Wildhood: The Astounding Connections between Human and Animal Adolescents

"Wildhood is a contemplation of how we are like, and unlike, other animals, using the widespread phenomenon of adolescence as an anchor... Readers will come away with an appreciation for a host of other qualities—friendship, social status, cooperation, leaving home and coming back—that are rooted in that one crucial stage of life."
—Marlene Zuk, The Wall Street Journal

“Natterson-Horowitz and Bowers go deep and wide in addressing the raft of species-spanning equivalents. The authors make clear that, in a fundamental sense, adolescent animals and teen humans encounter the same sorts of challenges — and that what may strike elders of any species as nutty, exasperating behavior is not only inevitable for most creatures in that stage of development but truly valuable.”
—Duncan Strauss, Washington Post

“The vivid storytelling and fascinating scientific digressions in Wildhood make it a pleasurable read. It’s also a book parents may find reassuring: The authors show that the often painful struggles human and animal adolescents go through are a way of developing the skills and experience that will make it possible for them to function as adults. But there’s also another implicit message in Wildhood about the interconnection among the planet’s species. The awareness that we’re all in this together ought to motivate humans to stop ravaging the planet so it can continue to be a place where adolescents of many different species can find their ways into adulthood.”
—Patrick J. Kiger, Los Angeles Times

"Adolescence may seem unique to the human experience, but Natterson-Horowitz and Bowers vividly reveal that many species navigate this crucial time between childhood and adulthood, which they so aptly call 'wildhood.'"
—Booklist, Top 10 Sci-Tech Books of the Year

"Adolescence isn't just for humans. Here an evolutionary biologist offers up rollicking tales of young animals navigating risk, social hierarchy, and sex with all the bravura (and dopiness) of our own teenage beasts."
—People

“All this time spent reading books on adulting can be harrowing for a worried parent who isn’t entirely sold on the survival skills of her teenage son. I needed some reassurance.... Luckily, I found it with a king penguin, a hyena, a humpback whale and a wolf.... Barbara Natterson-Horowitz and Kathryn Bowers...follow this cast of characters as they face the trials of making it into adulthood in their savage and competitive worlds. You don’t even need to anthropomorphize to find some of the similarities between animal and human teenagers uncanny, and the lessons they have to learn remarkably similar."
—Judith Newman,  New York Times Book Review

“Harvard evolutionary biologist Barbara Natterson-Horowitz and science journalist Kathryn Bowers draw fascinating connections between human and animal young adulthood."
—Meredith Wolf Schizer, Newsweek

"Bestselling authors of Zoobiquity, Barbara Natterson-Horowitz and Kathryn Bowers, paired up again to research what we can learn about adolescent behavior from their counterparts in the animal kingdom… in their newest book, Wildhood. Their five-year study found many similarities between the thrill-seeking and sometimes inexplicable-seeming choices of teens and those of adolescent animals developing in the wild."
—Laura Pearson, The Chicago Tribune, 28 New Books You Need to Read Now

“An enduring story plot finds a youth suddenly alone in the world, struggling to find shelter from the elements, safety from predators, food, and new friends. These struggles usually involve some tough lessons but ultimately lead to knowledge, a new identity, self-reliance, and maybe even love. In Wildhood, this theme comes to exhilarating life as evolutionary biologist Barbara Natterson-Horowitz and science writer Kathryn Bowers describe the challenges faced by adolescent animals. There is much here for the nature lover, the parent seeking advice, and the college freshman tackling “adulting.” By laying out the adolescent experience of so many species in rich detail, the authors normalize and celebrate the beauty and complexity of our own species’ journey into the big wide world.”
—Linda Welbrecht, Science Magazine

"Take the authors up on their invitation to observe animals in the wild and in your own household, and you’ll never look at other beings the same again. “Wildhood” is for parents, nature lovers, and the curious alike. You’ll be wild for it."
—Terri Schlichenmeyer, Times Record

"Reading [Wildhood], I was surprised to see that many of the adolescent behaviors humans exhibit are wired in for adolescents of most species. This may not provide much consolation for you as you try to guide your teen through the dangers of risk-taking, but it provides insights into how much your teen is exhibiting normal adolescent behavior shared with birds and monkeys. Most importantly, it’s a reminder that this is usually not about you."
—Mark Phillips, Marin Independent Journal

“An incredibly fascinating read, Wildhood illuminates what humans can learn from the animal world and how all species are more connected to one another than they may appear."
—Booklist, starred review

“Human teens have much in common with their counterparts throughout the animal kingdom – and those commonalities are eye-opening as described in the latest from biologist Natterson-Horowitz and science journalist Bowers. Reassuring ... should appeal to anyone who’s ever raised an adolescent, human or otherwise.”
—Publishers Weekly, starred review

