understood without taking into account all the vast and intricate coevolutionary processes that contribute to the evolution of sapient intellects. But not every aspect of our humanness is explained by bigger brains.
We have no basis for presuming an essential connection between sapient brains and the ability of both kin and nonkin to coordinate need-sensitive care and to provision young. In my view, cooperative breeding came before braininess. It set the stage so that apes with longer childhoods and greater intersubjectivity could evolve, and these traits in turn paved the way fo
r the evolution of big-brained, anatomically modern people. Brains require care more than caring requires brains.
To underscore this point, in the present chapter I broaden my scope beyond the Primate order (composed of monkeys and apes who do by and large have big brains relative to their body size) and examine alloparental caretaking in species that are taxonomically more remote: wolves, wild dogs, meerkats, bee-eaters, scrub jays, cichlid fish, paper wasps, and many others. In doing so, I emphasize how nonessential a sapient or, for that matter, even a primate mentality is for the sort of situation-dependent decisions cooperative breeders routinely make. Strategic alloparenting was well within the capacities of our prehuman ancestors long before our craniums started to expand. This is my primary reason for casting the net wide enough to include nonprimate cooperative breeders. But there is also another reason.
We have no time machine to return to the Pleistocene and observe hominins with shared care going about their lives. There are no firsthand observations to inform reconstructions of how early hominin mothers became less possessive of their newborns and began to relinquish exclusive access, or to explain why others were willing to accept such a charge. Yet, for several decades sociobiologists have been asking questions about a range of cooperatively breeding animals, some of them living under ecological and social conditions reminiscent of the challenges that Pleistocene hominins also faced. These studies can help us understand the processes through which cooperative breeding evolves.
From the outset, the study of cooperative breeding, particularly theoretical efforts to explain altruism toward the young of others, was central to the field of sociobiology.2 There is now a vast body of evidence and theory aimed at elucidating the evolution of alloparental care and provisioning in social insects, birds, and carnivores. We know far more about the behavior, ecology, and genetic relationships of these animals than will ever be the case for extinct hominins. Comparisons across cooperatively breeding animal species provide our best hope for understanding what sort of selection pressures induce individuals to help rear someone else’s young rather than attempt to breed themselves. Insights thus gained can also help explain why mothers among highly social apes living in Africa two million years ago might have abandoned their long-standing practice of mother-only care.
BIRDS OF A FEATHER, AND WHY WE NEED TO CONSIDER THEM
No one was giving much thought to the evolution of cooperative breeding (the term did not even exist) back in 1935 when an article “Helpers at the Nest in Birds” first appeared in the journal Auk.3 By the 1960s, primatologists were reporting on “aunting” behavior by females other than the mother in monkeys, and soon after terms like “communal care” were being used to describe co-mothering in rodents and lions.4 Meanwhile, the evolutionary theorist William D. Hamilton was still pondering a question that had puzzled Darwin a century before: How could such seemingly altruistic behavior evolve?
With the publication in 1975 of Edward O. Wilson’s Sociobiology: The New Synthesis, “cooperative breeder” became the umbrella term applied to any species with alloparental care and provisioning.5 By now we know that cooperative breeding occurs in a taxonomically diverse array of arthropod, avian, and mammalian species, including some 9 percent of roughly 10,000 species of birds and at least 3 percent of all mammals.6
The demographic consequences come as no surprise. Cooperatively breeding birds like Florida scrub jays manage to successfully rear young in exposed habitats that would be inhospitable to other jays. With alloparents to help guard them, their nests are less vulnerable to predation. Cooperative breeding and the flexibility it permits for rearing young successfully in a wide range of habitats, including otherwise adverse ones, allowed wolves, elephants, and lions (all of which were once much more widely spread around the world than they are today), along with various species like corvids, mice, and humans (all species that remain unusually abundant and widespread today), to move out of Africa or, as in the case of many cooperatively breeding birds, Australia, migrating to almost every continent of the world.7 Alloparental assistance means that mothers conserve energy, stay better nourished, remain safer from predation and other hazards, and survive to lead longer lives. Because mothers with help wean babies sooner, many reproduce at an accelerated pace. This means more young born over the mother’s lifetime and, even more importantly, more young likely to survive.8
Right from birth, an elephant calf is protected by his maternal aunts and grandmother, as well as by his mother. Any of these closely related allomothers may allow the infant to nurse. Siblings and cousins too young to lactate may engage in “comfort suckling.” Not surprisingly, calf survival is correlated with how many allomothers are in the family unit. (Oxford Scientific)
Mothers sufficiently confident of the benevolence of groupmates can entrust helpless and delectably edible offspring to their charge while they devote energy to producing more and bigger babies. With others, often including dads, to help, mothers are able to provision their youngsters, who in turn can afford the luxury of growing up slowly, building stronger bodies, better immune systems, and in some cases bigger brains without succumbing to starvation in the process. Among cooperatively breeding social carnivores—African wild dogs, gray wolves, red foxes, lions, banded mongooses, and meerkats—alloparents offer pups milk, meat, or both as well as protection. As with marmosets, alloparental pro-visioning—of mothers during pregnancy and of their young after parturition—permits significantly heavier litters. On average, each pup weighs proportionally more and grows faster than do pups in closely related, noncooperatively breeding carnivores.9 Across avian as well as mammalian taxa, the number of alloparents correlates with pup survival.10 In meerkats, Australian apostle birds, and white-winged choughs, chicks or pups do not survive without alloparental care.11 The same “obligate” care may have applied to child survival under the high-mortality conditions that characterized foraging populations among our Pleistocene ancestors.
