I am living through a unique experiment
Few photographs drew so much attention in biology, as one showing two female langur-monkeys chasing a male (click here). Sarah Blaffer Hrdy, who took the picture, is a primatologist and a feminist. Two of her books are The Woman that never evolved (1981) and Mother Nature (1999). The following interview took place in Tucson, June 1997.
Why did you study the langurs?
In the sixties I was very much interested in the effects of population-density on human behaviour. John Calhoun had just done his famous study on Norway rats. He allowed them to multiply, and he let them breed and breed until they were all crowded together, and then he began to see what he called 'social pathology'. I couldn't study this for humans, but I heard about these langurs, so I went to India to study infanticide in langurs because I thought it was being produced by crowding.
The first thing I found was that males in Langur-troops are usually quite tolerant of infants, instead of being infanticidal. Young infants come up and jump on the male as if he was a trampoline. They hang on his tail and swing on his tail when he is up in a tree, when he is mating with their mother the older juveniles will bat at him and harass him. And this is very annoying behavior, I think you have to call such a male tolerant. He might get fed up and threaten them, but he doesn't kill them.
What kind of monkeys are Langurs?
Langurs are colobines. The subfamily Colobinae is very widespread; you find them in Africa, as the black and white monkeys, all over India as Hanuman langurs and in South-east Asia as leaf eating monkeys and those wonderful droopy nosed proboscis monkeys in Borneo. By far the most common of the colobines, the most terrestrial, the easiest to study and best studied, is the Hanuman-Langur. They are named for Hanuman, the monkey-God, the loyal servant to king Rama in the Hindu epic, the Ramayana. Hanuman langurs tend to live in one-male groups, though you can sometimes find them in multi-male groups. So you have one male with six females to twelve or more females, up to fifty-nine females in the largest one-male group I know of. These are breeding units. Females stay in the same home-range for their entire lives, among their mothers, grand-mothers, sisters, daughters and aunts. Territories are passed down from mother to daughter. These home-ranges slightly overlap, and when females meet at the borders of their ranges you have territorial aggression, involving both the male travelling with the group and the females. The females are actually much more persistent in territory defence, and often quite aggressive. Females in intergroup encounters are extremely active, and it makes sense. They are defending the resources that are available to them to survive and reproduce. There is a photograph of two Langur-groups fighting one another, and the male is just sitting there as comfortable as can be, while the females are skirmishing at the border, lined up at one another and slapping.
Males either leave the group at maturity, or more often what happens is that they are driven out by another male. Immediately after a father and his sons are ousted from a troop, they travel together for a while, but they usually join with other males. There is extremely little aggression among these males, you think you could not hold all these males together, that they would not get along. In fact, you cannot even detect in an all-male band a dominance hierarchy.
So males are travelling together in all-male bands, there may be anywhere between two to sixteen males, these are fairly flexible groupings. These males roam around and cover tremendous distances, and they are very hard to study for this reason.
In the morning when the resident male in a group of females wakes up, he goes to the top of a tree, and he gives these long-distance shouts: whoop, whoop! It is like: "I'm here, don't even bother trying today!". And the all-male band whoops back. And you think: Why are they letting him know that they are trespassing in his territory? Why are they giving him this information about where they are? Basically I think they are saying: Do it again, let's hear a little more. I think it is information-gathering, and it is in the resident male's interest to answer, because if he doesn't, they are more likely to come. But they may come anyway. They are constantly probing and exploring.
But the resident male is trying to chase them away.
The resident male tries to keep them off. There is a reproductive advantage to being a resident male, -he has a disproportionate probability of siring the offspring-, and so there is intense competition to be that male. And the males in the all-male band will attempt to take over and to oust the resident male.
When the males that come in actually attack the resident male, there is a big chance that he will fight back, and one of them will be injured. In such a climate injury is a serious problem; it could lead to gangrene and an animal could die. So neither wants to inflict damage on the other that would then provoke a response of serious damage. They are held in check, rather like nuclear powers, by the threat of retaliation. But if there is a skewing of the odds, such that the resident male is weak, or there are so many males in the all-male band, the male band is sometimes able to drive out the resident male, and all the young males may follow later, or they all go at once.
