Male power is not the norm among primates. Conversation with Élise Huchard
Since Darwin and up until the late 1990s, research into animal reproductive strategies focused mainly on males. It was in this context at the beginning of the 21st century that Élise Huchard began her thesis on the reproductive strategies of female chacma baboons, a species she has been studying for some twenty years thanks to a research field in Namibia. She is now a research director at the CNRS and works at the Institut des sciences de l'évolution in Montpellier (Hérault). Her work was rewarded in 2017 with the CNRS Bronze Medal. The evolutionary biologist talks to Science columnist Benoît Tonson about what we know about male-female dominance in primates.
Élise Huchard, University of Montpellier
The Conversation France: Why, for almost 150 years, have scientists only been interested in males?
Élise Huchard: We can go back to Darwin and his book, The Sonship of Man and Sex Selection. This book follows on from the famous On the Origin of Species, and proposes the theory of sexual selection to explain certain observations that don't fit in with his theory of natural selection. The example Darwin uses is the peacock's tail. How can this tail help it survive in a world full of predators? From the point of view of the individual's survival, it looks more like a disadvantage, and this trait should not have been selected.
But survival isn't all that matters, and offspring must also be left behind. According to the theory of sexual selection, members of one sex - usually males - compete with each other for access to members of the other sex - usually females, who will choose from the winners. In this context, a trait that gives an advantage over other males - either for winning fights or for being more attractive than rivals - will be selected. According to this way of thinking, females are essentially passive, so the focus is on males.
This paradigm was to change in the 1990s, notably under the impetus of feminist philosophers who asserted that science is not as neutral as it claims, and is always part of a given societal context. Donna Haraway uses the example of primatology to argue that we have long projected our own biases onto the study of ape societies, by emphasizing dominant males and passive, subordinate females.
This is the context in which you start your thesis...
É. H.: Yes, back in 2005, there had been this introspection that, up to now, work had been biased in favor of males and that the female side of sexual selection needed to be documented. My thesis therefore focused on the study of baboons and, more specifically, on female mating choice and reproductive strategies. At first glance, they appeared to be very active, with a great deal of sexual solicitation from females to males, suggesting that they enjoyed a certain freedom in their sexuality.
How did you study these relationships?
É. H.: Right from my thesis, my research field was the Tsaobis study site in Namibia, which I now co-direct. It's been a continuous research field since 2000, with successive scientists studying two groups of chacma baboons. At the moment, we're following a troop of around 85 individuals and another of around 65. The core of our activity is to try to document the life histories of the individuals. That is, to follow each individual from birth to death, or, in any case, everything that can be tracked from his or her life and all the events that happen to him or her. It's very easy to follow maternal lines, because we can see babies being born and breast-fed, but we can also trace paternal lines using molecular biology and DNA for paternity tests.
We follow two groups at all times, so we send at least two people per group per day. It's very physically demanding: in the morning, you have to be with the baboons before dawn, because then they'll leave the cliff where they sleep and start moving. Then you have to follow them all day long. There are days when it's very hot and they won't do much, but other days when they're capable of moving 15 to 20 kilometers, so you have to follow them! It's a mountainous area, so they're hard to keep up with, as they're much more agile than we are. The views are absolutely spectacular, but it really takes a lot of effort!
And what have you learned?
É.H.: I tested the hypothesis that people choose their sexual partners on the basis of their immune genes, for example, with genes complementary to their own, so as to produce genetically diverse offspring. I spent hours and hours in the field observing mating patterns, trying to determine preferences. I also spent days and days in the lab genotyping baboons to test this hypothesis. And all to negative results! I couldn't detect any preference, any choice of females for males.
I therefore had to discuss these negative results in my thesis manuscript. Several of my observations were not consistent with our original hypothesis. Firstly, in baboons, unlike peacocks and many other species where males are more colorful than females, it is the females that display sexual ornaments: tumescences at the moment of ovulation. This suggested that, if it's the females who produce these ornaments, then it's also the females who are chosen by the males rather than the other way round. Secondly, I'd often witnessed unexplained violence from males towards females. This behavior can also be seen in chimpanzees. I didn't quite understand, because these potentially violent attacks occurred at times when the females weren't necessarily sexually receptive and weren't doing anything in particular. But when you know baboons well, it's interesting to note that, while there can be conflict in their large groups, it's rarely for no reason at all.
