July 25, 2011

The Benefits of Being a Beta Male

Photo of a male baboon Male baboon. Jeanne Altmann, Princeton University.

In male baboons, a higher social rank generally brings higher testosterone and lower stress hormone levels. But according to a new study, the highest-ranked (alpha) males have higher stress levels than the second-ranking (beta) males. The finding suggests that life at the very top can be more costly than previously thought.

Past studies have yielded conflicting results about the benefits of being an alpha male. A high social rank clearly has advantages in many animal societies. Alpha males, for example, have first choice of food and father the most offspring. But attaining and maintaining a high rank also brings conflict and stress, and stress can take both a mental and physical toll. Long-term exposure to high levels of stress hormones can suppress immune function and lead to cardiovascular problems like hypertension, coronary heart disease and stroke.

For 4 decades, a research team led by Dr. Jeanne Altmann at Princeton University, now along with Dr. Susan Alberts at Duke University, has been investigating the costs and benefits of social hierarchy among baboons in the Amboseli Basin in Kenya. The researchers have been collecting weather, life-history and behavioral data. They’ve also been collecting fecal samples since late 1999. The effort is unique in its ability to follow individuals through social hierarchy changes over time.

For their latest report, the researchers examined hormone levels in over 4,500 fecal samples from 125 males over a period of 9 years. They tested for metabolites of testosterone and the stress hormone glucocorticoid and compared these hormone levels with the animals’ social rank. Their work was funded by NIH’s National Institute on Aging (NIA) and National Institute of Mental Health (NIMH), along with the National Science Foundation.

The team reported in the July 15, 2011, issue of Science that high-ranking males generally had higher testosterone and lower glucocorticoid levels than other males. But there was one notable exception: Alpha males had much higher levels of glucocorticoid than beta males. This was true both when the social hierarchy was stable and when it was undergoing changes.

The researchers looked for differences that might account for the finding. Alpha and beta males were challenged by lower-ranking males at similar rates. They also received similar rates of grooming from adult females. But alpha males spent significantly more energy guarding fertile females. They also spent more energy displaying threatening, aggressive behavior toward other males to retain their alpha status.

In any given group, alpha and beta males do most of the mating and father most of the offspring. But this study yielded a surprising downside to being the alpha male. The insight will have implications for future studies of how social hierarchies influence health and wellness.

“Baboons are not only genetically closely related to humans, but like humans they live in highly complex societies,” says lead author Dr. Laurence Gesquiere of Princeton. “An important insight from our study is that the top position in some animal — and possibly human — societies has unique costs and benefits associated with it, ones that may persist both when social orders experience some major perturbations as well as when they are stable.”

— by Harrison Wein, Ph.D.

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