Zibby Owens: I’m here today with Barbara Natterson-Horowitz, MD, who is the coauthor, along with Kathryn Bowers, of best-selling book Zoobiquity and their latest, Wildhood: The Epic Journey from Adolescence to Adulthood in Humans and Other Animals. A visiting professor in the Department of Human Evolutionary Biology at Harvard University, Barbara is also a professor of medicine in the UCLA Division of Cardiology and president of the International Society for Evolution, Medicine, and Public Health. She currently splits her time between Cambridge, Massachusetts, and Los Angeles, California.

Welcome, Barbara, AKA Dr. Natterson-Horowitz. Take your pick of title. Welcome to “Moms Don’t Have Time to Read Books.”

Barbara Natterson-Horowitz: I’m so happy to be here.

Zibby: Thanks for coming. Can you please tell listeners what Wildhood is about?

Barbara: Wildhood is a book that looks at growing up very broadly. It tells a story of five hundred million years of reaching puberty and developing into a mature adult and why it’s so challenging and how much we can learn from animals who’ve been doing it for a long time, some well and some not so well.

Zibby: You outline four key life skills that all adolescents, both humans and animals, have to go through, how to stay safe, how to navigate social hierarchies, how to communicate sexually, and how to leave the nest and care for one’s self. I know this is basically what your entire book is about, but what are some of the most blatant ways that animals and people share these things? Tell me about how you discovered this.

Barbara: I have an unusual background for somebody to be writing about animals. I spent over twenty years as a physician, as a cardiologist, a professor of medicine at UCLA. One day I got a call from one of the veterinarians at the Los Angeles Zoo who wanted me to come and image the hearts of some of their animal patients. It was actually a chimpanzee who they thought had had a stroke. That experience led to a request a few weeks later to image a gorilla’s aorta and then a few months later, the heart of a lion they thought had metastatic breast cancer, and so on and so forth. What happened over the course of several years is that even though I was spending ninety-eight percent of my time at UCLA at the human hospital taking care of human beings with heart attacks and high cholesterol, etc., I was also going to the zoo periodically and joining the vets on rounds where they were talking about heart failure in a kangaroo or metastatic melanoma in a rhino, and even behavioral problems. They were talking about dosing fluoxetine, which is Prozac effectively, for some of their animals who had compulsions and anxiety.

I had this aha moment. It was really an aha moment. Here I had been a professor of medicine for twenty years taking care of very advanced cardiovascular disease. I’d been teaching medical students. I really had never thought much about the so-called human diseases in non-human animals. In other words, I thought about cancer and heart disease as diseases of civilization. Those are human. I just hadn’t really looked at it from a broader perspective. I started getting interested and got more interested, and eventually starting writing about this more than anything else, and now spend all of my time looking to the natural world across species and evolutionary time for insights into human health and development.

The genesis of Wildhood is really the story, for me, of my life over the last fifteen years. I work with a brilliant woman, Kathryn Bowers. For over ten years we’ve been research and writing and teaching partners. During that period we wrote a first book called Zoobiquity which looks across species at cancer and heart disease and eating disorders and anxiety and self-injury. At the same time, we both have kids who were entering adolescence. Mine are a little bit older. They were kind of in adolescence. We were hearing a lot about what everyone was worried about, depression, anxiety, substances. Social media wasn’t really a big thing at that point. We started wondering whether we could take this lens that we’d developed, which we were applying to health concerns, and see whether we could find commonalities or differences between animals in this phase of life. That is, puberty has started but a mature adult has not yet emerged. If we could, what could we learn? I don’t even remember what your original question was.

Zibby: That’s okay. I was basically asking you about the entire book. That’s how you answered, which is great, and how you uncovered all of this research.

Barbara: I think what your question, I’m realizing — there’s so many things that we learned. The thing that I can’t believe is how blindfolded I was before I started spending time with veterinarians and thinking in this species-spanning way. I like to say to my students on the first day of class, “Check your human exceptionalism at the door.” It’s a blindfold that’s prevented us from seeing connections and patterns that can help us better understand ourselves. Of all of the many insights and shivery inducing, revelatory things that came from five years of studying wild animal adolescence, it was essentially the thesis of the book, which is across the animal kingdom there are four universal, high-stakes tests that shape the destiny of every adolescent. That realization, that as different as we are from marsupials and amphibians and fish, fundamentally, that experience of being big in size but low in experience, it challenges every animal. It presents the same problems and opportunities for every animal. That’s been one of the most inspirational and humbling insights of the whole project.

Zibby: Did doing all this research make you look at your own children in a different way? I know that’s taking something so insignificant, probably, in the context of this ground-breaking research, but just wondering. You have your sample set of your own adolescents at home. Did learning all of this about the animal kingdom have implications for your parenting or day-to-day life, a greater sense of understanding or compassion for adolescence?

