If you’re a baboon, being in charge gives you a lot of advantages: you have better access to food, you get more action from the ladies, and your kids tend to grow faster and live longer. Low-rankers, meanwhile, must expend more time and energy to get food and mating opportunities. It makes sense to assume that the baboons at the bottom of their hierarchy might experience more stress than their high-ranking relatives.

But life at the top of a baboon troop isn’t all fun and games, since the alpha male must constantly struggle to maintain his social position. A new study in Science shows that alpha males suffer from much more stress than the second highest-ranking baboon, and tend to exhibit the same amount of stress hormones as baboons much lower in the hierarchy.

To study stress in a group of wild savannah baboons, the researchers collected 4,000 fecal samples from 125 adult male baboons over nine years in Amboseli, Kenya. Their goals were twofold: to determine how stress differed between high- and low-ranking baboons, and to assess whether this pattern changed in times of upheaval. The scientists tested each fecal sample for a group of hormones called glucocorticoids, which can indicate how much stress each baboon was dealing with at the time.

The researchers found that, with one notable exception, glucocorticoid levels decreased as rank increased; in other words, low rankers experienced much more stress than higher-rankers. The exception, however, was the alpha male’s stress levels, which were just as high as those of the low-ranking baboons. While the males that ranked second through eighth seemed to enjoy relatively low-stress lives, the alpha male experienced just as much stress as the baboons ranking ninth through fourteenth.

One of the most unexpected finding of this research is that the alpha males’ glucocorticoid levels were so different from the second-ranking males’. Two of the study’s other findings may account for the immense amount of stress that the highest-rankers experienced: they had a 17 percent higher rate of aggressive encounters with other baboons and they spent 29 percent more time mating than lower-rankers did. These physiological costs of maintaining the top spot are likely responsible for the sky-high stress levels exhibited by alpha males.

The stress patterns were consistent, no matter which individual held each rank; the study ran long enough for baboons to spend time at several different social ranks, and stress levels were a product of rank, rather than of consistent individual hormone profiles. In times of instability within the troop, this pattern didn’t change, but all the baboons’ stress levels went up a bit.

While short-term bursts of glucocorticoids are beneficial and can help individuals cope in stressful situations, lengthy exposure can be harmful. Over the long term, high glucocorticoid levels are known to suppress the immune system. In the study, alpha males exhibited much higher parasite loads than the baboons just beneath them in the hierarchy.

The researchers also noticed another interesting trend in their data: the alpha males tended to turn over more often than expected, and weren’t able to monopolize access to the troop’s females. Meanwhile, the second-ranking males managed to secure more matings than their rank suggested. While not directly tested, it’s possible that high stress levels negatively affected the health and performance of the alpha males, causing them to fall short of their reproductive potential.

Researchers often test for an effect of rank by lumping all high-ranking animals together and comparing those individuals to a group of lower-rankers. In light of this study, scientists may need to reevaluate this approach, and take into account the unique circumstances of various social ranks.

Source: Ars Technica

Image: Brandon Keim/Wired.com

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Science, 2011. DOI: 10.1126/science.1207120