At the same time, an American team published a scientific article on chimpanzees which, for the first time, tested the hypothesis of sexual intimidation to explain certain aggressions. According to this hypothesis, an aggression can be described as sexual coercion (1) if it targets fertile females more than non-fertile ones, (2) if it is costly for the latter - i.e., it's not simply a show of force, but a real aggression that leads to fear and injury for the females who are its victims - and finally (3) if this violence increases the reproductive success of the violent male. I've tested this hypothesis on baboons, and it works perfectly.
What forms can these acts of sexual coercion take?
É. H.: It's quite variable. A male may jump on a female and crush her to the ground. He may also pursue her for long periods, exhausting and terrorizing her. Sometimes he can push a female into a tree, blocking her and forcing her to take refuge at the end of a branch, potentially forcing her to jump from a great height, risking serious injury.
Does a male necessarily systematically attack the same female?
É. H.: Yes, and that's something we're currently working on. We're even wondering whether this behavior isn't an evolutionary path towards monogamy. It's interesting to note that these behaviours depend very much on the composition of the troop. When there are few males in the group, the dominant male mates with all the females in the troop, with a polygynous (or polygamous) reproduction system.
But when there are many males, a single male cannot monopolize all the females, so each male will focus on a single female, with whom he will maintain a fairly exclusive social relationship, combining affiliation, closeness and violence - a form of couple within the group. And it's with this female that he will mate during his fertile period, following her around for days at a time to prevent anyone else from approaching her and thus protecting his paternity. Males are thought to use violence to dissuade their females from mating with other males.
Once the female gives birth, the male is often very protective of both mother and child, and no longer violent towards them. Paternal care is observed - he may guard, carry and groom his young, and he will only do this towards his own young. This paternal care, which is very rare in non-monogamous species, is thought to have arisen precisely because baboon males, who manage to maintain sexual exclusivity even within these large social groups, have a strong certainty of paternity.
Can this type of behavior be found in other species?
É. H.: We're currently trying to understand the "landscape" of coercion. Which species are coercive, which less so, and why? It's a fairly recent field of research, and one that's still in the bud.
One difficulty is that, even if these behaviors haven't been described, that doesn't necessarily mean they don't exist. For example, scientists studied baboons for decades before realizing that these assaults by males on females were sexual in nature. This was not obvious, as these acts of aggression can take place days or weeks before the female's fertile period - so it's not easy to link them to sexual behavior. Sexual intimidation has been observed in other species, such as chimpanzees and mandrills. We suspect that this form of coercion is widespread in species that live in large multi-male or multi-female groups.
But there are also animal societies where this kind of sexual violence has never been observed, such as in bonobos, where females are socially dominant over males. Sexual coercion is undoubtedly much less frequent and intense when females are dominant over males. However, female dominance is not systematically protective, as sexual coercion has been observed even in societies where females are highly dominant, as in the case of catta lemurs.
So far, we've focused on chacma baboons, where males seem to be very dominant over females. Is this the predominant pattern among primates?
É. H.: No, and it would be very wrong to think that what happens with baboons can be generalized to all primates.
For too long, it was taken for granted that males dominated females, that this was a sort of "default" model. Then, in the 1970s, the discovery of female-dominated animal societies, such as those of spotted hyenas and many lemurs, came as a real surprise. It was seen as a kind of evolutionary accident.
One of the problems was that it was often assumed that males dominated females, but this was not necessarily studied quantitatively. We tended to establish a hierarchy of males and a hierarchy of females, without trying to put the two sexes in the same hierarchy. However, we have recently realized that the dominance of one sex over the other is not as binary a phenomenon as we thought, and that there seem to be species where neither sex is strictly dominant over the other, or species where the degree of female dominance can vary from one group to another, as in the bonobos. Here, females are generally dominant, but groups with an alpha male have also been seen.
This year you published a scientific article comparing male-female dominance in 121 species. What can we learn from it?
É. H.: In this study, we collected and analyzed all the data published in the scientific literature on dominance between the sexes. Initially, we didn't expect to find much because, as I said, primatologists tend to construct separate hierarchies for males and females. But, in the end, we found data for 121 primate species, which was beyond our expectations. We used quantitative, i.e. objective, measurements to estimate which sex dominated the other. To do this, we consider all confrontations involving a male and a female in a group, then simply count the percentage of confrontations won by females (or by males, it's all the same). This bibliographical survey took us five years!
