June 19, 2006 — Baboons, like humans, can have stressful lives, and a new study has determined what most bothers female baboons in the wild.
According to the study, the number one cause of stress to female chacma baboons, Papio hamadryas ursinus, is male immigration, when a new male enters a group and may threaten to bully and kill other baboons, especially infants.
"That suggests to me that females are highly aware of the risks presented by violent young males entering the troop," said Anne Engh, lead author of the study, which was published recently in the journal Animal Behavior.
Engh, a post-doctoral research in the Department of Biology at the University of Pennsylvania, and her team studied approximately 70 male and female baboons over a 16-month period in Botswana’s Moremi Game Reserve.
They collected fecal samples from females for stress hormone analysis. Prior research found that glucocorticoids, a class of steroid hormones, rise when apes, humans and other animals feel stress.
Besides male immigration, reproductive state, predation, rank instability and infanticide also triggered female stress.
The stress hormone rise during reproductive stages is normal, and indicates that female apes, like women, could experience a potentially cranky PMS phase.
Female baboons usually exist in stable, hierarchical groups, but the researchers observed two instances where low status females tried to pull rank and stress levels rose.
In one case, an adult female, her adult daughter and a sister all attempted to usurp the power of another unit. After a week of fighting and high stress, the lower-ranked group gave up.
In the second period of observed instability, an adult female named Cat ran off with a high-ranked male for several days. When she returned, a "Cat-fight" ensued with the other females. The fights not only lowered the status of Cat, but also placed the unfortunate baboon’s daughter, sisters, aunt and their children to the bottom of the hierarchy.
"Our best guess is that poor Cat just had very bad timing," Engh told Discovery News. "When she left the troop with the alpha male, more than half of the troop’s females were cycling, so there was probably quite a bit of competition between the females to catch the alpha’s eye. I liken it to teenaged girls vying for the attention of the captain of the football team during prom season."
Anxiety levels also rose when males, attempting to assert their power, killed infants. For several days after an infant’s murder, its mother would scream and flee whenever the killing male approached.
Engh added that friendships with less aggressive males and grooming with females seemed to greatly reduce the females’ fear and stress.
Among baboons, it's not only females who experience anxiety.
Robert Sapolsky, a professor of biological sciences at Stanford University, determined that rank instability causes a lot of stress among male baboons. Low-level males also tended to show signs of depression and anxiety more often than alpha males.
Sapolsky called the new study on female baboons an "excellent contribution to the field."
Joan Silk, a University of California at Los Angeles anthropologist noted for her work on the evolution of social behavior in primates, told Discovery News that she has worked at the same reserve where the new study took place, and that this "account seems consistent with what I know of the females there."
Silk added, "The results provide independent confirmation of the supposition that sociality has important consequences for females."
Despite all of the apparent stress that baboons in the wild experience, Engh suggests it may be worse for confined baboons in poorly maintained zoos and other facilities. Chronic stress from unnatural confinement could lead to chronic stress that may result in health problems.
Then again, Engh says she believes baboons in their natural habitat actually cope with stress better than humans do.
"Humans, in contrast (to baboons), get stressed out about many things, often things that are of little significance in our lives or are completely abstract threats," she said. "We would probably be better off aping baboons and worrying only about the important things in our lives."
