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THIS WEEK IN
CALIFORNIA WILD

interview

The Female Line:
Sarah Blaffer Hrdy

California Wild: I was intrigued to learn that you got started on your intellectual journey of the last 30 or so years during an undergraduate course at Harvard, at a time when the “man the hunter” paradigm had come to the fore as a model for human evolution. At what point did it become clear to you that traditional views of human evolution discounted the contributions of females?

Sarah Hrdy: Just learning about Darwin was an eye-opener, and my early writing about monkeys was well within the conventional male-centered view of contemporary evolutionists. Starting to examine selection pressures from a female’s point of view came later. At the outset I was entirely focused on this bizarre behavior, males killing infants among langur monkeys supposedly because the population was crowded. I had no idea that studying this behavior was going to cause me to rethink what I thought about my own sex.

CW: So what got you started?

SH: After graduating, I went off to Stanford to a film program. While I was there I started auditing Paul Ehrlich’s course on population biology. He was talking about population and crowding, and mentioned the experiments of John Calhoun [of Walter Reed Army Institute of Medical Research], who took a bunch of rats and let the population double, and double, and double again.

Among the things he was seeing was males killing infants, and he developed a theory he called “social pathology.” If I were going back and studying those same rats today, I would actually interpret it a little differently, I think, because there were little communities within the crowded rats where mothers were continuing to breed successfully. But then I thought, Whoa! That’s what must be going on with those langur monkeys I heard about in my undergraduate course. That’s when I decided to return to graduate school at Harvard in anthropology, to study infanticide among crowded monkeys.

I went to India to look for langur monkeys and found out fairly soon that my hypothesis was not going to explain what was going on. I had deliberately selected langurs with the highest density that I could find. Yet the male’s behavior towards infants was aloof, but very tolerant. Juveniles would jump on him like a trampoline and he would barely bat an eye. I expected to find infanticide, or at least some tension. But the langurs were very relaxed—until new males, usurpers, entered the breeding troops from outside. These were the males who targeted infants.

CW: What were the attacks like?

SH: They were very goal-directed, very single-minded. Nobody would mistake them for “shit happens.” There is no comparison between langur males attacking infants and, say, child abuse when babies are shaken to death in a tragic mixture of brutality and poor judgement. Langur males taking over groups of females stalk the mother, and when they get hold of the baby, they bite it in the skull if they can. The behavior is very deliberate. This did not seem like pathological behavior brought about by crowding or by stress. Something else was going on. So I had to rethink things.

I decided what I was seeing was the result of Darwinian sexual selection, male-male competition for access to females, in which the male who wins doesn’t necessarily kill the male who loses, but he sires more offspring. You had a male eliminating the offspring of his competitors in order to increase his own reproductive success. In terms of female choice: the females had made a choice. They made it with the last male, the father of their offspring. This new male was eliminating their last choice. He was canceling female choice. These were new interpretations, but squarely within the classic Darwinian framework of sexual selection and female choice.

When I suggested that infanticide might be an evolved behavior, the old guard within anthropology was very upset. Phyllis Dolhinow, a professor at Berkeley, wrote a now-famous letter to American Scientist saying that these could not be “normal monkeys.”

CW: But if you presented it as being a standard Darwinian paradigm, why did they find it so repellent?

SH: Because most anthropologists were still thinking in terms of group selection. The idea was that animals were only selected for behaviors that promoted the survival of the group and the perpetuation of the species. As Dolhinow put it, this looked like “destruction,” not “adaptation.” She did not see how you could have such selfish behavior counter to the good of the species or the group. It was the kind of classic confrontation that happens when a scientific paradigm is shifting. The paradigm shift during the 1970s was over group selection versus individual selection. My views were iconoclastic among primatologists and anthropologists, but within evolutionary biology, this focus on individuals was already becoming mainstream.

When I was inducted into the National Academy of Sciences, the citation read by the home secretary focused on the very ideas that, within anthropology, had gotten me into trouble: I had helped to show that selection operated at the level of the individual rather than the group. And frankly, looking back, in spite of all the controversy, I was lucky. The interpretations I ended up with fit well within a sociobiological framework, permitting me to establish a reputation as a hard-nosed evolutionist before I started writing about female reproductive strategies. Because that’s when I found myself criticizing traditional paradigms within evolutionary biology. It was not so much that I thought Darwin was wrong, but that I realized that so far as females were concerned, his theories, especially his theory of sexual selection, were woefully incomplete.