“A lucid, entertaining account of how creatures of many kinds learn to navigate the complex world that adulthood opens.”
—Kirkus

“This compelling account of how strongly human adolescent behaviors are rooted in our wild animal past should intrigue general science readers and fans of Zoobiquity.”
—Library Journal

"A fascinating and well-written book about the universal journey of adolescence. It is full of insights about humans as well as many other animals. There is much that human teenagers can learn from the experiences of adolescent penguins and hyenas."
—Yuval Harari, author of Sapiens

"It blew my mind to discover that teenage animals and teenage humans are so similar. Both are naive risk-takers. I loved this book!”
—Temple Grandin, author of Animals Make Us Human and Animals in Translation   

"The wild adventure of adolescence has never been analyzed in such depth. In lively personalized accounts that keep our attention, the authors explain how the transition to independence works in each species, and why it looks so similar across the board."
—Frans de Waal, PhD, author of Mama’s Last Hug and Our Inner Ape  

“Our teenage years can be many things, from fraught and frustrating to exhilarating and joyful.  In Wildhood, Natterson-Horowitz and Bowers show that these years are something else altogether—essential for humans and animals in general. Read their enlightening journey and you will never see the transition to adulthood the same way again.”
—Neil Shubin, Ph.D., author of Your Inner Fish and The Universe Within

“One of the most insightful books ever written about this critically important stage of life. Unfailingly fascinating—and sometimes downright mind-blowing—this a remarkably original account of the nature, meaning, and purpose of adolescence in today’s world.”
—Laurence Steinberg, author of Age of Opportunity: Lessons From the New Science of Adolescence

“A masterpiece. This is a spellbinding lens on the ways creatures with big bodies yet little life experience figure out how to survive and thrive. Read Wildhood!”
—Wendy Mogel, author of Voice Lessons for Parents and The Blessing of a Skinned Knee

“A deeply researched and beautifully written description of the fundamental tasks of adolescence. The authors’ account of the trials faced by teenagers across the animal kingdom inspires compassion for young people and a deep appreciation for what they must accomplish on the journey into adulthood.”
—Lisa Damour, author of Untangled: Guiding Teenage Girls Through the Seven Transitions Into Adulthood

“The authors offer a life-changing perspective on adolescents venturing out into the world. A treasure trove of scientific exploration and practical implications for how we understand and support youth.”
—Daniel J. Siegel, MD, author of Brainstorm: The Power and Purpose of the Teenage Brain 

“This fascinating book tells the compelling story of adolescence across species, framed in the convincing context of evolutionary and adaptive explanations.”
—Sarah-Jayne Blakemore, author of Inventing Ourselves: The Secret Life of the Teenage Brain

"Wildhood’s tour of the natural history of adolescence is original, entertaining, and constructive. The transition from youth to adulthood might never be easy, but this comparative biology is full of ideas for understanding it better."
—Richard Wrangham, PhD, author of The Goodness Paradox and Catching Fire 

“Those travails of adolescence?  It isn’t just you. Or your culture. Or even your species. Wildhood uses riveting stories about the challenges overcome by specific whales, wolves, and more to put the challenges of adolescence in a universal evolutionary context for the first time. Groundbreaking and fascinating.”
—Randolph M. Nesse, MD, author of Good Reasons for Bad Feelings

"Wildhood links coming-of-age neurobiology with ecology and evolutionary biology to create a powerful new lens for understanding the science (and art) of growing up. At times counter-intuitive, at times paradigm-shattering, this illuminating new book generates dozens of hypotheses for raising, educating, counseling and treating, and living life as an adolescent human."
—Gene Beresin, MD, Professor of Psychiatry, Harvard Medical School

Barbara Natterson-Horowitz, MD, is a Visiting Professor at Harvard University in the Department of Human Evolutionary Biology. She is also Professor of Medicine/Cardiology at UCLA and President of the International Society of Evolution, Medicine, and Public Health. She is the coauthor of Zoobiquity and Wildhood.

Kathryn Bowers is an animal behaviorist and writer who has taught at UCLA and Harvard. A Future Tense Fellow at New America in Washington, DC, she was an editor at the Atlantic Monthly, CNN-International in London, and at Zócalo Public Square in Los Angeles. She is the coauthor of Zoobiquity and Wildhood.

Wildhood

Prologue

Our quest to understand the nature of adolescence began on a cold California beach in 2010. We stood on a sand dune, contemplating a stretch of the Pacific, an expanse with an intriguing nickname: the Triangle of Death.

We’d been drawn there by a marine biologist’s unusual story. The Triangle of Death, he’d told us, owed its reputation to a horde of particularly lethal inhabitants: great white sharks. Hundreds of these colossal predators live in this region, and they’re so notoriously ravenous that even the local sea life has learned to stay out of their way. Lush kelp forests grow up and down the California coast, but not in the Triangle, so any animal foolish or unlucky enough to venture in there has nowhere to hide. So treacherous are those waters that even the scientists who work in them don’t get out of their boats.