All this work, published last June, has taught us several important things. Firstly, strict dominance of one sex over the other, when one sex wins more than 90% of confrontations, as observed in chacma baboons, is rare. There are less than 20% of species where males are strictly dominant over females, and also less than 20% where females are strictly dominant.
Secondly, this is not a binary trait. Dominance relationships between males and females extend along a continuum, with, at one end, species where males are strictly dominant, as in chacma baboons, and, at the other end, species where females are strictly dominant, as in the sifakas of Madagascar. Between these two extremes lies the whole spectrum of variation, with species where relationships are egalitarian, as in many South American monkeys(capuchins or tamarins, for example).
Finally, thanks to all this variation, we have been able to highlight the conditions under which females have become socially dominant over males (and vice versa) over the course of primate evolutionary history. Females become dominant more often in species where they exercise strong control over their reproduction, i.e. where they can choose with whom, and when, they mate. This is particularly often the case in monogamous, arboreal species - as they can escape and hide more easily in trees - and where both sexes are of equal size and physical strength (as in many lemurs). Females are also more often dominant in societies where they are in strong competition - symmetrically to the competition between males. This is often the case in species where females live alone or in pairs, where again they are territorial - characteristics which show that females do not tolerate the proximity of other females. On the other hand, male dominance is most common in polygamous, terrestrial, group-dwelling species such as baboons, macaques and gorillas, where males have a clear physical superiority over females.
These results reveal that power relations between the sexes in primates are variable, flexible and linked to specific social and biological factors. They offer new insights into the evolution of male and female roles in early human societies, suggesting that male dominance was not necessarily very marked.
Indeed, the violence and dominance you describe in baboons and other primates is reminiscent of our own species. From an evolutionary point of view, can we draw any conclusions?
É. H.: It's a complex question, and a sensitive one too. We need to start by clarifying a key point: we must never confuse what is "natural" (i.e. what we observe in nature) with what is moral (i.e. what is deemed good or bad in human societies). Just because a behavior is natural doesn't mean it's morally acceptable. There were lively debates on this subject in the 1980s. For example, when the first primatologists reported that males dominated females in chimpanzee societies, they tended to deduce that male dominance is natural in humans, and therefore difficult to combat.
Donna Haraway, whom I quoted at the beginning of the interview, warned against this kind of shortcut, which can lead to the fallacious justification of the social hierarchies observed in humans. While evolutionary biology can help us understand certain human behaviors, it can't and shouldn't be used to justify them. A parallel can be drawn here with criminology: just because a criminologist explains a crime on a psychological or scientific level does not mean that he or she justifies it on a moral level.
Having clarified this, as an evolutionary biologist and primatologist, I of course believe that humans are just another primate. There are major patterns common to all animals, from which we cannot escape. For example, I believe that sexual violence responds to the same psychological motivations in humans and non-humans, linked to a desire for sexual possession and reproductive control by males over females.
I also pointed out above that there is enormous variation in male-female relationships in animal societies, so we must be wary of hasty generalizations. The example of chimpanzees and bonobos, the two species closest to our own, is enlightening: the former are male-dominated and coercive, while the latter are female-dominated and do not exhibit sexual coercion.
So it's impossible to say what's "natural" for humans when it comes to dominance of one sex over the other and sexual coercion! Above all, since these two species are genetically very close, this example also shows that our behaviours are not implacably "determined" by our evolutionary heritage and our genes. Increasingly, we're realizing that these coercive and domineering behaviours vary from one individual to another, and depend on the social context. We therefore need to understand the factors, particularly social or cultural, that promote or inhibit such behavior in different primate societies. This is what we are currently working on.
We can end by recalling that one of the specific features of human societies is that there are powerful social mechanisms for regulating hierarchical and violent behavior, through cultural norms and institutions. Even if our societies are not free of sexual violence and gender inequality, it's up to us as humans to actively combat them through our cultural and institutional mechanisms. And some non-humans, like bonobos, are even there to show that it's possible, as primates, to live together and form a society without sexual violence!
Élise Huchard, CNRS Research Director, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.