You have to understand that women in science are always at special risk of being marginalized. And people who are saying things that are very much against the grain are even more at risk. Add to that a taint of feminism—well, normally, that would have been the kiss of death. This is why I say that if I had not established my reputation based on ideas firmly within an established Darwinian paradigm, I very much doubt that you would be interviewing me today about what I think about mothers.

CW: Now that we’ve gone from the “man the hunter” through the “woman the gatherer” hypotheses and the “grandmother hypothesis,” how close do you think we are to having a comprehensive theory about early hominid social relationships that might help us deal with social issues that we grapple with today?

SH: We’re better off than we were when we assumed we already knew how man the hunter lived in humankind’s “environment of evolutionary adaptedness.” Today, at least human behavioral ecologists and sociobiologists, after 25 years of tossing and turning, have put to rest the notion that there’s got to be one prehominid human family system. There are a lot of behavioral ecologists out there who now assume that early hominids were really flexible, that they were seeking opportunities, that they were migrating away from adversity and that they preferred to be with kin. But beyond that, we’re not out there saying, “oh, they lived in polygamous harems,” or “oh, they lived in monogamous groups.” We’ve moved away from typologies.

We are talking about very flexible primates. If you think of the langur monkeys I used to study, or you look at chimps, the variation within each species is enormous. Langur monkeys high in the Himalayas live in multimale groups and exhibit very little aggression. When you look at langurs at lower altitudes, living at higher population densities, there’s usually only one male, and a lot more aggression.

Take the intraspecific variation in langurs or chimps, multiply it several times over, and you have humans. Humans are extraordinarily variable: a “weedy” species that survives and prospers in a wide range of different habitats and uses their cooperative social system as an important tool in helping them to adapt. My guess is that some of our ancestors were in chronically shifting breeding systems, sometimes polyandrous, sometimes polygynous, other times monogamous relationships. Maybe the monogamous relationships were most stable. The female has something to gain, and the male has something to gain. But in polygamous relationships you’re going to have the female pushing against having these extra wives around. Polyandrous relationships—they are going to be tense for other reasons.

The predictions of Steve Emlen [of Cornell University] about cooperative breeding in birds were, in fact, lovely summaries of some of the things we already know are going on in humans. I think the model of cooperative breeding is going to be extraordinarily useful for researchers trying to understand the selection pressures on mothers and infants during the Pleistocene. But we are first going to have to broaden our definition of cooperative breeding.

CW: Can you elaborate on how you would rework the definition?

SH: In the classic cooperative breeder model, other group members help to rear the offspring. Classically, cooperative breeding was thought to be linked to suppressed ovulation as in social insects (where only the queen lays eggs) or as in wolves or marmosets, where only the dominant female ovulates. Today we are developing a broader definition of cooperative breeding that focuses on the outcome: anytime other group members help rear infants with the result that mothers can breed faster, produce bigger young, or young that survive better. The emphasis needs to be on availability of “allomothers,” individuals other than the mother who, for whatever reason, can be recruited to help a mother rear her young.

One way to ensure nonbreeding help is available is through suppressed ovulation. For a long time this was viewed as the defining feature of cooperative breeding. In those cases, the subordinate female doesn’t ovulate, for if she does and conceives, there is the risk that the dominant female will kill her infants, as Leslie Digby and others have reported for wild marmosets.

Suppressed ovulation is not the only thing going on that makes nonreproductive helpers available. Any species with delayed maturation, for example, is going to have babysitters on hand. So another route is through long postmenopausal life spans. This is one of many respects in which members of the genus Homo are especially well equipped to reap the benefits of cooperative breeding. Add to that a kind of general primate propensity to find infants fascinating, to look out for them, and nurture them.

