Male dominance is not the norm among primates. A conversation with Élise Huchard
From Darwin’s time until the late 1990s, research on animal reproductive strategies focused primarily on males. A shift in perspective then took place, as researchers realized that it might be worthwhile to study more than just half of the partners… It was against this backdrop at the start ofthe 21stcentury that Élise Huchard began her dissertation on the reproductive strategies of female chacma baboons, a species she has been studying for some twenty years through fieldwork 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 with Benoît Tonson, Science Editor, what we know about male and female dominance in primates.
Élise Huchard, University of Montpellier
The Conversation France: Why, for nearly 150 years, did scientists focus exclusively 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 followed his famous *On the Origin of Species * and proposed the theory of sexual selection to explain certain observations that did 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 perspective of the individual’s survival, it seems more like a disadvantage, and this trait should not have been selected.
But survival isn’t the only thing that matters; leaving descendants is also essential. According to the theory of sexual selection, members of one sex—usually males—compete with one another for access to members of the opposite sex—usually females, who will choose from among the winners. In this context, a trait that provides an advantage over other males—whether to win fights or to be more attractive than rivals—will be selected. According to this line of thinking, females are essentially passive, and so we will focus primarily on males.
This paradigm shifted in the 1990s, largely driven by feminist philosophers who argued that science is not as neutral as it claims to be and is always embedded in a specific societal context. Donna Haraway thus uses the example of primatology to argue that we have long projected our own biases onto the study of monkey societies, highlighting dominant males and passive, subordinate females.
It is against this backdrop that you are beginning your dissertation…
É. H.: Yes, it was in 2005. There had been a growing realization that, up until then, research had been biased toward males and that we needed to document the female side of sexual selection. My thesis therefore 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 females making frequent sexual advances toward males, which suggested that they enjoyed a certain degree of freedom in their sexuality.
How did you study these relationships?
É. H.: Since my dissertation, my research site has been Tsaobis in Namibia, a site I now co-direct. It has been an ongoing research site since 2000, where successive teams of scientists have studied two groups of chacma baboons. Right now, we’re tracking one troop of about 85 individuals and another of about 65. The core of our work is trying to document the life histories of the individuals. So we track 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 track the maternal lineages—that’s very easy because we see the babies being born and nursed—but we’re also able to trace the paternal lineages using molecular biology and DNA to conduct paternity tests.
We track two groups at all times, 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’ll leave the cliff where they sleep and start moving around. Then we have to follow them all day long. There are days when it’s very hot and they don’t do much, but on other days they can travel 15 to 20 kilometers, so we have to keep up with them! It’s a mountainous area, and they’re hard to track because they’re 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 suggested people choose their sexual partners based on their immune genes—for example, by selecting partners with genes complementary to their own in order to produce genetically diverse offspring. I spent hours and hours in the field observing mating patterns, trying to identify 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 the females.
I therefore had to address these negative results in my dissertation manuscript. Several of my observations did not align with our initial hypothesis. For one thing, in baboons—unlike peacocks and many other species where males are more colorful than females—it is the females who display sexual ornaments: swellings at the time of ovulation. This suggested that, if it is the females who produce these displays, then it is also the females who are chosen by the males rather than the other way around. Furthermore, I had often witnessed fairly unexplained instances of male aggression toward females. This behavior can also be observed in chimpanzees. I didn’t quite understand it, because these attacks—which could be very violent—occurred at times when the females weren’t necessarily sexually receptive and weren’t doing anything in particular. Yet, when you know baboons well, this raises questions: certainly, there can be conflicts within their large groups, but they rarely occur without reason.
Around that time, a scientific paper was published by an American research 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 ones, (2) if it is costly for the latter—that is, if it is not merely a display of strength, but actual aggression that causes fear and injury to the female victims—and finally (3) if this violence increases the reproductive success of the aggressive 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 exhausts and terrifies her. Sometimes he can drive a female to take refuge in a tree, corner her there, and force her to retreat 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 actually something we’re currently working on. We’re even wondering if this behavior might be a step toward monogamy. It’s interesting to note that these behaviors depend heavily on the composition of the group. When there are few males in the group, the dominant male mates with all the females in the group, following a polygynous (or polygamous) reproductive system.
But when there are many males, a single male cannot monopolize all the females, so each male focuses on a single female, with whom he maintains a fairly exclusive social relationship that combines attachment, closeness, and aggression—a form of pair bonding 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 female from mating with other males.
Once the female gives birth, the male often becomes very protective of the mother and the young, and he is no longer violent toward them at all. Paternal care is observed—he may guard, carry, and groom his offspring, and he will do so only for his own young. 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 regarding paternity.