But, the biologist said, that wasn’t the most interesting part. Counterintuitively and at great danger to itself, one animal does regularly enter the Triangle of Death: the California sea otter. But not all of them. Only one specific kind joyrides into the death zone, and it’s not the mature adults. It’s certainly not the baby pups. No, the magnificent knuckleheads that swim into the cold, barren, shark-filled Triangle of Death are adolescents. Sometimes they die in a flash of teeth and a swirl of blood. But more often than not, these thrill-seeking animal “teens” emerge with hard-won experience, newfound confidence, and more sea smarts than they had as parent-protected, dependent juveniles.

At the time, we were researching our first book, Zoobiquity, which explores the ancient and essential connection between human and animal health. (We work as a team: Barbara is a visiting professor in the Department of Human Evolutionary Biology at Harvard and professor of medicine in the Division of Cardiology at UCLA. Kathryn, a science writer, is a certified animal behaviorist. Together we’ve designed and taught courses at Harvard and UCLA.) Gazing over the Triangle of Death, we were struck by how these adolescent otters sounded a lot like teenagers we knew: taking risks, seeking out danger, doing scary things their parents had grown out of. After regarding the ocean for a few more moments, we walked back up the beach, over a dune, and onto a spit of land overlooking a different scene.

In a cove protected from the whitecaps, kayakers slowly paddled through the calm water. This inlet, called Moss Landing, is a prime location for observing wildlife, including sea otters. The extended families of the adolescents drawn to the Triangle of Death come here to feed, relax, and socialize.

On that day there were dozens of the sleek creatures floating on their backs, twisting and twirling through the water. The panorama resembled open-swim hour at a public pool—with young and old otters cavorting together. Older animals swimming leisurely made way for splashing groups of youth. We saw otters diving for sea urchins and learning to break them open, play-fighting in pairs and groups, and testing out the nose-grabbing behavior these animals use when they court. Although it looked like carefree recreation, we’d later learn that this was actually a cove full of teachable moments for the younger members of the group.

As we watched, pandemonium suddenly broke out. The water exploded in a churn of white as a group of otters powered at top speed from one end of the inlet to the other. What just happened? we asked our biologist guide. Was it a shark? Had a predator entered the shallow bay?

No, the biologist responded, pointing. That kayak got too close. And look—they didn’t all get spooked. There, still floating comfortably, undisturbed, was one cluster of otters. The gray fur on their heads showed they were mature adults, experienced and discerning. The skittish ones who’d taken flight were the adolescents who couldn’t yet tell the difference between a great white and a Sea Ghost 130.

Swimming up to sharks one moment, then fleeing from a plastic boat the next: these inexperienced adolescents were both overly bold and overly cautious. But we observed that these animal adolescents were also exuberantly socializing with their peers, experimenting with sexual behaviors, and fumbling with how to feed themselves. The parallels with our own species, and even with our own younger selves, were remarkable.

It also crossed our minds, as it has often since we started researching animal-human overlaps, that we might be anthropomorphizing this otter behavior—reading too much into the antics of these wild mammals. From the beginning of our research together, we’d made it a point to avoid projecting human qualities onto other species, thinking it a central scientific danger. But as we learned more about work from fields including neurobiology, genomics, and molecular phylogeny, we realized the bigger danger might be denying humans’ real and demonstrable connections with other animals, in body and behavior. The real threat, we recognized, wasn’t perhaps anthropomorphism, but its opposite, what primatologist and ethologist Frans de Waal calls “anthropodenial.”

Over and over in our work we’ve refuted claims of human uniqueness: Wild animals can and do get so-called human diseases like heart failure, lung cancer, eating disorders, and addiction. They can develop insomnia and anxiety. Some overeat when they’re stressed. They’re not all heterosexual. Some are timid; some are bold. Nearly every time we’ve encountered claims of human exceptionalism, we’ve found them to be incorrect.

And there, in the water right in front of us, was another striking parallel. Although it happens at any point from a few days to many years after they’re born, all animals have a “teenage” period. Boys and girls don’t become men and women overnight. And the transition from foal to stallion, joey to kangaroo, or sea otter cub to sea otter elder is just as distinct, just as necessary, and just as extraordinary. All animals need time, experiences, practice, and failure to become mature adults.

That day at the Triangle of Death we caught a glimpse of animal adolescence. And once we’d seen it there, we began to see it everywhere.

A NEW WAY OF SEEING

It was, almost quite literally, as if we’d taken off a blindfold. Our physical vision hadn’t changed, but our perception had, and suddenly a whole new way of understanding what it means to grow up revealed itself. Flocks of birds, pods of whales, groups of young people, our own children—even memories of our own adolescence and young adulthood—would never look the same.

