Stanford Report Online

Stanford Report, February 21, 2001
We can all relate to stressed-out baboons
Physiologist says humans can learn from how primates handle pressure


Every summer since the late '70s, Stanford physiologist Robert Sapolsky has traveled to Kenya to study the stressful lives of baboons, whose competitive, stratified society resembles our own. By linking baboons' behavior with their health, he has learned that some individuals handle stress better than others. And those coping strategies from the Serengeti may prove relevant here in the Silicon Valley.

"Some baboons have a Type A personality, and they pay for it in terms of disease," Sapolsky told an audience Feb. 18 at a symposium on stress and health at the annual meeting of the American Association for the Advancement of Science (AAAS). The baboons that handle stress best, in contrast, are those who have formed stable social connections.

Over the short term, stress hormones help baboons deal with harsh realities: competition for sexual partners and aggression from bullies, coalitions or beaten animals looking for a third party to pummel. But over the long term, aggression and even psychological stress itself can exact a toll in the form of elevated stress-hormone levels, a poor immune response, elevated resting blood pressure, an unhealthy ratio of "good" to "bad" cholesterol, hardening of the arteries and perhaps even premature death.

Because baboons are rarely threatened by famine, plague or predators, they are good models for socialized disease, Sapolsky says: "Baboon societies are ironically a lot like Westernized humans. We're ecologically privileged enough that we can invent social and psychological stress. Baboons in the Serengeti, who only work three hours a day to meet their caloric needs, are similarly privileged. They ulcerate because of social complexities."

Mental stresses show up in the body. But unlike zebras, whose bodies are well-adapted to dealing with short-term emergencies like running from hungry lions, primates also experience psychological stresses that can elicit physiological responses that, evoked over time, can make us sick.

Sapolsky observes baboons in the wild to determine their ranks, personalities and social affiliations. Then he anesthetizes them with a blowdart to collect blood samples that reveal levels of stress hormones, antibodies, cholesterol and other indicators of health status. So far, most data are from male baboons, as at any given time, 80 percent of the females are pregnant or lactating and cannot be anesthetized without risks.

Sapolsky has found a key to handling stress may be cultivating friendships. Males who spend the most time grooming and being groomed by females who are not in heat (that is, are not of immediate sexual interest) and playing with infants have the lowest levels of stress hormones.

And when keeping calm, perspective helps. Baboons who cannot tell if a situation is a real threat have twice the stress-hormone levels of those who can discern hazards from histrionics. And if a situation poses a real threat, baboons who sit there waiting for a fight have higher stress-hormone levels than those who take control of the situation and strike first. Baboons who cannot tell if they are winning or losing a fight have much higher stress-hormone levels than those who know whether their lots are improving or worsening.

Lastly, rank matters. Top baboons in a stable dominance hierarchy have lower stress-hormone levels than do subordinates. But throwing a new monkey in the mix sends stress hormones soaring as troop members jockey for dominance.

Have you hugged your rat today?

When Sapolsky is not in Africa watching baboons behaving badly, he studies stressed-out rats in his lab at Stanford, where he is a professor in the Department of Biological Sciences and in the Department of Neurology and Neurological Sciences. His books include Why Zebras Don't Get Ulcers, The Trouble with Testosterone and most recently A Primate's Memoir (Scribner, 2001).

Some physiological functions are more vulnerable to stress than others, he has found. Stress exerts its strongest effects on gastrointestinal function, sleep, sex drive and blood pressure. For example, blood pressure will rise only two seconds after a stressful event -- an especially dangerous reaction for Type A's, who see stressors everywhere. In fact, he says, Type A behavior is a greater risk factor for cardiovascular disease than smoking.

Stress has lesser effects on diabetes, immune function and brain aging. Says Sapolsky: "During stress, immunity is suppressed to the point where you're more at risk for colds, mononucleosis, herpes flare-ups. But you must absolutely flatten the immune system to hurt tumor defense, and the effects of stress on the immune system are never that severe."

The good news is that your mind is a powerful tool for de-stressing your body. The bad news is that the brain cells that turn off the body's stress response are the first neurons to succumb to stress.

"The brain needs a thermostat, or negative feedback device, to determine, 'Do we have enough hormone?'" Sapolsky explains. That thermostat is a region in the brain called the hippocampus, which tells the adrenal glands to stop making the chemical messengers needed to turn on the body's "fight or flight" response. Its predominant functions are learning and memory.

Stress-hormone levels tend to rise with age, and hippocampal cells express fewer hormone receptors on their surfaces to protect themselves from the excess. The result? The stress response becomes harder to turn off. Continued exposure to stress hormones can kill hippocampal cells, Sapolsky says.

Stress-hormone levels are elevated in half of people suffering from major depression. Magnetic resonance imaging reveals shriveled hippocampal regions, with longer bouts of very serious depression linked to greater shrinkage.

But stress need not lead to brain damage, according to Michael Meaney, a psychiatry professor at McGill University. Moderate stress, or stimulation, is good for the brain. A study he and Sapolsky conducted showed that early experiences in young rats had strong, lasting effects: Rats handled by humans as newborns had finely tuned stress responses that may have lowered their lifetime exposure to stress hormones compared to unhandled rats.

Gene therapy someday may protect the brain from stress too, according to Sapolsky. Even after brain trauma begins, viruses that have been rendered harmless may transport protective genes into brain cells, reducing the numbers of cells killed. Glucose transporter genes, for instance, may provide energy to remove the excess excitatory amino acids and calcium that flood into traumatized cells and damage them irreparably. The trick is to keep the window of opportunity open for preventing cell death, Sapolsky says.

Physiologist Robert Sapolsky with Rock, one of the Serengeti baboons Sapolsky travels to Kenya each year to study. By linking baboons' behavior with their health, Sapolsky has learned that individuals cope with stress in different ways. Photo credit: Lisa Share.