From Charles Darwin to Edward O. Wilson, great naturalists have been intrigued by societies with divisions of labor and levels of cooperation as extreme as those found in honeybees and humans, and have sought their evolutionary rationales. By behaving like “superorganisms,” Wilson proposed, such creatures have been able to occupy their respective “pinnacles of social evolution” through better survival and preemptive exclusion of competing organisms, thereby spreading around the globe with spectacular success.12 In an influential paper entitled “An Evolutionary Theory of the Family,” the ornithologist Steve Emlen detailed finer-grained similarities between human families and cooperatively breeding birds known as African bee-eaters.
It is no accident that the language of ornithology has always been rich in anthropomorphic descriptors for the behavior of avian “husbands” and “wives.” Emlen’s parallels included “adultery” and “incest avoidance,” problems with “stepparents,” as well as Freudian-style father-son conflicts over who gets to breed, with dads chasing away their sons’ prospective mates to force their sons to work for the family unit instead.13 Yet until recently, mammal researchers studying cooperative breeding were surprisingly silent about where big-brained, bipedal human mammals fit in. When lists of cooperatively breeding mammals were drawn up, humans were rarely included.14
By the close of the twentieth century, however, this situation began to change. My own investigation of cooperative breeding was piqued by an interest in maternal emotions and infant needs. In 1999 I argued that unless early hominin mothers had been able to count on significant alloparental as well as paternal contributions for the care and provisioning of extremely costly, slow-maturing young, the human species simply could not have evolved.15
Today, comparisons with humans are increasingly cited as reasons for studying mammals with cooperative breeding. Such justifications can focus either on the highly specialized division of labor and group-level arguments offered by Wilson or on Emlen’s arguments about how complex families work.16 But whether one approaches cooperative
breeding from the perspective of the “superorganism,” the family’s internal workings, the mother’s interests, or child well-being, the same evolutionary conundrum pops up: How could natural selection favor alloparental behaviors leading individuals to care for and provision someone else’s young?
THE CRITICAL IMPORTANCE OF SHARING FOOD
Part of the explanation for the evolution of alloparental care is that these behaviors are not always as self-sacrificing as they appear. In many instances, babysitting is occasional, engaged in for the most part when an animal has little else to do. Gifts of food may be proffered only when not actually needed by the donor. Over lifetimes, alloparents strategically schedule assistance so as to reduce the cost, volunteering only when helpers have energy to spare or when they are still too young or too dis-advantageously situated to be able to reproduce themselves.17 In animals where practice is critical for learning how to parent, as is the case in many primates, babysitters derive valuable experience from caring for someone else’s young.18 But what about cases where care actually is costly, as when allomothers provide hard-earned food or give up their lives altogether?
The easiest way to get bitten by a dog or other animal is to reach for its food. Yet alloparents routinely volunteer food. In many cooperative breeders, allomothers even provide breast milk, which is, metabolically speaking, the costliest substance a mammal produces. Milk is so precious that in herd-dwelling mammals like sheep or elephant seals, mothers resolutely refuse to share it—viciously butting aside orphaned or separated infants who attempt to pirate their “white gold.” How odd then that an animal who had taken risks and gone to so much effort to catch or collect food, especially after converting the food into milk, would then deliver this hard-won prize to someone else’s offspring. And yet humans and marmosets are far from the only creatures where allomothers not only routinely guard, defend, keep warm, groom, or carry about infants other than their own but also provision or suckle them.
Mothers and Others Page 20
