RESOURCES:
"Why Zebras Don't Get Ulcers" by Robert M. Sapolsky
TRANSCRIPTION:
Caroline MacGregor: You're listening to All Things Considered here on 89 one WEMU. I'm Caroline MacGregor, and I'm speaking today with U of M Professor of Psychology and Anthropology, Jacinta Beehner, one of the lead authors of a study about stress. And while it can be life-saving for wild animals, this is not often the case for human beings. Thank you for joining me today to discuss this fascinating topic!
Prof. Jacinta Beehner: Sure! It's really a pleasure to be here!
Caroline MacGregor: Professor, we're all familiar with stress. First of all, tell me about the study and your team of researchers at U of M and what you discovered about stress in primates versus human beings.
Prof. Jacinta Beehner: Well, sure! So, the team of researchers actually derived from both UCLA, as well as the University of Michigan. And we've known for a long time that the stress response is adaptive. This is something that is in every textbook. Everyone who writes about stress always says this is an adaptive response. But they always then shift into saying it's an adaptive response gone wrong. In other words, we are killing ourselves with chronic stress. And that is entirely true for humans. It's entirely true with respect to possibly captive animals in cages. It's possibly entirely true for anthropogenic populations where we are encroaching on their environments. So certainly, chronic stress, it can lead to atherosclerosis. It can lead to diabetes. It can lead to mental illness--many of the things that we associate with the stress response as being a bad thing. Now, that said, while this is true for humans, we think that the biomedical bias of all of these bad things has now seeped into the literature in behavioral ecology. So, in other words, we are now looking for all the bad things that happen in wild animals due to the stress response. And while that is true, having a stress response can cause an animal to have weaker immune systems. It might cause them to live less long. The alternative to not having a stress response is death. You get eaten by the lion. And there's so little research that really looks at that. So, in other words, we always look at the costs of the stress response, partly because they're easier to measure. But we never look at the advantages. Why did it evolve in the first place? And this was something that has bugged me always in studying behavioral ecology and learning about the stress response. And it was sparked by Sapolsky's book "Why Zebras Don't Get Ulcers." And so, this is a book about the idea that, yes, humans have chronic stress. Animals don't. But the evidence isn't out there of how the stress response is adaptive. So, that was the motivation. We wanted to go out and collect data on the stress response and possibly look at the consequences of one stress response versus another stress response.
Caroline MacGregor: I see. And so, your research basically suggests that a stronger stress response promotes survival in wild animals or primates. This is why it's so fascinating when it comes to human beings. Why has it become the complete opposite? Like you said, it's almost killing us with different diseases.
Prof. Jacinta Beehner: That's true. And this is because, in recent years, probably since the Industrial Revolution and Agricultural Revolution, so going back maybe a thousand years, our stressors are no longer what they once were. So, while in the past, our stressors were running from predators, surviving famines. Now, our stressors are mortgages, public speaking, someone at work who's causing you grief. None of these are life-threatening. And none of them really require a physiological response that releases energy, causes our heart rate to go up, causes our digestion to go down. There are all of these physiological changes that go on in our body when we are under a stress response that help us run from a lion, but they don't help us pay the mortgage. And so, while we're getting all the costs of the stress response, we're not getting any of the benefits as humans.
Caroline MacGregor: Is this physiological response a difference in the actual hormones being released to the various types of stress--the day-to-day stress that human beings endure versus the survival stress hormones we would have experienced a long time ago and that these animals are experiencing?
Prof. Jacinta Beehner: That's a really good question. The stress response itself is enormously conserved. And what that means is, is going back in the history of vertebrates, the stress response of a lizard is, for the most part, the same as a stress response in humans. So, we all release a hormone, either cortisol or corticosterone, which are known as glucocorticoids. For the most part, you can think of them as stress hormones. They get our heart rate going. They release glucose from our liver. They cause us to maybe focus a little bit more in our cognition. They shut down all nonessential processes. So, the same stress response that causes our heart rate to go up also causes our immune system to maybe be depressed. It makes us not digest, our body not ovulate, maybe. So, all of these nonessential processes that we can do tomorrow, if we're still alive, get shut down.
