Male dominance is not the norm among primates. Conversation with Élise Huchard
From Darwin until the late 1990s, research on animal reproductive strategies focused mainly on males. Then came the realization that it might be interesting to study more than just half of the partners... It was in this context at the beginning ofthe 21stcentury that Élise Huchard began her thesis on the reproductive strategies of female chacma baboons, a species she has been studying for some 20 years thanks to a research field in Namibia. She is now a research director at the CNRS and works at the Institute of Evolutionary Sciences in Montpellier (Hérault). Her work was recognized in 2017 with the CNRS bronze medal. The evolutionary biologist discusses what we know about male and female dominance in primates with Benoît Tonson, head of the Science section.
Élise Huchard, University of Montpellier
The Conversation France: Why, for nearly 150 years, have scientists focused solely on males?
Élise Huchard: We can go back to Darwin and his book, The Descent of Man, and Selection in Relation to Sex. This book follows on from the famous On the Origin of Species and proposes the theory of sexual selection to explain certain observations that do not fit 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 individual survival, it seems more like a disadvantage, and this trait should not have been selected.
But survival isn't the only thing that matters; it's also important to leave descendants. 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 among the winners. In this context, a trait that gives an advantage over other males—either to win fights or to be more attractive than rivals—will be selected. According to this line of thinking, females are essentially passive, so we will focus mainly on males.
This paradigm changed in the 1990s, notably under the influence of feminist philosophers who argued that science is not as neutral as it claims to be 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 monkey societies, emphasizing dominant males and passive, subordinate females.
It is in this context that you are starting your thesis...
É. H.: Yes, it was in 2005. There had been some introspection suggesting that, up until then, research had been biased in favor of males and that the female side of sexual selection needed to be documented. So my thesis focused on the study of baboons and, more specifically, on the mating choices and reproductive strategies of females. At first glance, they seemed very active, with a lot of sexual solicitation from females towards males, which suggested that they enjoyed a certain freedom in their sexuality.
How did you study these relationships?
É. H.: Since my thesis, my field of research has been the Tsaobis study site in Namibia, which I now co-direct. It has been an ongoing research site since 2000, where scientists have been studying two groups of chacma baboons. We are currently tracking one troop of around 85 individuals and another of around 65. The core of our work is trying to document the life histories of individuals. This means tracking each individual from birth to death, or at least as much of their life as we can track, and all the events that happen to them. We follow the maternal lines, which is very easy because we see the babies being born and breastfed, but we are also able to trace the paternal lines thanks to molecular biology, using DNA to perform paternity tests.
We follow two groups continuously, so we send at least two people per group per day. It's very physically demanding: in the morning, we have to be with the baboons before dawn, because after that they leave the cliff where they sleep and start moving around. We then have to follow them all day long. Some days it is very hot and they don't do much, but on other days they can travel 15 to 20 kilometers, so we have to follow them! It's a mountainous area, and they are difficult to follow because they are much more agile than we are. The views are absolutely spectacular, but it really takes a lot of effort!
And what did you learn?
É.H.: I tested a hypothesis that said we choose our sexual partners based on their immune genes, for example, with genes that complement our own so that we have 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 that for negative results! I detected no preferences, no choice of males by females.
I therefore had to discuss these negative results in my thesis manuscript. Several of my observations did not fit with our initial hypothesis. First of all, unlike peacocks and many other species where males are more colorful than females, it is the females that display sexual ornamentation in baboons: swelling during ovulation. This suggested that if it is the females who produce these ornaments, then it is also the females who are chosen by the males rather than the other way around. Secondly, I had often witnessed situations of rather unexplained violence by males towards females. This behavior can also be found in chimpanzees. I didn't really understand this, because these potentially very violent attacks occurred at times when the females were not necessarily sexually receptive and were not doing anything in particular. However, when you know baboons well, this is striking: of course, there can be conflicts in their large groups, but it is rarely without reason.
At that time, a scientific article was published by an American team studying chimpanzees, which, for the first time, tested the hypothesis of sexual intimidation to explain certain acts of aggression. According to this hypothesis, an act of aggression can be described as sexual coercion (1) if it targets fertile females more than non-fertile females, (2) if it is costly for the latter—that is, if it is not simply a show of strength, but a real attack that causes fear and injury to the female victims—and finally (3) if this violence increases the reproductive success of the violent male. I tested this hypothesis on baboons, and it works perfectly.
What forms can these acts of sexual coercion take?
É. H.: It varies quite a bit. A male may jump on a female and pin her to the ground. He may also chase her for long periods of time, which will exhaust and terrify her. Sometimes he can force a female to take refuge in a tree, block her there, and push her to the end of a branch, potentially forcing her to jump from a great height, at the risk of seriously injuring herself.
Will a male always attack the same female?
É. H.: Yes, and that's something we're currently working on. We are even wondering whether this behavior might be a step towards monogamy. It is interesting to note that these behaviors depend greatly 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, using a polygynous (or polygamous) reproductive system.
But when there are many males, a single male cannot monopolize all the females, and each male will then 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 is with this female that he will mate during her fertile period, following her everywhere for several days in a row to prevent anyone else from approaching her and thus protect his paternity. It is believed that males use violence to deter their females from mating with other males.
Once the female gives birth, the male often becomes very protective of the mother and child, and is no longer violent towards them. Paternal care is observed—he may guard, carry, and groom his young, and he will only do this for his own offspring. It is believed that this paternal care, which is very rare in non-monogamous species, has emerged precisely because male baboons, who manage to maintain sexual exclusivity even within these large social groups, have a high degree of certainty about paternity.
