Many studies in humans and animals suggest that chronic stress is bad for one's health, in part because it suppresses the immune system. But nearly 30 years of data on wild baboons shows that top-ranking males, despite showing signs of increased stress, recover more quickly than low-ranking baboons from wounds and illness. The results may help explain why some people escape from the negative effects of stress while others do not.
Most studies in humans have shown a clear correlation between higher socioeconomic status and lower risk of death or illness from stress-related diseases such as heart attacks and diabetes. Some of the most famous of these are the so-called Whitehall studies of the British Civil Service, which showed that death and illness rates decreased in a step-wise fashion the higher an employee was on the service’s 6-grade pay and responsibility scale. These and other studies also have found that being at the bottom of the totem pole leads to greater stress as a result of increased work loads and time pressures, as well as more job insecurity.
But studies of animals, especially other primates, have shown that the relationship between stress and status largely depends on the social organization of the species in question. For example, in species such as baboons that have rigid social rankings and hierarchies, with so-called alpha males dominating other males and females over extended periods of time, it can apparently be more stressful at the top. In a study reported last year in Science, a team that included ecologist Jeanne Altmann of Princeton University revealed that baboon alpha males had the highest levels of glucocorticoid hormones, such as cortisol, as well as testosterone in their feces, indicators that they were under greater stress than lower-ranking individuals.
To try to tease out the relationship between social rank, stress, and health, Altmann teamed up with Elizabeth Archie, a behavioral ecologist at the University of Notre Dame in South Bend, Indiana, and Susan Alberts, a behavioral ecologist at Duke University in Durham, North Carolina, to analyze data collected from 1982 through 2009 in the Amboseli region of Kenya, home to a large population of wild baboons.
Working with three Kenyan field assistants who observed the baboons 6 days a week, 52 weeks each year, the team noted 633 cases of either illness or injury in 166 adult male baboons over the 27-year course of the study. Illnesses included digestive and respiratory problems, while injuries included cuts, slashes, and puncture wounds. The team found that the highest incidences of illness occurred in the oldest and lowest-ranking males, rather than the highest-ranking alpha males, as might be expected if the alpha males' higher stress levels were suppressing their immune systems. And the highest rates of injury were found in middle-aged, mid-ranking males, the authors report online today in the Proceedings of the National Academy of Sciences.
To further investigate the effect of stress levels on the baboons' immune systems, the team analyzed healing rates in 144 of the baboons. The researchers found that low-ranking males typically required about 31 days to recover, whereas high-ranking males required about only 25 days.
Since age and rank are correlated—older baboons are less likely to be high-ranking—the team corrected for the effect of age, but found that rank was still the most important factor in predicting wound-healing time. The researchers also identified a correlation between speed of wound-healing and the size of the social group the baboon belonged to: Males from larger groups recovered more quickly than those in smaller groups.
These results are "somewhat surprising," the team writes in its report, because a number of studies with laboratory animals and captive primates have shown a clear relationship between stress levels—as measured especially by fecal glucocorticoid concentrations—and immune suppression.
The researchers suggest that primates such as baboons and humans have benefited from an "evolutionary flexibility" in how they respond to stress and that immune suppression is not always the result. Thus the higher levels of stress hormones in alpha males, the team contends, are probably due to the stress of expending the energy necessary to stay competitive and on top of the hierarchy, whereas low-ranking males are stressed out by social factors such as being the targets of aggression by alpha males.
Nevertheless, the researchers argue, alpha males and lower-ranking males seem to benefit from the social support they receive by being in larger groups.
"Our results suggest that even though alpha males experience high stress, they seem to escape from the negative consequences," Archie says. "In humans, we probably see similar patterns. Although people of high socioeconomic status experience stressful events, they probably also have better access to resources and coping mechanisms." In contrast, Archie says, "low-ranking baboons and people of low socioeconomic status experience long-term stressors with little chance to escape." Nevertheless, Archie and her co-workers say, more research will be necessary to determine the precise relationship between health and rank—that is, whether healthy animals achieve higher ranks or higher-ranking animals achieve better health.
Michaela Hau, an evolutionary physiologist at the Max Planck Institute for Ornithology in Germany, says that the new study is "immensely valuable" because it was carried out with a large number of baboons who lived in the wild rather than a captive population, which might be suffering from different kinds of stresses due to captivity, social isolation, or variable food quality. Hau adds that while the alpha males show higher levels of stress-related hormones in their feces, this might be due to numerous short spikes of acute stress episodes rather than one long, continuous state of chronic stress such as humans low on the totem pole might face. Such acute spikes in stress hormones, Hau says, have been associated in previous research with higher healing rates, especially in the skin.
Bruce McEwen, a neuroendocrinologist at Rockefeller University in New York City, agrees. "An acute stress hormone response is likely to enhance adaptive immunity and wound healing, whereas the sluggish and prolonged elevation" of stress hormones "is likely to impair immune function."