Caroline MacGregor: If you're just tuning in, you're listening to 89 one WEMU. I'm Caroline MacGregor. And today, I'm talking with U of M Professor of Psychology and Anthropology, Jacinta Beehner, about her findings that suggest while a stronger stress response promotes survival in animals, this is not the case in human beings. Professor, why is this wear-and-tear that produces health problems in humans so detrimental and a uniquely human outcome?
Prof. Jacinta Beehner: That's another good question. The difference is, is that the wild animals are getting the benefit of the stress response. So, in other words, they activate a stress response which helps them escape the lion. They are now alive the next day. That's a big advantage in terms of evolutionary fitness compared to the human who activates the stress response. They were never in danger of dying, and they only experience the wear-and-tear the next day without the benefit of survival. And so, when we talk about wear-and-tear, this is something that is always present with a stress response. There is no free lunch. You don't get to activate a stress response without taking that energy from some other place. And so, if you think of it as energy as your currency, you take it from one to put it in another. There is a cost, but, again, there also is a benefit if it's causing you to survive. The problem with the benefit is how do you measure a slight reduction in mortality across an event for a given population. You really need either experiments where you basically push animals maybe to mortality, or at least you push them towards something that is pretty stressful and then you measure the outcome. But in a wild primate, this is unethical for very good reasons. And instead, we have to wait for a natural disaster to come, which, unfortunately, these are happening more and more. And so, we took advantage of an El Nino event. It was actually the strongest El Nino on record from May of 2014, and, really, it went until about May of 2016. So, we had two years of data where a wild population of white-faced capuchins that has been studied for decades by Susan Perry and her colleagues at UCLA. These animals were dying at an unprecedented rate. And Dr. Perry called me and she said, "This is terrible!" These monkeys are dying! And I'm not sure what to do!" And she and our colleagues had been collaborating on other things. And I said, "I realize this is terrible, but could we maybe go in and look at the stress response that these animals are exhibiting to see if we can test which parts are adaptive and which parts aren't?" I just looked at a famine, really kind of a situation where these animals were exposed to El Nino. They had less food than usual. What did they do? But maybe it's a very different kind of stress response. It's adaptive if it was a flood or if it were a predator situation. And so, trying to understand the human side of it is a really different endeavor than what it looks like in animals.
Caroline MacGregor: It does take an enormous toll on the human species.
Prof. Jacinta Beehner: It does. And right now, we have evidence that being exposed to the same thing can be more stressful, like sensitization, where getting exposed to the same stressor actually makes you more stressed about it. Then there's also similar evidence from humans that the more you experience a stressor, the less stressful it is. It's a high variation in how humans respond to stressors. We have resilient people who just, for whatever reason, are much more resilient and don't exhibit a stress response to the same degree as others who might get really stressed out by psychological stressors. So, I would like to know what the stress response in these hormones should look like if you're going to survive a certain stressor. And I would like to know what does it look like? How long do you keep these hormones elevated? When do you go back down to baseline?
Caroline MacGregor: It's so interesting! And the last few points you made just bring to mind some human beings. Did they evolve differently from other human beings generations ago? Did they go through events that today led to their descendants handling stress better than in some other human beings?
Prof. Jacinta Beehner: That's a great point! It's really important for all of us to remember that anxiety kept us alive. You know, in a world where there is danger at every turn, which is what our ancestors evolved under, anxiety was always adaptive. Rarely did anxiety cause you to ruminate and worry about things that weren't actually going to kill you. And so, there has been heavy selection for anxiety across the years in many species, especially prey species. So, keeping that in mind might help you feel a little better about your own anxiety.
Caroline MacGregor: I've been talking today with U of M Professor of Psychology and Anthropology, Jacinta Beehner. We've been talking about stress and how it differs in wild animals and in human beings where it can produce more deadly results. Thank you so much for joining me today!
Prof. Jacinta Beehner: Sure! It was a pleasure to be here!
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