Over the next several years we focused our research on understanding animals in this in-between phase, the ones too physically grown to be juveniles but not yet possessing the experience to be considered mature.

Watching a herd of wildebeest crossing a crocodile-infested river, we noticed that the first ones in the water were big but gangly adolescents. Oblivious to danger, exuberant with inexperience, they leapt right in while their more prudent elders held back, swimming across safely once the crocodiles were occupied chasing the adolescents.

In Manhattan, Kansas, of all places, we came face-to-face with two young adult hyenas and observed how one bullied the other although they were the same age and size. All it took were two young individuals to form a clear social hierarchy.

Approached by a group of wide-eyed lemurs in a forest preserve in North Carolina, we were charmed by the one that came right up to us. He was an adolescent named Nacho, and his fearlessness both endeared him to us and—had we been poachers instead of scientists—endangered his own safety.

We listened to orphaned wild wolves learning to howl, their changing adolescent voices warbly and cracking. We watched panda adolescents learning to peel bamboo, a first step in eventually feeding themselves. One extraordinary afternoon, we observed herds of wild horses, white rhinos, and zebras. We zeroed in on the adolescents within those herds and saw how they postured and shoved each other as they jockeyed for a place in their groups.

Some of our searches were more successful than others. Adolescent Canadian bison in Prince Albert National Park near the Arctic Circle stayed invisible in spite of the twenty miles we hiked through mud and mosquito-dense wetland hoping to spot them. The young adult bear whose warm scat we found on the trail during that same trek also didn’t materialize. We came close while tracking an adolescent mountain lion in Los Angeles. As we paused to rest, our guide opened a trail camera and showed us that we were standing in the very spot the lion had slinked through just a few hours earlier.

A PLANETWIDE TRIBE

Biologists have long been aware that animals—human and not—go through physical and behavioral changes between infancy and adulthood. But the risk-taking, the socializing and sexual experimenting, the leaving home to seek one’s fortune or to find oneself, not to mention the angst and mood swings, the romantic and turbulent emotions—even the raging hormones and rapidly changing “teenage” brain—surely all that was uniquely human? No, we would learn, it is categorically not.

While every individual’s adolescent experience will differ in its details—some will be triumphant; some will be tragic; most will be somewhere in between—when we started looking at adolescence across species, a universality presented itself. Regardless of the animal, its position on Earth, or the historical era it lives in, all individuals on this journey face the same core challenges. And successfully surmounting those challenges, we argue, is the definition of maturity.

While on this journey, adolescents from bottlenose dolphins to red-tailed hawks, clownfish to humans, have, in many ways, more in common with one another than with their mature parents or immature younger siblings. They share what author Andrew Solomon has called a “horizontal identity.” In his book Far from the Tree, Solomon contrasts vertical identities, those between you and your ancestors, with horizontal identities, those among peers with whom you share similar attributes but no family ties. Expanding Solomon’s concept to include other species, we suggest that adolescents share a horizontal identity: temporary membership in a planet-wide tribe of adolescents.

This global journey, and the ways successful adolescents navigate it, is the subject of this book. Its premise: human adolescence is rooted in our wild animal past, and the joys, the tragedies, the passions, and the purpose aren’t inexplicable; they make exquisite evolutionary sense.

COMING OF AGE ON PLANET EARTH

At Harvard in the spring of 2018 we first offered “Coming of Age on Planet Earth,” a course for undergraduates based on the research in this book. On the first day of class we had our students grab their backpacks and follow us through the Peabody archaeology museum, past the cases of kachina dolls and towering Mayan steles to the Tozzer Library of anthropology. Waiting for us, mounted and elevated on a long wooden table, was a first edition of Margaret Mead’s Coming of Age in Samoa. In 1925, at the age of twenty-three (by today’s measure an adolescent herself), Mead traveled to the South Pacific nation to study adolescence in another culture as a way of better understanding it in modern Americans. Mead’s comparative approach completely transformed the field of anthropology, particularly her focus on culture, rather than biology, as the primary shaper of human individuals and societies. While her work was later criticized (many say unfairly) for relying on methods that were at times more impressionistic than data-driven, she remains a leading intellectual force of twentieth-century understanding of human development, especially adolescence.

In the late nineteenth century, scholarly interest in adolescence had been sparked by an American psychologist, G. Stanley Hall, who borrowed the German literary term sturm und drang (storm and stress) to describe the age. Throughout the twentieth century, psychoanalysts, including Sigmund and Anna Freud, Erik Erikson, and John Bowlby, advanced nurture-based explanations for the challenges of childhood and adolescence, while cognitive psychologist Jean Piaget saw more of a role for biology along with environment in shaping adolescent minds. The Nobel laureate Nikolaas Tinbergen, a founder of the field of animal behavior and an ornithologist by training, saw animal roots in human development. In this era, adolescence was often viewed as a malady: those afflicted with it were studied as if some disease had caused their restlessness, rebellion, risk-taking, and unhappiness.