Barbara: I don’t even know how to start. First of all, I look back and I wish that, particularly when my daughter was going through adolescence because she was in her late teens when this was all happening, that I had understood the connection between status. By that I mean, all vertebrates live in hierarchies. What that basically means is that every individual in the group has a perceived rank relative to every other individual. Status is kind of an ugly word. In proper society, it has some negative tones to it. Essentially, it’s a word to describe the perception of where you stand relative to the other fish or birds or wildebeests that you’re living with. One of the biggest insights was what happens in a fish’s brain, for example, when she experiences status descent. These serotonergic networks are activated. There’s a reduction in serotonin. The behavior, you can watch a fish who’s had status decent, and they stop moving around as much. They stop initiating activity. They get very withdrawn, actually. On the other end of it, what’s called the winner effect, the fish that keep being compared positively, they’re moving around a lot. They’re initiating stuff.

Zibby: The fish who’s not that cool anymore is now acting depressed.

Barbara: Again, to be careful not to anthropomorphize, I would say — I’m winking at you because I don’t think we need to be that worried anymore about anthropomorphizing.

Zibby: I was distilling in case that went over anybody’s head. I was just giving a dumbed-down example in case.

Barbara: The idea that mood — why am I saying mood? Mood is really complicated, lots of factors. To break it down, the most popular antidepressant drugs are the serotonin reuptake inhibitors. We know that serotonin is entrained with human mood. It turns out those same serotonin networks are really detecting and signaling a shift in status. Because high status is so fitness enhancing — that is, it increases survival and reproduction in the animal world — the burst of serotonin is probably a pleasure reward for whatever it was the animal was doing, a kind of “Attagirl, do it again.” A fall in status may be a kind of chemical reprimand. As we were doing this research and finding these connections — how we studied it scientifically, we can get into — but the parenting point of it all was I started thinking about how I used to respond when she would be concerned about what some girl or a couple of girls who I didn’t think were any great big deal, about what they thought of her. I would say things like, which I wish I hadn’t in light of what I know now, “Why do you care about what she thinks? What do you care about what they think?”

Knowing that in the animal world, status is literally a matter of life and death, and status descent is, and what happens to a fish when there’s — by the way, not just a fish, but a lobster. We have the whole set of animals in here. What happens when they rapidly tumble down that social ladder is that their neurobiology is changed. Their behavior is changed. Again, I can’t ask a lobster or a fish how they feel. I believe what we call human mood, what we use our human language to describe as happy, sad, humiliated, depressed, derives from those serotonin-based, status-signaling networks that are telling an animal when there is a fall in status, “This is really bad.” In order for the signal to work, it has to be uncomfortable for the animal and almost, let’s just say, painful. As I was working on this, I was thinking about how I’m a physician. I know about pain. Like a lot of physicians and a lot of people, even though I know psychological pain can be very, very, very painful obviously, still, physical pain seemed more real to me. Knowing the purpose of psychological pain, that is, the evolutionary origins of it, that it’s to really shape an animal’s behavior to catch their attention like, “This is really bad. You’re at the bottom of the ladder. Do something about it,” it has to be very painful for them to pay attention. I wish I had known that. I think I would be more compassionate about what it feels like to lose a little status or a lot of status.

Zibby: In the book, it was so interesting how you compared it to social media and friend groups today. Not to take it away from the animals at all, but just in the way that it has real-life implications. The comparing in the social network, you talked about, in the book, how it used to be. As a teenager, you could go home. You would go to school. You would have all this status comparison at school, stressful, stressful, kind of like the animals roaming around. Then you could go home and regroup. You could let your brain rest. Now you go home and it’s constant 24/7. You’re on Instagram all night. The kids are seeing each other’s friends and monitoring their status basically constantly, which is leading to X, Y, Z, fatigue and depression, anxiety. I don’t know. What do we do about that?

Barbara: It’s really interesting. What is social media? To sum it into one sentence, I guess it’s lots of things, but a big part of it is about comparison. It’s about, who am I relative to everyone else? Usually, I think the people who are on social media ultimately feel less than. Not only are you competing against the whole world, you’re competing against a filtered whole world of images. The biology of comparison, as I said, I think that’s the building blocks of mood. We see in the last several years, this uptick in depression and anxiety in adolescence, which is interesting because rates of mental illness among mature adults has remained pretty stable. We’re seeing this real uptick. People are properly observing that there seems to be an association — I don’t think we’ve gotten to cause — between social media use and anxiety and depression. Not only that, there seems to almost be a dose-response curve. The amount, the number of hours per week that kids are on social media, relates to the likelihood of having depression, anxiety.