Even males—highly macho gorillas, or male langurs—have a nurturing component to them that gets activated under the right conditions. It’s just that the threshold for nurturing is set very high in these guys. And then you have the species like titi monkeys where the opposite is true: the threshold for nurturing is set low, even lower perhaps than in females. Something has changed. Perhaps there are more receptors for oxytocin in the brain that have been selected. If a mother titi monkey is in a cage with her mate and infant, she is more upset if you take her male out than if you take the infant out, whereas the male is more upset if you take the infant out than if you take the female out.

CW: What about humans?

SH: I suppose there may be some men out there who cannot be primed, but as in most primates, men can get interested in babies, responding to infant need, dependency, and long periods in proximity to a baby. The threshold for responding is lower in women—they are more likely to come over and go “koochy-coo”—but sooner or later, guys get into it. I think any human—reproductive or nonreproductive, man or woman—who has ever raised a baby becomes emotionally very involved and really cares about that baby. And that’s what you need for a cooperative breeder. An individual with the emotional underpinnings to get involved in caretaking, a mammal that can be hormonally altered by exposure to infants and rendered more nurturing.

CW: But why are you so convinced that humans evolved as cooperative breeders?

SH: Imagine this apelike female, perhaps the ancestor of Homo ergaster, who’s giving birth to one baby at a time. She’s nursing it for a long time like a chimp does, four or five years. How could there ever have been selection on this female to produce a baby so far beyond her own means to rear? Why set herself up for failure by producing a human baby when she could continue to produce a chimplike baby that would be independent at weaning and would survive. The only way there could have been selection on any mother to produce a baby so far beyond her means to rear would be if she could count on others to help her.

What I think I know about human men is that they do help and that they are terribly important for infant survival. The well-being of children goes way down if they don’t have fathers. But men are not always reliable. Fathers defect, they die. They take up with a new mate. And there must have been high mortality, not just for infants but for adult men, who were hunting and perhaps getting into fights with one another, especially if men were looking for additional opportunities to mate. Whether I am scanning the ethnographic record for hunter-gatherers or for modern post-industrialites, there is nothing about the human case that makes me think that hominid males evolved to behave like titi monkeys, putting babies first.

So back to the question: How could there ever have been selection pressure on this chimplike female to produce such a costly offspring? The only way I can think of is that she had backup, that she was eliciting help from the father if she could get it, but otherwise, help from her brother, from other men she might have had sex with, from her own kin, from grandmothers, cousins, aunts and uncles. I just don’t see any alternative solutions to the human mother’s dilemma.

As behavioral ecologist Hillard Kaplan calculates, a kid growing up among foragers needs on the order of 13 million calories before they’re independent at around age 18. And think about a young girl reaching menarche. Hunter-gatherer girls are not fertile until age 15 or 16. In many tribal societies, girls are not giving birth to their first baby until they are 18 or 19. Today, menarche is age 12 ¹/₂, very low. So we have this capacity for earlier menarche and earlier birth, but a girl would never have enough fat on board to have that surge in hormones triggering ovulation unless she had other group members helping to provision her.

Today, because we’re so much more sedentary and girls are so much better fed, a young woman can be fat enough to ovulate without having anything like good social support. We’ve altered the conditions of human existence. Maybe we ought to think seriously about compensating for what we’ve done. Girls are not ready to be mothers at that age, or if they are going to be mothers they need a tremendous amount of social support, and it just isn’t there. I see in this way of looking at the world things that I wish policy makers and politicians were talking about.

CW: It seems like a lot of people are not ready to be fathers either. I heard you say that in California alone, fathers owe 14 billion dollars in unpaid child support. How do you think we can turn these cads into dads and get them to behave more like titi monkeys?

SH: I’m a big fan of companionate monogamous marriage and fortunately have the luxury to live that way, since I don’t need to scrape together help from several men to support my children. I am a fan not because I think it’s more natural than other lifestyles, but because I think long-term relationships are one of the few blessings in this vale of tears. I think monogamous pairs embedded in extended families (real kin, or artificially constructed “fictional” kin groups) are a great way to raise kids. But I’m not interested in projecting my mating preferences onto the world at large.

Clearly, if we’re interested in child rearing, we have to think of ways we can provide extended family-like settings. Drawing older generations in is one way, but it’s sometimes impractical in a highly mobile society.