Advances in neuroscience changed that, starting in the 1960s. Marian Diamond’s work on brain plasticity and Robert Sapolsky’s on the coevolution of social and emotional brain development shifted the view of human adolescence from a fraught stage with fixed characteristics to a dynamic period crucial for normal development. Frances E. Jensen, Sarah-Jayne Blakemore, Antonio Damasio, and others have connected genetics and environment to the remarkable and terrifying aspects of the period such as risk-taking, novelty-seeking, and peer influence. Linda Spear, a developmental psychologist, has examined adolescent brain biology in relation to temperament, and Judy Stamp, an evolutionary biologist, has explored how environments, whether physical or social, shape adolescents’ destinies. Psychologist Jeffrey Arnett has popularized the term “emerging adult” and exposed the power of modern culture in shaping the adolescent experience. And, in addition to illuminating this often turbulent time of life for parents and educators, psychologist Laurence Steinberg’s work on adolescent neurobiology is being used to question whether younger defendants in criminal cases should be punished as harshly as fully mature adults.

Following in the tradition of these thinkers, but especially inspired by Mead, we use a comparative approach in our research, our teaching, and in this book. However, we push beyond human comparisons to examine the central challenges of adolescents across species. Our focus is not the two-hundred-thousand-year history of Homo sapiens, but rather the six-hundred-million-year history of animal life on Earth.

JURASSIC PUBERTY

The terms “adolescence” and “puberty” are sometimes used interchangeably, but although they’re related, they’re not the same thing. Puberty is the biological process, kicked off by hormones, resulting in an animal’s ability to reproduce. Puberty describes strictly physical development—a growth spurt and, among other things, the activation of ovaries and testes to begin the production of eggs and sperm. Great white sharks go through puberty. Crocodiles go through puberty. As do pandas, sloths, and giraffes. Insects go through puberty (it’s part of metamorphosis). Every adult Neanderthal went through puberty, as did Lucy, the famous female hominid Australopithecus afarensis whose 3.2-million-year-old bones were found in present-day Ethiopia. Dinosaur puberty hit Jane, an adolescent Tyrannosaurus rex, sixty-seven million years ago in Montana. She died before she completed it, according to the paleontologists who unearthed her skeleton and gave the young T. rex her name.

While details vary across species, the basic biological sequence of puberty is remarkably similar. The same hormones kick it into high gear in hummingbirds and ostriches, giant anteaters and miniature ponies. Nearly identical hormones get it started in snails and slugs, lobsters and oysters, clams, mussels, and shrimp.

The dazzling array of most life on Earth today erupted 540 million years ago during a period called the Cambrian explosion. But puberty is older even than that. It’s part of the life cycle of one of Earth’s most ancient life-forms, single-celled protozoa. Protozoa still exist today, and one, Plasmodium falciparum, finds its way into human blood by way of a mosquito bite. Once there, the physically immature organism floats harmlessly around the body until it passes through its protozoal puberty and becomes a leading cause of death worldwide: Plasmodium falciparum is the parasite that causes malaria.

Despite its sex-specific connotation, puberty exerts its hormonal effects on every organ system in the body. Hearts grow, dramatically increasing cardiovascular performance. Lungs expand in capacity, giving young athletes more endurance (and asthmatics more attacks). Lengthening skeletons provide gangly-limbed pre-adult bodies thrilling new acceleration, but this rapid bone growth is also behind the increased incidence of bone cancers at this age. Child-sized skulls enlarge to adult dimensions, something not only seen in human children but also noted in dinosaurs. Jaws change shape, and so do the teeth within them. In fact, great white sharks are incapable of administering their deadliest bites until after they’ve gone through puberty.

So puberty is an ancient process of physical transformation. But to attain adulthood, a physically developed young creature must go through a second phase. This one combines body and behavior. It is about learning to think, act, and even feel like a mature member of a group. It’s a period of collecting crucial experiences, a time to absorb information from mentors, and test oneself against peers, siblings, and parents.

This phase is adolescence, and it lasts as long as it takes to create a mature adult. In fact, for a species to produce mature adults, as opposed to just physically grown individuals, an adolescence is essential. The quest for maturity through experience is the universal purpose of adolescence in nature.

And the journey can spark astonishing innovations. One of the most famous fossil finds of recent decades is a fish called Tiktaalik unearthed by the University of Chicago paleontologist Neil Shubin. These 375-million-year-old creatures bore a clue to our evolutionary past: four small limbs that served as both fins and feet. Those four appendages are evidence of Tiktaalik’s pioneering role in one of the most epic stories of life on Earth—the transition from water to land.