What’s the connection? I think that not only is serotonin maybe the common language, that common molecule that can help us decode the ups and downs in our kids’ moods and by the way, our own moods — even though I don’t like saying it’s about status, again, I don’t have another word for it. The other way that this lens can help us understand and maybe do something about it is this. You can use an evolutionary lens to understand lots of things. Let’s talk about obesity. We have these skyrocketing rates of obesity. One of the ways we talk about it is we say we’re walking around in these bodies that we have inherited. We have this ancient, not only human, but we have this animal legacy. A lot of our metabolic machinery that we have is shared with not only the other mammals, but other vertebrates. The systems evolved in settings where calorie scarcity was a given. When you’re a wild animal or early man, you’re living in nature, you know one thing. There are going to be periods where there’s not enough to eat. As our metabolic systems were evolving and being optimized and tuned to the environment, they were being optimized to a calorie-scare environment. Now we’ve got this mismatch between this physiology that’s adapted to that environment, and now this obesogenic, whatever we want to call it.

Kathryn and I started wondering about taking that idea and applying it to the skyrocketing rates of anxiety and depression, especially knowing that comparison seems to be a big part of mood, or at least fluctuation in mood. We started wondering whether our social brain networks — the social brain network, there are anatomical regions of the human, the avian, the reptilian, the amphibian, the fish brain, that’s responsible for detecting changes in status and then signaling what to do about it. There’s a lot of brain real estate that’s being occupied. It’s important. Maybe animals in the wild, or early man or whatever frame you want to use for brain evolution, assessments and competitions were periodic, seasonal. For example, there are a bunch of species where they compete during a certain premating season, and that’s it. Then the hierarchy happens. Boom, they can move along. Maybe what we have now is a mismatch between social brain networks that evolved for some or intermittent, maybe even seasonal, assessment. Now it’s you walk into school, it’s in the cafeteria. It’s in the classrooms, academic. It’s sports. It’s looks. It’s blah, blah, blah. Then there’s no sanctuary. You walk out of class. You’re on the bus home or whatever. You reach in your pocket. Did I get any likes on my Insta post?

It’s this tsunami of assessment. It’s what we call assessment overload. What an evolutionary frame would say is if the reason we have all this obesity is because we’ve got bodies that are evolved for one situation and we’re in a completely different situation, let’s change the environment so that it’s more aligned with the physiology. The parallel here would be let’s align the environment to be more attuned to this physiology and decrease the amount of assessment. Social media’s a big part of it. Now how to accomplish that, I don’t know because it’s so addictive. That’s at least our working hypothesis about the connection between all these emotional symptoms and particularly anxiety, which is that feeling like you don’t know where you’re going to land on the hierarchy. If depression is that sad fish and euphoria is that winning fish, anxiety is that walking into a party and not knowing people are sizing you up and down, and that feeling.

Zibby: In my senior year of college — this is going back in the day here, 1997, 1998 — my thesis, I was a psychology major, and I did a study on the application of social comparison theory to eating disorders. I tested for comorbidity of depression and anxiety with social comparison theory, which by the way never got published. My advisor and I tried to put it in some psychology journals and all the rest. That was my theory. It’s how much people compare what they eat, how they depressed they are, and it leads to eating disorders. I noticed in the college setting, of course, like so many girls and myself included — I didn’t have an eating disorder. Although, I might as well have. I was always noticing what everybody was eating and saying, “She’s having this dessert and I’m having this.” Anyway, I know you’re going through your slides here to show me the same thing.

Barbara: I completely agree. There’s so many things. In Zoobiquity, we wrote a chapter about eating disorders basically just to point out that how we feel about what’s going on in the world, whether it’s social stress, we as animals, it affects our appetite. Clearly, there’s really good reasons for it. I have these videos of these Slovenian red deer grazing. When there’s lots of predators around, they can smell it. They eat like this. They take a bite, and then they look up. They’re hypervigilant. Then they take a few bites, and they look up.

Zibby: You look like my daughter. That’s exactly how my daughter eats, oh, my god. Sorry, go on.

Barbara: Then when they cull the predators — I have a video in my class. I teach a course based on — you see them. They’re grazing. They’re eating more. They’re relaxed. Clearly, the perception of danger and risk is affecting their eating. The other piece is social. That’s predators. What about the social experience? I’m just saying that we live as these social animals and that hierarchies are affecting how we feel. By the way, one of the big advantages about being a high-ranking fish, let’s say, is they often restrict access of food. The subordinates don’t get to eat as much. It’s so interesting. For birds, when you’re high ranking you know when there’s food you’ll be able to eat. They don’t overeat. They eat just enough. The reason it’s important as a bird not to overeat is, again, wild animals balance starvation and predation. Your flight initiation time, if it takes too long, you’re going to be the one the predator gets. A lighter body mass leads to a faster flight initiation time.