Is this type of behavior found in other species?
É. H.: We are currently trying to understand the “landscape” of coercion. Which species are coercive, which are less so, and why? It’s a relatively new field of research that is currently flourishing.
One challenge 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 attacks by males on females were sexual in nature. This wasn’t obvious because these acts of aggression can occur several days to weeks before the female’s fertile period—so it’s not easy to link them to sexual behavior. This behavior 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 multi-male or multi-female groups.
But there are also animal societies where such sexually violent behaviors have 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 are reports of sexual coercion even in societies where females are highly 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 most common pattern among primates?
É. H.: No, and it would be completely wrong to assume that what happens among baboons applies to all primates.
For too long, it was assumed that male dominance over females was a given, 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. This was viewed as a sort of evolutionary accident.
One of the problems is that it was often assumed that males dominated females, but this wasn’t necessarily studied quantitatively. There was a tendency to establish a hierarchy of males and a hierarchy of females, without attempting to place both sexes within 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 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 is the case with bonobos. Females are generally dominant in this species, but groups with an alpha male have been observed.
You published a scientific article this year in which you compared male-female dominance in 121 species. What are the key takeaways?
É. H.: In this study, we compiled and analyzed all the data published in the scientific literature on sexual dominance. At first, we didn’t expect to find much because, as I mentioned, primatologists tend to construct separate hierarchies for males and females. But in the end, we found data for 121 primate species, which exceeded 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 within a group, then simply calculated the percentage of confrontations won by females (or by males—it’s the same thing). This literature review took us five years!
This research, published last June, taught us several important lessons. 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. In fewer than 20% of species are males strictly dominant over females, and in fewer than 20% are females strictly dominant.
Furthermore, this is not a binary trait. Dominance relationships between males and females exist along a continuum, with, at one end, species where males are strictly dominant—such as chacma baboons—and, at the other end, species where females are strictly dominant—such as the sifakas of Madagascar. Between these two extremes lies the full spectrum of variations, including species where relationships are egalitarian, such as many South American monkeys (capuchins or tamarins, for example).
Finally, thanks to all this variation, we were able to identify the conditions under which females became socially dominant over males (and vice versa) throughout the evolutionary history of primates. Females are more likely to become dominant in species where they exercise strong control over their reproduction—that is, where they can choose with whom and when they mate. This is particularly common in monogamous, arboreal species—since they can escape and hide more easily in trees—and where both sexes are of similar size and physical strength (as in many lemurs). Females are also more often dominant in societies where they face intense competition—mirroring the competition among males. This is particularly common in species where females live alone or in pairs, where they are also territorial—these characteristics indicate that females do not tolerate the presence of other females. Conversely, male dominance is observed primarily in polygamous, terrestrial, group-living species, such as baboons, macaques, or gorillas, where males possess a clear physical superiority over females.
These findings reveal that gender power dynamics among primates are variable, flexible, and linked to specific social and biological factors. They thus offer new insights into the evolution of male and female roles in early human societies, suggesting that male dominance was not necessarily very pronounced in those societies.
Indeed, the violence and power dynamics 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 issue, and a sensitive one as well. We must begin by clarifying a key point: we must never confuse what is “natural” (that is, what we observe in nature) with what is moral (that is, what is judged to be 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 too quick to generalize to humans what was observed in non-human primates—for example, when the first primatologists reported that males dominated females in chimpanzee societies, there was a tendency to conclude that male dominance is natural in humans, and that it is therefore difficult to combat.
Donna Haraway, whom I quoted at the beginning of the interview, has specifically warned against this kind of oversimplification, which can lead to a false justification of the social hierarchies observed among humans. While evolutionary biology can help us understand certain human behaviors, it cannot and must not under any circumstances justify them. Here we can draw a parallel with criminology: just because a criminologist explains a crime from a psychological or scientific perspective does not mean they are justifying it morally.
That being said, as an evolutionary biologist and primatologist, I naturally believe that humans are simply one species of primate among many. There are broad patterns common to all animals, from which we are no exception. For example, I believe that sexual violence stems from 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 earlier that there is enormous variation in male-female relationships across 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 when it comes to the domination of one sex over the other and sexual coercion! And above all, since these two species are genetically very close, this example also shows that our behaviors are not inexorably “determined” 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. The goal, therefore, is to understand 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 noting that one distinctive feature of human societies is the existence of powerful social mechanisms for regulating hierarchical and violent behavior through cultural norms and institutions. Even though 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 a society without sexual violence!
Élise Huchard, Research Director at the CNRS, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.