Shubin has found that Tiktaalik fossils reveal something else. They have been found in a range of sizes, some the length of tennis racquets, others longer than a surfboard. This means something as profound as it is obvious: These ancient fish grew up. And during that process, like adolescents today, just-through-puberty Tiktaalik individuals would have been especially vulnerable, lacking not only size but experience with predators, with competitors, with sexuality, and with finding food. Vulnerability and inexperience regularly push younger animals into unfamiliar settings. We wrote to Shubin and asked whether he thought it possible that adolescent Tiktaalik fish were the ones leading the charge to land. He saw this as plausible and wrote back: “Tiktaalik was an animal with big carnivorous adults, so near the top of the food chain, but juvenile stages would be exposed to predation and may have benefited by being partially terrestrial. Likewise, maneuvering on land would be easier in smaller fish, rather than larger ones, at least in incipient stages.”

While this remains only a hypothesis, it’s consistent with everything we know about the risk-taking and novelty-seeking behavior of adolescents across time and place. Driven by necessity, adolescents explore frontiers. They innovate new ways to survive. And when they do that, they can create the future.

THE “TEENAGE” BRAIN

Among the organs undergoing radical change during puberty and adolescence is the brain. Transitioning “teenage” brains are marvels of upheaval, markedly different from the child’s brains they were and the adult brains they will be.

Every brain makes memories, but the teenage brain in particular is storing away huge numbers of them that will shape who we are and how we approach the world for the rest of our lives. Psychologists call this the “reminiscence bump,” the especially deep and enduring memories formed during this period (in humans it happens roughly between the ages of fifteen and thirty).

The impulsivity of adolescents, their drive to experiment and seek novelty, and their immature decision-making have been linked to the brain’s executive function center, particularly the prefrontal cortex, which matures late in brain development. Adolescents’ preference for being with peers and even their conflicts with their parents have also been traced to unique neurobiology in regions of the brain supporting emotion, memory, and reward. So have their mood swings from stratospheric highs to subterranean lows. Vulnerability to substance abuse, self-harming behaviors, and mental illness have also been attributed to the still-developing brain, which doesn’t fully finish its transformation until well into a person’s late twenties and perhaps even early thirties.

The mysteries of the human teenage brain have been widely chronicled in recent decades, and this research has helped us understand why adolescents behave as they do. Yet this groundbreaking science largely ignores a much bigger revelation: during adolescence, the brains and behaviors of other animals are also going through a massive transformation.

Adolescent birds have a brain region that, like the developing prefrontal cortex in humans, helps young animals gain self-control. The brains of adolescent orcas and dolphins continue to grow after physical and sexual maturity, as do our own. And the changing adolescent brains of other primates and smaller mammals drive tendencies like risk-seeking, sociality, and interest in trying new things. Even adolescent reptiles show unique neurological shifts between juvenile and adult life, as do adolescent fish.

Whether our bodies are covered with skin, scales, or feathers, whether we move by running, flying, swimming, or slithering, we share common biology that builds and shapes our adult selves. This book explores the universality of the period between childhood and adulthood—what we decided to call “wildhood.” Looking across the world of animals over hundreds of millions of years of evolutionary time allows us to separate out which aspects of adolescence are unique to a single animal species or human culture, and which are the norm on planet Earth.

THE FOUR CORE LIFE SKILLS

The central insight is this: four fundamental challenges of wildhood are the same for a fruit fly coming of age in the bananas on a kitchen counter, a lion roaring into adulthood on the Serengeti, and a nineteen-year-old balancing work, school, friends, relationships, and other responsibilities. They are:

How to stay safe.

How to navigate social hierarchies.

How to communicate sexually.

How to leave the nest and care for oneself.

Each of these four essential challenges is encountered throughout an animal’s life, but adolescence and young adulthood are when they’re faced together for the first time, and usually without parental support or protection. The experiences of wildhood build necessary life skills and shape the adult fates of individuals.

Avoiding danger. Finding a place in groups. Learning the rules of attraction. Developing self-sufficiency and purpose. These skills are universal—because they support the survival of young animals moving into the wild world. Learning them is mandatory for a successful life.

Safety. Status. Sex. Self-reliance. The four skills are also at the core of the human experience and the basis of tragedies, comedies, and epic quests.

Millions of things can go wrong for an adolescent animal on the road to adulthood. But when the journey goes well, and a mature adult emerges, it always means the same thing. During its wildhood, that individual faced the four challenges and developed competency in each one. These individuals didn’t just grow older; they grew up. The journey of wildhood has been undertaken for more than six hundred million years, by countless animals. We believe the ancient legacy of those combined experiences can become a modern atlas for surviving and thriving into adulthood.

COMING OF AGE IN A DIGITAL WORLD

As we’ll see, animals develop what, for lack of a better word, we’ll call “culture” around transmitting these four life skills to up-and-coming adults. Even within animal species, cultural specifics can vary from region to region and from group to group, just as human cultures have their own endless permutations.