Day care is another, constructing an artificial extended family. We just need to pay very close attention to stability in having the same individuals involved in care-taking. I have followed with great interest the very large study on day care being done by the National Institute of Child Health Care and Development [NICHD]. They find no harmful effects from day care per se, provided children feel secure and wanted in the first place and providers are sensitive and responsive to infant needs, and provided the day care is small-scale and stable—in short, day care that functions like extended family. I would say that the NICHD recommendations get a lot of support from anthropological findings.

CW: You were quoted in The New York Times as saying that everyone’s worried about rainforest destruction, global warming, ozone and so forth, and no one is really worrying about day care issues and how they protect the future of Homo sapiens. Can you elaborate on that?

SH: Of course I’m delighted that people are interested in the rainforests and habitat conservation, because without habitats we’re in bad shape. But I’m distressed that family planning has become so controversial that politicians are no longer willing to talk about it. Politicians pay some attention to environmental issues, but candid discussion of birth control, family planning issues, and unwanted children is virtually nonexistent.

Here’s how I look at it. Humans clearly have a capacity for compassion and for caring about other people. One of the things that helps people get along is our empathetic capacity, with a real emphasis on sharing, a real emphasis on someone not getting too much because this will destroy the fabric of relationships within groups. Humans are unusually good at cooperation, and this was central to the evolution of Homo sapiens. This capacity to empathize has a genetic component, but it is only expressed under particular rearing conditions. Children learn what psychologists call “perspective taking” in the first few years in the course of relating to their mother and other caretakers.

Children develop what evolutionary psychiatrist John Bowlby called “an internal working model” of how the world works. I can expect my kin to look out for me. Out of this developmental substrate develops an individual that looks out for others. Kids who grow up not knowing anyone is going to look out for them, who are not trusting or securely attached to their mother or other group members, are less likely to develop this uniquely human capacity for compassion. In the most extreme cases, you find someone with no compassion at all—the kind of people we call sociopathic.

Other animals do have a rudimentary capacity for compassion, such as chimps and dolphins, but humans really take intelligent empathy to a different level. They can articulate how someone else is going to feel. I can worry about AIDS orphans in Africa even though I have never seen them. Chimps don’t do that. Obviously this has to do with language too. But the bottom line is, humans have a capacity to put ourselves cognitively and emotionally in someone else’s shoes, and presumably we have it because this trait was important to the survival of individuals who depended on others and needed to cooperate with others.

The problem is, unless a trait is expressed in the phenotype, in the walking, talking individual, there is no way for a trait—no matter how advantageous—to be selected for. Genes are invisible to selection unless they are expressed in the phenotype. So how does the genetic predisposition to become a compassionate human being get expressed? It comes from developing within certain social contexts. So if we have children who are raised under conditions where they simply never develop these extremes of cooperative capacities that have really been a hallmark of our species, if they’re not expressed or if they’re expressed in only a small proportion of individuals, we are really changing the terms on which our species is evolving. Traits not selected for over evolutionary time have a way of getting lost—like the capacity for sight in cave-dwelling fish.

CW: It sounds like you don’t think evolution has stopped yet in the human species.

SH: Evolution simply means changes in gene frequencies. And gene frequencies in human populations are changing all the time. What’s going on in Africa right now provides a classic example. In parts of the world where there is a 40 percent infection rate, women who are somewhat resistant to being infected with AIDS are going to have more offspring survive.

CW: So you think we are still evolving, and that we have changed the conditions under which selection operates on our species?

SH: Yes. We have changed the rules. In our evolutionary past, children without a number of people looking out for them— mother and allomothers—were very unlikely to survive. For the most part, having a supportive kin group was synonymous with surviving, so that most individuals had to develop within a fairly cohesive and supportive network of cooperative relationships. But today, for the first time in human history, there is the prospect of a large proportion of the population surviving to reproductive age in spite of not having been reared by caring kin. We’re changing the relative frequencies of people whose cooperative tendencies are expressed.

We are changing the rules of social evolution for the human species. I actually don’t know if anyone else is worrying about this. When I hear people speculating about environmental disasters that will destroy the human species, I find myself wondering: How do you know we will still be human? Tens of thousands of years down the line we’ll be smart and technologically sophisticated bipeds, of course, but will we still be human as we currently define that term?