It is a very chaotic time. Everyone is fighting with everyone else, in the sense that the females are still perhaps exhibiting antagonistic behaviour toward these outsiders, and the males themselves are still fighting. And this is one of the reasons why people said that infanticide is just an accident, the infants are killed incidentally during this chaos. It is very hard to collect information when this is going on.
When these new males come into a troop, previously there were no dominance differences perceptible among them. After they take over a troop, -and now you might have five adult males resident in the troop-, the competitive behaviour to construct a dominance hierarchy begins to emerge, and one of these males in general emerges as the new alpha male. Occasionally two or three males will stay in for a time, and you will have briefly a multi-male group.
Are they brothers?
We don't know. We never obtained DNA, and it is unlikely that we will get it for India because these are sacred monkeys, and it is difficult for Americans to collect these data; we had a number of political problems working in India. What may be the case though, is that if the ousted male is still very powerful, it might make sense to tolerate some competition. You'll share some copulations in order to have the alliance of males.
You took a picture that became quite famous; Two female langurs chasing a male who has just captured an infant. What actually happened?
It is a very gripping image of a male who has an infant in his mouth. The image is blurred, you can just see the tail of the infant, its body flying off in space. And two females are charging the male to try to get that infant back.
The fascinating part is that neither female is the infant's mother. They are both older females in the group. The one in the forefront, a female called Sol, was almost certainly a close relative of the infant, perhaps a grandmother, perhaps a great aunt. We estimate that females in langur groups on average are related some place on the order of 1/16 of their genes by common descent, like first or second cousins. This was a female who did not reproduce during all the years of the study where she was observed. Whether one wants to call her menopausal or not, she was for all practical purposes post-reproductive. I am assuming that both are close relatives and therefore intervening and trying to save the infant.
What about the mother of the infant?
The mother of the infant herself was a young female at the peak of her reproductive career. Much more cautious, sitting on the sideline, letting the older females protect the infant.
In fact there are other cases where a mother for example allows her infant to drop from a tree, the male will rush to it, and it is not the mother who goes down to save it, it is again these older females.
Why these older females instead of the mother?
What I believe is going on is that females with a lower reproductive value have a different cost-benefit ratio for taking chances on behalf of relatives. This can be seen in other contexts. So for example McCarthy and Bugos, when they collected data on maternal infanticide among the Ayoreo Indians in South America, found that very young mothers and mothers with high reproductive value were much more likely to give up, and not go on with an infant under bad circumstances. Whereas an older mother will go ahead, no matter what, because she is getting near the end of her reproductive opportunities. So in other words, females are making very different decisions based on where they are in their life-history, and this is an important point when we come at a problem with pre-conceptions about what is maternal behaviour. Biologically, what is a good mother? Well, the truth is there is no one good mother, no one good solution. As post-enlightment Western human beings we have very set ideas about what is a good mother. But in biological terms, maternal investment is very facultative.
Why is the male trying to kill the infant?
As one male emerges as the new troop-leader, he will start to stalk females with young infants, and disproportionably the targets of his assaults are six months and under. Before a male is ousted, a langur male is extraordinarily tolerant of offspring, many of whom are likely to be his own. It is only when males enter the troop from outside the breeding-system, and encounter females that they have not mated with, that they exhibit this aggressive behaviour.
The male's behaviour is very goal-directed. He will stalk an infant for hours or days. He looks in different ways, in every direction except at the mother-infant pair, pretending not to be interested while moving closer. Then suddenly he grabs towards them. The behavior of the females also tells you something.
Females without infants don't particularly avoid this male. Females with young infants are moving away. They are very, very skittish of him. The fights that happen after the male has taken over, between females and the male, tend to be when the male has attacked someone's infant. And other females then come back and fight. In other words it does not appear to be a male attacking females in an effort to hurt the females. Rather the females attack the male because he is trying to hurt an infant. When you are watching this, no-one who has ever seen this happen has ever come away questioning the idea that this is goal-directed behaviour on the part of the male, it is not some incidental byproduct of social disturbance or chaos.