However, one specific area in which humans do in fact stand out from our animal cousins lies in how our teenagers must now traverse two distinct worlds to reach adulthood, one in the real-life communities where they live, the other online.

The four core life skills apply just as much to the internet as they do offline, but these two cultures can be radically different, requiring many modern teens to make two simultaneous journeys to adulthood.

For instance, as we’ll explore in Part II, social animals, from fish swimming in the sea to high schoolers rushing to class, must learn to navigate hierarchies of peers. One of the ways they do this is called “association with high-status animals.” This term makes immediate sense to anyone who’s ever been to school, had a job, or had a social life—you can improve your own status by hanging out with more powerful people. We’ll explore the fascinating intricacies of how this works in groups of other animals, but spare a thought for modern human teens and the number of additional hierarchies the internet brings into their lives. If they spend time in multiplayer games or on social media, they’re being assessed, sorted, and ranked, sometimes invisibly and sometimes explicitly, alongside an entire universe of other people on those platforms. Imagine the status boost of being praised by a sports or pop star; imagine too the crushing humiliation of being called out by an idol.

Parents and other elders have plenty of experience guiding adolescents and young adults through the real world. But no one has yet aged a full life in the digital world. The four life skills can help sort this new terrain into more easily fathomable categories, because the real-life ones have online correlates: How to stay safe from trolls and predators. How to move through virtual hierarchies. How to express sexuality. How to shape, nurture, and maintain a digital self or identity.

WHY WILDHOOD

When we teach “Coming of Age on Planet Earth,” we include an informal poll: Raise your hand if you think you’re an adolescent. Next, raise your hand if you consider yourself an adult. Our students are all between eighteen and twenty-three years old, but rarely has a hand shot up immediately or confidently for either question. Often, our students respond with “yes” and “yes”—we’re both.

If adolescents don’t use the term “adolescent” to describe themselves, what should we call emerging creatures who are fully grown (or almost) but not quite fully grown up? Who are large in size but small in experience and who may be sexually mature but whose brains won’t be for many more years?

The term “adolescentia” derives from the Latin word adolescere, meaning to grow up, and it appears in medieval texts dating back to the tenth century, describing a religious turning point in the young lives of saints. In North America, the New England Puritans of the mid-1600s considered the age to be a “chusing time,” when frivolity was to be left behind and adult employment taken up, but the people in this time were generally called “youth” until the late 1800s, when “adolescents” came into common usage.

Flapper, hipster, bobby-soxer, teenybopper, beatnik, hippie, flower child, punk, b-boy, valley girl, yuppie, Gen Xer—these terms offered ways of talking about young people in specific American cultural contexts throughout the twentieth century. The word “teenager” first appeared in print in 1941 and soon dominated the lexicon. Even today, nearly eighty years later, “teenager” remains the go-to synonym for “adolescent,” even as it became scientifically inaccurate when neuroscientists revealed that adolescent brain development starts before thirteen and continues well beyond nineteen. For the past decade or so, “millennial” has neatly covered people in this stage of life, but at this point most millennials have aged out of the adolescent young adult period. “Generation GWoT” is U.S. military parlance for those who have come of age during the Global War on Terrorism. In North America, we’ve often heard “kids” used as a default term—even by adolescents themselves—but it sounds too young once they’re in later high school.

We searched for a better descriptor for both humans and nonhumans in this phase, a word that would cover its ancient commonality. Some terms were too clinical (“pre-adults,” “emerging adults,” “dispersers”). Some were off-putting or even insulting (“sub-adults,” “immatures”). Some were poetic (“fledglings,” “deltas,” and “elvers,” which is the term for adolescent eels). The world’s languages held marvels such as the Japanese term seinenki (meaning green ones, saplings), or the Russian lishney cheloveki (odd people), but we balked at choosing one culture’s term over another’s.

Our term needed to describe the phase of life in which biology and environment come together to shape mature individuals across all species. It had to be unbounded by a specific age, physiologic sign, or cultural, social, or legal milestone. And it had to capture the vulnerability, excitement, danger, and possibility of this distinct phase of life. We had coined the title of our first book, Zoobiquity, by bringing together the Greek root for “animal” with the Latin for “everywhere.” For this book, we again created our own term and title. We chose “wild” to capture the unpredictable nature of this life-stage and acknowledge the shared animal roots. And we added the old English suffix “hood,” which means both a “state of being” (boyhood, girlhood) and a “group of persons” (neighborhood, sisterhood, knighthood), to indicate membership in the planetwide tribe of adolescents. The phase of life before adulthood, following childhood, across species and evolutionary time, became “wildhood.”