Is this type of behavior found in other species?
EH: We are currently trying to understand the "landscape" of coercion. Which species are coercive, which species are less so, and why? This is a relatively new field of research that is currently blossoming.
One difficulty is that even if these behaviors have not been described, it does not necessarily mean that they do not exist. For example, scientists studied baboons for decades before realizing that these attacks by males on females were sexual in nature. This was not obvious because these acts of aggression can occur several days to weeks before the female's fertile period, making it difficult to link them to sexual behavior. This type of sexual intimidation has been observed in other species, such as chimpanzees and mandrills. It is suspected that this form of coercion is widespread among species that live in large groups with multiple males or females.
But there are also animal societies where such sexually violent behavior has never been observed, such as among 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 always protective, as there have been observations of sexual coercion even in societies where females are very dominant, such as among ring-tailed lemurs, for example.
So far, we have mainly discussed chacma baboons, where males appear to be very dominant over females. Is this the predominant model among primates?
É. H.: No, and it would be completely wrong to think that what happens with baboons can be generalized to all primates.
For too long, we thought that male dominance over females was a given, a kind 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 is that males were often considered to dominate females, but without necessarily studying this quantitatively. There was a tendency to establish a hierarchy of males and a hierarchy of females, without attempting to place both sexes in the same hierarchy. However, it has recently been realized that the dominance of one sex over the other is not as binary a phenomenon as previously believed, and that there appear to be species in which neither sex is strictly dominant over the other, or species in which the degree of female dominance can vary from one group to another, as in bonobos. Females are generally dominant in bonobos, but groups with an alpha male have also been observed.
This year, you published a scientific article comparing male-female dominance in 121 species. What are the key findings?
E. H.: In this study, we collected and analyzed all the data published in scientific literature on dominance between 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, and therefore objective, measures to estimate which sex dominated the other. To do this, we considered all confrontations involving a male and a female in a group, then simply counted the percentage of confrontations won by females (or by males, it's the same thing). This bibliographic survey took us five years!
All this work, published last June, has taught us several important things. First, strict dominance of one sex over the other, where one sex wins more than 90% of confrontations, as observed in chacma baboons, is rare. Less than 20% of species show strict male dominance over females, and less than 20% show strict female dominance over males.
Secondly, it is not a binary trait. Dominance relationships between males and females exist along a continuum, with species where males are strictly dominant at one end, such as chacma baboons, and species where females are strictly dominant at the other end, such as sifakas in Madagascar. Between these two extremes lies the entire spectrum of variations, with species where relationships are egalitarian, such as many South American monkeys (capuchins and tamarins, for example).
Finally, thanks to all this variation, we were able to highlight the conditions under which females became socially dominant over males (and vice versa) during the evolutionary history of primates. 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 often the case in monogamous, arboreal species—because they can escape and hide more easily in trees—and where both sexes are the same size, with equal physical strength (as in many lemurs). Females are also more often dominant in societies where they compete fiercely with each other, mirroring the competition between males. This is particularly often the case in species where females live alone or in pairs, where they are also territorial—characteristics that show that females do not tolerate the proximity of other females. Conversely, male dominance is mainly observed in polygamous, terrestrial species that live in groups, such as baboons, macaques, and gorillas, where males have a clear physical superiority over females.
These findings reveal that gender power relations among primates are variable, flexible, and linked to specific social and biological factors. They thus offer new insights into understanding the evolution of male and female roles in early human societies, suggesting that male dominance was not necessarily very pronounced.
Indeed, the violence and dominance relationships you describe among baboons and other primates inevitably bring to mind our own species. From an evolutionary perspective, can we draw any conclusions from this?
É. H.: This is a complex and sensitive issue. We must begin 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 considered good or bad in human societies). Just because a behavior is natural does not mean it is morally acceptable. There were heated debates on this subject in the 1980s. Some people were quick to generalize to humans what was observed in non-human primates—for example, when early primatologists reported that males dominated females in chimpanzee societies, there was a tendency to conclude that male dominance is natural in humans and therefore difficult to combat.
Donna Haraway, whom I quoted at the beginning of this interview, warned against this kind of oversimplification, which can lead to the fallacious justification of social hierarchies observed in humans. While evolutionary biology can help us understand certain human behaviors, it cannot and must not be used to justify them. A parallel can be drawn here with criminology: just because a criminologist explains a crime in psychological or scientific terms does not mean that they are morally justifying it.
That being clarified, as I am an evolutionary biologist and primatologist, I naturally believe that humans are just one primate among many. There are major patterns common to all animals, 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 of females by males.
I also pointed out earlier 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, which are the two species closest to our own, is illuminating: the former are male-dominated and coercive, while the latter are female-dominated and show no evidence of sexual coercion.
Therefore, it is impossible to say what is "natural" for humans in terms of the domination of one sex over the other and sexual coercion! Above all, since these two species are genetically very similar, this example also shows that our behaviors are not "determined" in an inexorable way by our evolutionary heritage and our genes. Increasingly, we are realizing that these behaviors of coercion and domination vary from one individual to another and depend on the social context. It is therefore a question of understanding which factors, particularly social or cultural ones, promote or inhibit such behaviors in different primate societies. This is what we are currently working on.
We can conclude by recalling that one of the distinctive 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 from sexual violence and gender inequality, it is up to us, as humans, to actively combat them through our cultural and institutional mechanisms. And some non-humans, such as bonobos, are even there to show that it is possible, as primates, to live together and form societies without sexual violence!
Élise Huchard, Director of Research at the CNRS, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.