The sexual selection hypothesis that I proposed is based on Darwin's theory of sexual selection. You have competition between males for access to females, and in some cases female choice. Males are in competition with other males to breed with females. If a male has taken over a group of females, he has only a brief window of opportunity, twenty-seven months on average. If a female weans the offspring she already has, if she continues to lactate, she will not ovulate again for perhaps a year. By that time the male could be driven out by another male. So he is basically trying to subvert female choice. By eliminating the offspring of the last male who sired her infant, the new male creates a circumstance whereby the female is going to be under pressure to ovulate again soon.
Why don't females simply refuse to mate with an infanticidal male?
Because females are in competition with other females for representation in the next generation. If a female refuses to mate with an infanticidal male, and therefore waits, say two years before she breeds again, she leaves fewer progeny in the next generation then will her sister who does go ahead and breed. The second point is that if she is in a population where male take-overs are happening, her son, once grown up, will be at a disadvantage in competition with other males if he is not infanticidal. So females are under tremendous pressure to make the best of this appalling situation by breeding with an infanticidal male.
How do females defend their young?
The most obvious thing of course is that they try to stay clear of the male. Mothers threatened by infanticide are much more restrictive of their infants.
Females with an infant may leave the group and travel apart. If she has an infant approaching the age of weaning she may try to leave that troop with the male and the all-male band. Furthermore, females can sort of pre-emptively reduce the probability of infanticide if they have bred with a male before he comes into the troop. It is the mother who is the cue to whether or not he attacks or tolerates the infant. I have seen females kidnap infants from other groups and bring them back to their own, but so long as the female is familiar, the male does not attack these little strangers. So I hypothesized that females solicit outside males in addition to her resident male as a defensive strategy in case one of these comes in. In this way she manipulates information available to males about paternity.
Why was your interpretation of infanticide in langurs so controversial?
Part of the controversy that has come up is that people have assumed that I am talking about 'a gene for infanticide', a male either has it or he doesn't. I have never said anything about it, -I don't know. The same male is infanticidal in one context and not in another. And I know that some strains of mice are more infanticidal then others, so we have good reason to think that this is something that is inherited. But even in an infanticidal strain of mice, such a male may only engage in infanticidal behaviour 70% of the time when he has an opportunity to do it; so clearly context matters. The fact that the mechanism is still unknown was one problem. But the other, larger issue, was a reluctance to accept that something so obviously detrimental to the species or the group could be adaptive for individual males.
When my langur research was published, some people drew a parallel with human child abuse and the stepfather-phenomenon. It is structurally similar, but the rationale for it I now believe to be quite different. A human male who kills his stepchildren is usually not enhancing his reproductive access by that female. I agree completely with Martin Daly and Margo Wilson that the issue here is best explained in terms of competition for resources. The stepfather who assaults an infant belonging to his mate's previous partner is responding to the demands that an infant is making on him, on the mother, and on the household-resources.
Infants of course have been selected to be basically insatiable in their demands, and you have a situation that is more nearly comparable to another kind of langur behavior. Langurs are infant-sharers; females take and carry other females' infants. The only time you see infants abused by female langurs is when a female takes an infant and is subsequently no longer motivated to carry it. A female who has taken an infant that is not her own, and grows tired of holding it and no other female comes up to take it from her; -she tries to push it off herself. For an infant not being held by a female is paramount to death! If it is on its own, it is subject to predation. Infants are selected to cling like glue to whoever has them. So you have an allomother, -a female other than the mother-, pushing the baby off, the infant trying to stay on, and this is when you see langur females sitting on the baby, pushing it against a rock, this kind of abuse. This is much more relevant to what is going on in human child-abuse, than is infanticide by adult male langurs. It is a stepfather who is not motivated to invest as much in the infant as the infant is asking for. He is pushing it off, and that is when he is losing his temper and inflicts mortal damage on the infant.
Why is it the females that stay in their natal group, instead of the males?
Most Old World monkeys, as among most mammals, if they are social, are matrilocal. The advantages to a female of having her relatives around are very great.