A CROSS-DISCIPLINARY APPROACH

The scientific evidence we’ve assembled and summarized, and which we present here, represents the product of five years of scholarship at UCLA and Harvard. Because our work falls at the intersection of evolutionary biology and medicine, we used research tools from both fields, developing large systematic reviews of comparative adolescence and using the results to create phylogenies. (Systematic reviews are comprehensive, tightly targeted surveys of the world’s scientific databases, powered by advances in search technology over the past twenty years. Phylogenies are diagrams of evolutionary relationships among different species, which can be simple family trees or complex computer models containing thousands of data points.) We also conducted fieldwork observing animal adolescents in natural settings and sanctuaries around the world, and interviewed experts in human adolescence, wildlife biology, neurobiology, behavioral ecology, and animal welfare.

We believe our research has important implications for multiple groups, and we’ve chosen to describe it in ways that can inform both scientific and other audiences. References directly linked to the text are provided as endnotes. An expanded bibliography, including links to our research, source material, and content of interest is available online for readers who parent, teach, study, treat, mentor, coach, or work with adolescents, and most important, for those who are adolescents themselves.

We’re writing in early-twenty-first-century America, and our work will reflect that; we don’t presume to understand the specifics of every person’s adolescent experience. That said, we did have one personal motivation while writing this book. During the whole process, we were parenting adolescent offspring of our own. Kathryn’s daughter was thirteen when we started, and Barbara’s daughter and son were sixteen and fourteen. All three are older now but being mothers of adolescent humans gave us a practical advantage: we could observe wildhood up close. After field trips to the Arctic Circle, Chengdu, the Gulf of Maine, and North Carolina, we would come home to our own exuberant teens and be reminded of the complex yet fleeting wonder of this age.

A COMMON QUEST

Our office in Harvard’s Museum of Comparative Zoology, where we wrote the bulk of this book, has a secret passage that connects it to another world. Up one particular stairwell, if you turn right instead of left, you end up in the Peabody Museum, an institution dedicated to preserving human cultural heritage. Sometimes, immersed in our work, we’d emerge from one world and get lost in the other. On one side, the legacy of comparative zoology, from dinosaur bones to molecular genetics. On the other side, physical objects testifying to millennia of human ingenuity, persistence, collaboration, and love. Both sides—zoology and anthropology, animal and human—a reflection of the diversity of life on our planet.

After crossing this symbolic divide many times, we became as good at identifying the signs of human adolescence in the Peabody’s collections as we were at seeing them in animals in the field. We came to feel connection with, almost affection for, these artifacts of growing up. Whether it was a suit of armor from a tiny island in the middle of the Pacific, a youth’s golden pendant from fifth-century Mesoamerica, a Lakota courtship blanket, or an Inuit snow shovel, these human touchstones further bridged this unique yet universal phase of life.

As you know from every coming-of-age story you’ve ever read, youths go on quests. They’re kicked out of the house, they escape after a conflict, or they’re orphaned, and they head out into the wild world. They’re dangerously unprepared, sometimes hilariously, sometimes fatally. On their journeys away from home, they fight off predators and exploiters. They meet friends and learn to identify foes. They might fall in love. And they learn to fend for themselves—finding their own food, making their own homes, and then usually at the end of the story deciding whether to rejoin the community they were born into or reject it and forge a new one of their own.

Our science is told through the real-life coming-of-age stories of four wild animals tracked by biologists over months and years. Our protagonists are not human, but they are all adolescents. Ursula, a king penguin born and raised on South Georgia Island off Antarctica, faces almost-certain death from a monstrous predator on her first day away from her parents. Shrink, a spotted hyena in the Ngorongoro Crater in Tanzania, battles bullies and forms friendships as he navigates the hierarchical hyena version of high school. Salt, a North Atlantic humpback whale born near the Dominican Republic who spends every summer in the Gulf of Maine, confronts sexual desire and learns how to communicate what she wants, and doesn’t want, from her partners. And finally, on a harrowing but exhilarating journey away from home, Slavc, a European wolf, nearly starves, drowns, and dies of loneliness as he tries to hunt his own food and find a new community.

We’ve chosen to tell their stories in a narrative style, which we hope captures the real drama each experienced on the journey from adolescence to adulthood. However, every detail that we provide in these stories is based on and validated by data from GPS, satellite, or radio collar studies, peer-reviewed scientific literature, published reports, and interviews with the investigators involved.

Separated by hundreds of millions of years of evolution, these four wild animals are connected to one another, and to us, through the common experiences, challenges, and ages of wildhood.

Whether experienced in the treacherous waters off Antarctica, the grasslands of Tanzania, a shimmering Caribbean bay, or the Triangle of Death, wildhood extends throughout nature and into our human lives. It shapes and sometimes determines our adult destinies. Wildhood is the common inheritance of all creatures on Earth, an ancient and ongoing legacy ready to be claimed.