Whereas the subordinates who are often hungry, when they do find food, they overeat. I say overeat because it actually affects their survival. Even though the dominants have more access to food, the subordinates tend to weigh more. They’re actually at greater risk of being eaten by the predators which works in favor of the dominants. It’s this whole interesting piece. What happens in social settings is lots of different eating. This is a captive example. Captive great apes and some marine mammals, orcas, walruses, when they have social stress, when they feel isolated or even bullied, some of them engage in this, it’s self-induced vomiting. It’s a cycle of vomiting. Then they ingest the emesis, and they vomit again. It’s not very pretty to talk about, but it’s a response to isolation and social stress.

Zibby: It’s like bulimia for animals.

Barbara: I wouldn’t go that far. I would never go that far, but I think there’s some shared mechanism. My hunch is that there are bulimic patients, people who have bulimia, who talk about both the binge phase and the purge as it’s like a relief or release. The binge part kind of makes — when you eat food, it can calm you down. The purge part is a little harder for me to get on top of. It’s interesting. I wonder if there’s some shared mechanism, some self-soothing vagal mechanism that they share.

Zibby: Maybe your next book could be all about the animal implications of the way we eat and the way we — I know you have some of that in all your books, just to give you an idea for another one.

Barbara: I’ve been working on a paper with an eating disorder specialist. It’s the paper that will never get done. We keep on changing it and finding new and different things. I came across a study — it was done years ago — of goby. I think they were brown goby. Goby are these pretty common fish. Completely interesting, there are dominants and subordinates. The dominants that were studied tolerate the subordinates. They’re fine. They’re going about their life. They’re getting bigger. Subordinates are usually younger. Let’s call them adolescents. They tolerate them until they get within five to ten percent of their size. The dominants are like, “That’s fine. She can swim there.” Once they’re in five to ten, their percent, they’re perceived as a threat. The dominants go after the subordinates and try to cast them from the area, which is a hundred percent predation. What these adolescence gobies do is they have access to food, but they self-restrict to keep themselves from gaining weight and getting bigger to avoid conflict with the dominants. I was shocked that the investigators, they called it a diet. That’s not my anthropomorph-ed word. I thought that was so interesting because it’s really an example of using body size in a fish. Body size itself can create competition and tension between two individuals. An animal can decrease the amount of conflict in a group by controlling how much they’re eating.

Zibby: Interesting. Honestly, this stuff is so fascinating. We’ve only gotten a pinprick of the mountain of material in the book. I didn’t even know which to pick to discuss today. I hope that listeners get a sense of just how deep your research goes and how it makes us rethink everything that goes on in our day-to-day world. It’s a really good sanity check to remember that the world does not revolve around us as people. We can learn so much from all the other species. Your pointing that out is super helpful.

Barbara: I have to say I feel really qualified to be sitting here, not for any of my academic training or anything but because I really was not — my husband says I should not admit this to people. I really was not much of an animal person. We didn’t have pets. We had a cat, I think, as a kid. It’s so funny because I’m keynoting the American Veterinary Medical Association. In vet world, I’ve become this figure. Now I have dogs. I love them. I have a completely different appreciation. I was the prototypic academic physician with one point of view, probably arrogant. I don’t know. Maybe that’s for somebody else to say. I had no idea. I studied evolutionary biology as an undergraduate. Then I went to medical school. This whole experience for me, first of looking at cancer across species — I did a study of breast cancer in other animals. I’m an Ashkenazi Jew. I was being screened for BRCA1 mutation, which I am negative for it, but increases the risk of breast cancer.

It was around that period in our research that we learned about Venezuelan jaguars who have a pretty high rate of breast cancer and ovarian cancer. Some of them have BRCA1 mutation. The connections, first it was about the medicine because that’s what I was professionally. Increasingly, and Wildhood reflects this, this lens of looking across species in a very systematic, scientific way — I have a methodology. I create these models. You can take this lens and you can literally put it on anything. We put it on adolescence for five years. We think we’ve learned a lot and have a lot of insights. I’m really excited about putting it on lots of things where that human exceptionalism, that arrogance — you know what it is? Human exceptionalism, the worst part of it is it’s all these unexamined assumptions about our uniqueness. That’s the blindfold.

Zibby: It’s true. Thank you for taking our blindfold off with Wildhood and for coming on “Moms Don’t Have Time to Read Books.” Thank you.

Barbara: I’m so happy to be here.