It is usually males that disperse because females, -probably to avoid inbreeding- exhibit a preference for mating with novel males. So that a male who remains in his natal troop is at a breeding-disadvantage with other males, because he is often not the preferred male. I believe that he moves to a new troop to advance his breeding-options. The other possibility is that he himself is leaving to avoid inbreeding.
There are however circumstances when males can't afford to move, as when they are threatened by male alliances in competing groups. When fathers, brothers and sons remain together, as among chimpanzees, it can force females to move instead. But it is very costly to a female to lose the social support of her relatives. If her mother's feeding grounds are particularly good, an advantaged daughter may refuse to move as Jane Goodall has reported for Flo's matriline at Gombe. Once patrilocality emerges though, females are at a real disadvantage. Control of feeding grounds and resources fall increasingly under patrilineal control.
The key phrase here is being able to control the resources. If parents in a patrilocal situation are aware that their sons are going to have allies around, while their daughters will not, they risk territory and resources passed on to daughters being diverted into her mate or husband's patri-line. So parents bias toward sons in intergenerational transmission, because their sons are better able to protect and maintain it. I am obviously thinking here of human examples.
Why are a few primate-species patrilocal?
Yes, why do males sometimes stay? Why are there male patrilocal groups? Well, lets think about where we find them. We find them in species like chimpanzees. Chimp males have a special problem, because they are in competition with the males in competing communities, males who will come in and if they have the chance wipe them out and take their females. How can they protect the resources they have, which consist of the females in it? They need alliances with other males, and the best allies are relatives. You can depend on them the best, a relative has a different cost-benefit ratio for helping you versus competing with you than does a strange male. So males stay with their fathers and grandfathers and brothers in order to protect themselves, the females they want access to, and the offspring they produce, from other males. The males won't leave because if a male goes off he'll be killed by the males of competing communities. So the female leaves, and as Anne Pusey suggests, she uses her sexual receptivity as a passport to enter new communities. She tries to settle there, she may lose her first infant, it may be killed by the resident males, but her later offspring will probably get to survive.
Mariko Hiraiwa-Hasegawa has noted that there is a bias in terms of males attacking sons in chimpanzees. It is one of the few cases outside of humans where you have sex-biased infanticide. The sample-sizes are still quite small, I think it is twelve cases of infanticide in chimps, and nine of the twelve are males rather than females, killed by males. And you have to think, why on earth is this going on? I think that the males who control the community, this brotherhood of males, have much more to lose by allowing another male's son to grow up in their community. A female if she grows up in their community, might leave, and go off and breed elsewhere, or they might have a chance to breed with her. But if a male that is not related to them grows up and joins the brotherhood, they will be sharing the resources of their community, the females in it, and sharing sexual access with this unrelated male. And this is a tremendous dent in the inclusive fitness of these males, to tolerate an alien male among them.
What is your opinion about the idea that in some distant human past, females were dominant over males?
I think it is feminist invention, a psychological antidote against myths of male dominance. It is an effort to invent an alternate reality. But the archaeological or ethnographic evidence is just not there to support it. For instance fertility figures, they say look as if people worshipped females, so females must have ruled. But all this proves is that somebody in those societies was fascinated by female fecundity. These statuettes tell us nothing about political power.
Hunter-gatherers are often held up as being egalitarian, but according to my reading of hunter-gathering monographs, -and I read quite a few of them-, they are more egalitarian then most, but even so males are dominant. I don't find even among hunter-gatherers a very convincing case where females are dominant or even completely equal.
In the way I see the world today, as a Western woman living in the late twentieth century, I am living through a unique and really wonderful experiment for females. I have access to education, I own property in my own name, I have a degree of self-determination, to be able to marry and reproduce and still be able to pursue a career, sustain an independent life intellectually. This is unheard of for the genus Homo, utterly novel. What tends to be overlooked in some feminist mythologies about how ‘we are regaining a matriarchal past’, is that this is an illusion, and it is dangerous. I think we need to understand that this is an experiment, and that it is fragile. Rights that so many now take for granted, no one familiar with our evolutionary history would sensibly take for granted.