CW: Then it’s a much bigger problem than just a mother being able to find other mothers to help share the burden?

SH: It is a larger social problem. If mothers don’t find other mothers and we rear a bunch of these extraordinarily self-centered takers, it probably has implications for conservation. I suppose you might have ardent conservationists who basically just hate human beings, but I think most conservationists are people who feel they have a stake in the future. They care about future generations, what kind of planet we are going to pass onto our children. I think these, by definition, are people who’ve grown up to be compassionate and cooperative, who are going to care about their family, are going to care more about their environment. My term for such people is “green mothers,” and they can be members of either sex.

I don’t think you find many sociopathic personalities worrying about global warming except as a kind of “the world is a bad place” general paranoia. One of the interesting things about people in tribal societies, hunter-gatherers, is you often have an ethic of, “the world is a wonderful and generous place, the forest is my mother, the forest gives to me.” It’s a metaphor for the social unit in which they grew up.

CW: You are critical of the idea that there is a single, overriding, self-sacrificing maternal instinct. What sort of instincts do you think evolution has provided mothers?

SH: I’m trying to move people away from this sort of lump-sum “the maternal instinct” because this is never going to apply to humans or any other mammal. So it’s going to lead to gender stereotyping and unrealistic expectations, as in the faulty logic behind those who imagine that there can’t be maternal instincts because some mothers don’t care for their babies. I want to remind people what makes a mother committed to her baby. Think about things that are going on during pregnancy: hormonal changes; estrogen and progesterone secreted by the placenta lowering a woman’s threshold for responding to babies; all making her even more responsive to babies than female primates normally are. But at birth the placenta is ejected along with the baby so that little hormone factory inside is no longer there.

At that point, other systems chip in. You find oxytocin generated at the time of birth rendering the mother a little more mellow. Then, once lactation gets going and prolactin is being secreted, the mother is really on a leash. Among the many things that this jack-of-all-trades hormone does is produce protective and nurturing kinds of behavior. Add to that all the stimuli from the baby itself, as the mother responds to very specific cues: how the baby smells; how it sounds; how it looks. A lot of different systems are at work, transforming the mother, committing her to care for this infant. Every time she responds, she is further transformed. I love that line from George Eliot: “Our deeds determine us as much as we determine our deeds.”

CW: What you are saying is very different from the old idea that a woman has this little kernel inside her, “the maternal instinct,” that makes her do things.

SH: Exactly. The entire process is interactive and dynamic; you can pick it up at any phase along the way.

CW: What about mothers who adopt?

SH: Let’s say a woman missed the birth experience, or missed the first days after birth; she can come in late and still pick up from there. It just might take more stimulation. For example, an adoptive mother with a three-day-old baby who says, “I really want to breast feed,” can breast feed, but it’s going to take a lot of kneading of her breasts, maybe some hormone injections. But then, from that point on, the processes really start to work. We primates are remarkably flexible this way. One of the things I’m so sorry about is that we have this terminology, “the birth mother,” meaning the biological mother. In fact, we have genetic and gestational mothers and we have biological mothers. A lot of women who haven’t given birth can be perfectly good “biological” mothers because their hormone profile and their neural pathways change in response to this baby. That’s surely what it means to be a biological mother.

CW: In Mother Nature, you talk about the impossibility of legislating maternal love. What did you mean?

SH: We have politicians who say, “If we could just shake the bushes enough, we could get enough foster parents for all these children. We don’t need birth control. We don’t need informative sex education for teenagers, or abortions for teenage mothers, because we can always find good foster families to rear their babies.” They’re not thinking, because you can’t legislate these kinds of biological responses. You can’t make a teenage mother love her baby just by telling her she’s supposed to. You can’t make foster homes good places for children just by putting children in them. That’s why we have to recognize what a delicate, dynamic, and interactive process the creation of bonds between adults and infants is. This is what I mean by needing more biological literacy among policy makers.

Former California Wild Senior Editor Blake Edgar is a science editor with the University of California Press. He is co-author of the forthcoming book The Dawn of Human Consciousness.

Spring 2001

Vol. 53:2