But in February 2004 lions killed Sierra and a male she was consorting with.
Predation by lions and leopards is the most common cause of death among Botswana's baboons.
Lions attack in groups of three or more, scattering the baboons into smaller groups that remain apart for several days before they reunite.
The researchers found that the baboons' stress-hormone levels were higher during these temporary separations.
They also found that predation is especially stressful for females whose close relatives are killed.
Female baboons do most of their grooming with close kin, so researchers speculated that Sierra's death would lead to a decrease in the amount of grooming Sylvia engaged in.
But the opposite turned out to be the case.
"For a week or two [after Sierra died] Sylvia sat alone staring at her feet and really didn't interact with the others very much," Engh said.
The researchers didn't waste any sympathy on the seemingly grieving baboon.
"We don't feel that sorry for Sylvia, because she has been known as the Queen of Mean her whole life," said. "She's very high ranking and very nasty to the lower-ranking females.
"But what was very surprising was that after about two weeks she started being really nice to lower-ranking femalesquite a change in behavior.
"She started initiating grooming with them, and settled into a friendly relationship with a low-ranking female she'd never given the time of day to before."
The scientists also found that Sylvia's glucocorticoid levels, which had tripled after Sierra's death, were down to normal by the second month after the incident.
Frans de Waal is a professor of primate behavior at Emory University in Atlanta, Georgia, and was not involved in the study. He says this hormone research in baboons is "wonderful work."
"This is an important study, the sort of thing that needs to be done more often," de Waal said.
"We have a lot of anecdotal evidence of grieving in primates, especially with mothers who lose offspring. They carry around the infant until the body falls apart, or go into depression, stop eating, sit in a corner the whole day.
"But this is the first study I've seen of traumatic social experiences where the researchers measured the impact hormonally."
De Waal also pointed out one aspect of the study that the researchers themselves did not emphasize.
"There's an absence of response by the other members of the group. In monkeys you don't find evidence of sympathy. You do in apes and, of course, in humansalso in elephants. They respond to the distress of others."
But with the troop of baboons, de Waal added, there is "an absence of an empathic environment."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES