If we were not under their sleeping nests when the chimpanzees woke up, then we would miss out on the pee—the key to the chimps’ testosterone levels. So I prepared, as I did almost every morning during the eight months I spent with the chimps, for a walk through the predawn jungle.
Evolution has fashioned an elegant system that motivates us to start our days with the benefit of the light (and heat) emitted by our star. Like all diurnal animals (those that are active during the day), we sync our sleep-wake cycles with our planet’s twenty-four-hour rotation around its axis. When the morning sun is sensed by the photoreceptor cells in our retinas, the information is transmitted to the pineal gland, a tiny pine cone–shaped organ, deep in the center of our brain. In response, the gland decreases its production of the “sleep hormone” melatonin, which nudges us toward a particular behavior—waking up.
At least, this is how it worked before we humans got used to artificial light. But since chimps are sticking to the old schedule, I had to get my butt out of bed while my melatonin was still high. I attempted to counteract this groggy state of affairs with a dose of caffeine delivered via coffee, which I made with rainwater on the field site’s propane-powered stove.
Armed with my Wellies to protect from such inconveniences as army ants, mud holes, and black mamba snakes, a flashlight, and a foot-long machete (for bushwhacking), I headed out to meet my Ugandan field assistants. This was a usual day of chimping—tagging along with the chimps and taking notes on their lives and activities in the Kibale Forest of Western Uganda.
After a hike of about an hour, I rested on the forest floor near the base of one of the trees in which the chimps slept, high in the treetop nests they’d constructed the night before. I tried to soak up every detail of the dramatic transformation of the night forest. The steady hum of trilling insects was drowned out by the growing cacophony of bird and monkey calls, and slivers of sunlight pierced through the undergrowth, turning dewdrops into glowing gold beads that clung to the green foliage. I was waiting for one sound in particular, that of rustling from above, the first stirrings of chimps waking up. That was my cue to prepare.
Chimps aren’t much different from humans in terms of their first morning needs—they need to go! But while we stumble out of bed and head to the bathroom (or outhouse or pit), chimps just stick their rear ends out, over the side of their nest. I did my best (which was not always good enough) to be far enough away to shield myself from the urine that rained down about thirty feet through the leaves, but close enough that I could catch some. I did that with the help of a long stick with a forked end, over which I had tied a plastic bag.
In this way I was making a small contribution to the data, both behavioral and physiological, that was collected by researchers from the Kibale Chimpanzee Project. This treasure trove of information allows scientists to gain insights into the origins of all kinds of behaviors. But we were particularly interested in sex, aggression, and dominance, which are all affected by the subject of this book: testosterone, or, as insiders call it, “T.” With human subjects, we can just ask them to spit into a tube. But wild chimps are less cooperative, so we measure T in their urine (and feces) instead.
I carefully pipetted what little pee I managed to catch from the plastic bag into test tubes, to be carried back to the field site for later transport back to the endocrinology lab at Harvard. After a few minutes of rustling and relieving, the chimps shimmied down the tree trunks to begin their day, with the field assistants and me in tow.
A BULLY GIVES A BEATING
Chimpanzees live in “communities,” usually comprising about fifty chimps. In some ways, each community is like a group of people living in a small town, with well-defined and defended borders, and hostile relationships with the neighboring towns. Imoso was like the mayor—the alpha—of this town, called Kanyawara. It was just one of several such communities located within the vast forest near the border with the Democratic Republic of the Congo. Imoso was temperamental and despotic—a leader who must have been more feared than liked. Each day, smaller groups of chimps from the community, called “parties,” would congregate and spend the day together, and I’d follow one of the parties. When Imoso was part of a party I followed, I could count on lots of grunting, screaming and hooting, threatening, slapping, stick dragging and throwing, and chest pounding. There was one sure way to make things even more exciting, and that was to add a fertile (estrous) female to the mix. Lots of sex, and even more aggression, as the males competed for the right to mate with her.
Other days were punctuated less with bouts of high drama, and more with the ebb and flow of nurturance and play. Little ones would cling and cuddle, nurse, tumble and chase their siblings and friends, or perch like royalty on mom’s back, as they traveled from one feeding patch to another. Those were the times I followed parties without the adult males.
On one January day, Imoso seemed calmer than usual. And on this day, unusually, he decided he’d like to hang out with only one female and her two small children. Leaning against a tall fig tree, I opened my notebook. Outamba sat behind Imoso on a large downed tree in a clearing. She expertly searched through his thick, dark hair, separating and flattening it, examining the area for dirt or parasites, deftly removing what she found and popping the tasty bits into her mouth. The baby Kilimi and her older sister Tenkere frolicked in a patch of grass in the heat of the equatorial midday sun, amid the din of birds and insects.
Outamba’s ear-piercing screams jolted me out of my tranquil state and sent my heart racing. I bolted upright. Imoso jumped up so that he stood on the fallen tree and began to pound Outamba with his fists while also kicking her. She tumbled to the ground, and tiny Kilimi quickly hopped into the refuge of her arms. Outamba huddled over her daughter in a protective embrace, her back exposed to Imoso’s assault. I tried to accurately record all that was happening—who was doing what to whom, and for exactly how long. (I was lucky to have been with one of the project’s experienced field assistants, John Barwogeza, who gave me a thorough debriefing on all that I had missed.) After a few minutes of this brutality, already the longest and most severe beating I’d ever seen, Imoso picked up a large stick and began hitting Outamba’s head and back with it. Tenkere, just three years old and no more than two feet tall, raced around Imoso and pounded at him with her impotent little fists as the giant beat up her mother. But kicking, hitting with fists and sticks wasn’t enough—Imoso got even more creative and hung from a branch, leaving his feet free to stomp on and kick her with greater force. Nine stunning minutes later, it was over.
The beating left Outamba bleeding from the tender, hairless skin on her bottom, but at least her kids were unharmed and she was able to scamper off with them.
Although I knew other researchers had observed prolonged—even murderous—attacks, this was new for me. The episode was gut-wrenching, but also, as a scientist, it was thrilling and confusing. Sure, the big males would routinely harass and beat the adult females, but from what I’d seen before, these beatings were brief and mild by comparison.
Richard Wrangham, the world-renowned Harvard primatologist who founded and ran the field site, happened to be visiting that week. I raced the two or so miles back through the forest to the field station to describe what I’d seen. I was breathless and bursting with emotion and questions, but his initial response was to simply hold out his hand to shake mine. He told me that I was the first researcher to observe such weapon use by a nonhuman primate in the wild. Time magazine even ran a story, accompanied by a large picture of Richard, me, and the now famous stick (later retrieved from the clearing by the field assistants), under the title “Wife Beaters of Kibale.” That anthropomorphic title made me cringe, but there was no denying the similarities between Imoso’s disturbing behavior and domestic violence among humans. Why did he do it? I didn’t have any answers that day, but research on testosterone and reproduction from the field site would later supply them.
My journey to Uganda wasn’t exactly a straight line. An interest in human behavior drove me to major in psychology in college. I enjoyed classes like Freud and Jung, Abnormal Psychology, and Personality and Individual Differences. But it wasn’t until my senior year that I had to restrain myself from jumping out of my seat, barely able to contain my excitement about the lecture material. I will never forget that course (Biological Psychology), the professor (Josephine Wilson), and the day she introduced me to neurons and neurotransmitters and how their actions and levels affect all kinds of behavior. I remember her standing tall, raising her outstretched arms above her head, and wiggling her fingers around to bring a neuron and its dendrites—little branches that communicate with other neurons—to life. A new, powerful way to understand the origins of behavior was opening up for me, and it felt tremendously satisfying. I knew I wanted more of that feeling, but graduation loomed and I had no job.
As one does with a BA in psychology, I landed a job in financial software. (Mostly I just wanted a job where I could “work with computers.” This was 1988, after all.) I told myself that I would do this for a couple of years until I figured out a grand life plan. But I still had lots of learning to do, and my job was comfortable. So two years turned into ten. I took classes that I had missed out on in college, like molecular biology and genetics, and discovered that—contrary to the impression I’d gotten throughout my early schooling—I loved biology. I traveled widely, to places like Israel, Tanzania, Costa Rica, and China, and became curious about the origins of the diversity of cultures and ecologies across the globe. And I read popular science books, like The Selfish Gene by Richard Dawkins, that introduced me to how evolutionary theory could help to answer my questions about life on earth.
These experiences intensified my desire to find the deepest, most powerful explanations for human behavior and converged on one question: how has evolution shaped human nature?
Then I read the book that suggested a path I could take to pursue my questions: Demonic Males: Apes and the Origins of Human Violence. It wasn’t violence, specifically, that drew me in; it was the approach that the two authors used to investigate large questions about how we got to be the way we are. I decided that I wanted to do what the lead author did, which was to study chimpanzees to learn more about ourselves and our evolutionary origins. So I quit my job and applied to graduate school.
I do not recommend doing things in that order.
The lead author of that book was Richard Wrangham. Fortunately he happened to teach at Harvard, in my home city of Cambridge, Massachusetts. I eagerly mailed in my application to the program in his department, which at the time was called Biological Anthropology. The rejection note was disappointing, but in hindsight I should have expected it. It’s tough to get into such a program without a shred of research experience “in the field,” as it’s called. But in some cases, naivete can be an asset. I persisted in my efforts, and eventually Richard—we were now on a first-name basis—offered me an opportunity to spend a year in Uganda at the Kibale Chimpanzee Project. He had started the field site in 1987 to study the behavior, physiology, and ecology of wild chimpanzees. My job was to manage the site and learn to conduct some research of my own. I could hardly believe it. Of course I accepted.
SEX AND VIOLENCE IN TWO KINDS OF PRIMATES
And that’s how I found myself in the forest on that January day in 1999, catching chimp urine and watching a big male beat up a smaller female while she tried to protect her kids. Their interaction had dramatically exemplified the contrasting patterns of chimp behavior that had already captivated my interest—relatively peaceful, nurturing females and sex- and hierarchy-obsessed aggressive males.
I watched adult male chimpanzees use aggression in different situations for different purposes, only some of which had a clear explanation. They used it to show who was in charge and to demand what we might think of as respect. A lack of respect meant that one’s dominance rank was not being observed, and a beating might be the way to increase the chances that proper deference be paid to the dominant male in the future. Two males, close in dominance rank, might duke it out over a sexual opportunity—fighting to win a mating with a sexually attractive female (those who are in estrus and who can thus conceive are the focus of much male attention), or to keep other males away from her (known as “mate guarding”). And what about Imoso’s assault on Outamba, at a time when she wasn’t in estrus? As the data would later suggest, such aggression tends to increase a female’s sexual compliance in the future. Males tend to target females who are in the best reproductive condition, and females preferentially mate with and bear offspring fathered by males who have been especially aggressive toward them. (I should emphasize that this doesn’t mean that men’s aggression against women has a similar evolutionary rationale, still less that such behavior is inevitable or excusable. And in any case, other animals—including other primates with different social systems—can also offer clues about the evolutionary origins of our own behavior.)
All this is not to say that every male chimp is a bully or that they are violent 24/7. They have different personalities—some shy, some sweet, some brutish. The big males, even Imoso, could be gentle and patient. They played with the youngsters, lightly wrestling and biting, and allowed their bodies to be used as jungle gyms as they tried to catch some shut-eye; they spent lots of time in their social groups, with the females, kids, and each other, traveling, relaxing, eating, and grooming, with little or no brutality. And although I saw very little aggression from the females, it does occur, sometimes with intensity.
And, of course, the same goes for adult males of the human community, who are capable of extreme acts of heroism, tenderness, and generosity, but also of violence and cruelty. I spent long hours every day as the lone woman in a group of local men, and I trusted them with my life. But during that time other men from the very same region of Africa were carrying out brutal acts against civilians.
The BBC World Service kept me company every night, and the lead story often featured the planet’s alpha male, President Bill Clinton, and his affair with a young White House intern, Monica Lewinsky. As many men had done before him, and continued to do after, Clinton had risked everything for a few fleeting sexual encounters. Although this was a titillating distraction, I listened carefully for mention of the Congolese rebels, trying to glean any information on whether any were headed in the direction of my field site. A civil war was under way next door in the Congo, and the area was a hotbed of political violence. I learned of gruesome attacks, involving men using their machetes to attack villagers including children, cutting off hands, limbs, or heads, and raping the women. There were regular threats against Westerners, specifically of beheadings. I felt like a sitting duck, alone in my small bungalow at night, my own machete, tucked under my pillow, providing little comfort.
One horrific and widely publicized attack in March 1999 triggered the evacuation of most Westerners (including the Peace Corps) out of the region. Rwandan rebels had invaded a Ugandan national park, 250 miles to our south and also on the DRC border. The rebels killed four park employees and kidnapped fifteen tourists, who were marched into the mountains. The rebels slaughtered eight of them, from the UK, New Zealand, and the United States, with machetes and clubs. At least one woman showed evidence of a severe sexual assault.
I stayed on at my field site for another few months, but eventually, because of increased threats to Westerners and rebel movements in our area, the U.S. embassy ushered me out.
My experience in Uganda left me with the ambition of knowing more about how the shared biology of humans and nonhuman animals can help to explain why males and females are often so different. Really, I longed to understand men. Testosterone promised to be a key part of that explanation. So when my second application to Harvard was successful, and I started work on a PhD in biological anthropology, I learned everything I could about it.
Testosterone is present in our blood in minute quantities. Both sexes produce it, but men have ten to twenty times as much as women. Despite its insubstantial physical presence, T has managed to achieve a substantial reputation, dwarfing that of any other corporeal chemical. After all, T is an “androgen,” from the Greek “andro”—man—and “gen”—generating. If the Y chromosome is the essence of maleness, then T is the essence of masculinity, at least in the popular mind. Bill Clinton was assumed to have plenty of it, but with Donald Trump we got actual numbers.
Just before the 2016 presidential election, Trump appeared on Dr. Oz’s national TV show to reveal the results of his latest physical. Oz read off the various numbers—weight, cholesterol, blood pressure, blood sugar. While the doctor seemed quite positive about what he described as “good numbers,” only one number seemed to move the audience: 441 (nanograms per deciliter). Presumably, the audience’s enthusiastic applause indicated that they saw Trump’s T level as scientific proof that he possessed not just the spirit but also the constitution of a strong, masculine leader. And while the precise nature of the molecule itself is not especially enthralling to most people (its chemical formula is C19H28O2), the same cannot be said for its ostensibly masculinizing powers—sometimes titillating, but sometimes toxic.
The writer Andrew Sullivan told readers of New York Magazine that he got “a real sense of what being a man is … [with] the rush of energy, strength, clarity, ambition, drive, impatience, and, above all, horniness” from his biweekly testosterone injections. A Psychology Today article suggests that “women are attracted to toxic masculine male phenotypes that correlate with testosterone … and who exhibit patterns of behavior that will allow them to ascend the social hierarchy and defend their positions from encroachers.” According to the left-wing Huffington Post, Trump’s presidency is “testosterone-fueled,” making it “an extremely dangerous one” that could lead to war. According to the right-wing American Spectator, the problem is not too much T, but too little, among some prominent conservatives: “There is also a low-testosterone, dilettantish strain of conservatism that has overdeveloped in the ‘mainstream’ media … to create such sterile hybrids as Michael Gerson and George Will and David Brooks,” who were, during Trump’s first presidential campaign, “sipping tea” while Trump’s base was “fighting a war.” And in another piece from Psychology Today, the author describes the “testosterone curse” in which high T induces “a biological urge that sooner or later demands expression.” According to him, while we can’t forgive the sexual transgressions of Harvey Weinstein, Bill Cosby, and other male celebrities, we should understand that “men are just animals who, when under T’s influence, have great difficulty perceiving females other than one-dimensionally, as objects for lascivious gratification.”
So not only are powerful men suffering from the curse of hypermasculinity that leads them into war and rape, T is to blame, and we women can’t help but love it! Apparently, too much T is toxic, too little is emasculating, and just the right amount leads to vigor and success.
Is any of this remotely accurate? Or is it just a popular myth—and perhaps one with a suspect sexist history? A proper answer to that question needs a whole book, and you are holding it.
There’s no doubt testosterone is responsible for the human male’s reproductive anatomy and physiology. As we’ll shortly see, whether it’s responsible for much more than that has been hotly disputed. The consensus of experts is that testosterone’s main job is to support the anatomy, physiology, and behavior that increases a male’s reproductive output—at least in nonhuman animals. And men are no exception—T helps them reproduce, and directs energy to be used in ways that support competition for mates. How that works is the subject of the rest of this book.
SEX DIFFERENCES AND SEX HORMONES
Sex differences are simply differences between males and females—in humans, chimps, or other species—and noting a difference says nothing about its cause. Some differences are small, or inconsequential, at least for the purposes of this book: for instance, women are somewhat better than men at performing mathematical calculations like adding up numbers in a column. And women’s names are usually different from men’s names. Others are both large and meaningful. Men are much more likely than women to be sexually attracted to women, and they are far more physically aggressive than women in every pocket of the earth, at every age. For example, they are responsible for around 70 percent of all traffic fatalities and 98 percent of mass shootings in the United States, and worldwide commit over 95 percent of homicides and the overwhelming majority of violent acts of every kind, including sexual assault. One important point about sex differences, illustrated by these examples, is that almost any feature that differs between the sexes isn’t exclusive to males or to females. Some men are called “Shirley”—and indeed the name was a man’s name a few centuries ago. Women murder and sexually assault, they enjoy sex with other women, and many are slower and less accurate than most men when totaling up the household budget.
Let’s look more closely at an obvious and uncontroversial sex difference: height. In the United States, the average height of women is less than the average height of men, by about five and a half inches. Following the pattern of many other sex differences, there is significant overlap: there are women who are taller than most men and men who are shorter than most women. If we picked hundreds of men and women at random and recorded their heights, the distribution of heights would look something like this:
Sex differences in height: different average, different variation
The vertical axis (or “y-axis”) represents the number of people from the sample who fall into each height category (in inches), which is indicated along the horizontal axis (or “x-axis”). The curves over each set of bars are just a clean way of approximating the (inevitably) messy data. (Only some bars are shown.) The dark bars represent women and the light bars men. Looking at the longest dark bar, it tells us that we found slightly fewer than sixty women who are sixty-five inches tall. We found more than twenty women who are seventy inches tall, and so on. The average height of women (at the top of the dark curve, around sixty-five inches) is clearly less than the average height of men (at the top of the light curve, around seventy inches), but there is a lot of overlap in height across the sexes.
The distribution of male heights is also wider than the distribution of female heights. The women cluster more tightly around their average than the men do around theirs. That is, there is more variation in male heights than female heights. This means that there are more men on the extremes of height, leading to more very short and very tall men, and fewer such women. More of the women are closer to the average female height than men are to the average male height.
A sex difference can be a difference in the average only (as we see in some tests of reading ability, in which females are higher), a difference in the variation only (as we see in IQ, in which there is greater variation in male scores), or a difference in both, as we see with height. The first two cases are illustrated in the graphs below.
Group differences: different average, same variation
Group differences: same average, different variation
Sex differences are everywhere. Some are big, some are small, some are uninteresting, and some are striking and in need of explanation. One very large sex difference is the level of testosterone over a lifetime. What role—if any—does that sex difference play in all the others? One uncontroversial role of T is to increase the height of men relative to women. (Although, as we’ll see in the next chapter, cutting off a boy’s testicles before puberty increases his height.) Testosterone’s role in the sex difference in complex behaviors like violence, however, is more controversial. In their 2019 book Testosterone: An Unauthorized Biography, Rebecca Jordan-Young, a professor of women’s, gender, and sexuality studies, and Katrina Karkazis, a cultural anthropologist, express skepticism about the idea that T does much of anything when it comes to sex differences in behavior. According to them, the notion that “T drives human aggression” is a “zombie fact”—a hypothesis that rises from the dead despite being killed again and again. And elsewhere Jordan-Young writes that exposing this myth is crucial for “denaturalizing violence and opening up the remedies we can pursue or even imagine.”
Copyright © 2021 by Carole Hooven
Copyright © 2021 by Felix Byrne