In addition, every female within each matriline has a rank.
The hierarchical structure is very stable within a communityrarely do reversals occur in which one family switches ranks with another. Similarly, rank reversals within a family are also uncommon.
Threat-Grunts and Screams
Bergman followed the baboons for six months, tape-recording calls. Calls consist of a series of "threat-grunts" and frightened "screams." Threat-grunts are usually issued when a dominant female threatens a subordinate. The subordinate, in turn, issues a scream in a submissive response.
Bergman collected calls from many females within the group and manipulated them to generate call sequences between individuals of specific rank and matriline.
The team would then play a call sequence to an adult female baboon when she was alone, via hidden speaker, and film her response.
"Baboons are fairly nonchalant. It is pretty difficult to get them to respond to anything," Bergman said. "A glance is actually a big response." The scientists evaluated responses to the calls by timing how long the baboons stared in the direction of the hidden speakers.
The technique relies on a well-established fact that animals pay more attention to stimuli that is unusual or surprising. If calls were unexpected, the baboons stared for much longer than if the call was familiar or unimportant.
Each adult was exposed to three different calls. The first consisted of a threat-grunt by a female from a low ranking matriline and a scream by a female from a high-ranking family.
Foundation for Language
This rank reversal between families aroused the most attention from the baboons, which isn't surprising, since such an altercation could impact all the individuals in both families.
A similar role reversal between low- and high-rank females within the same family was interesting but not as captivating. Internal family squabbles would only affect a couple of individuals.
Calls that involved no reversal of rank registered as status quo and barely triggered a glance.
Bergman's experiments could have broad implications, Dunbar said.
The hierarchical system that baboons use to judge kinship and rank may be similar to the way in which humans decode language. "Humans deduce the meaning of sentences by arranging words into nested, hierarchical groups," Bergman and his colleagues wrote in their report, which appeared in the November 14 issue of Science.
Baboons live in some of the largest social groups of any primate, and they are known to have a large neocortexthe region of the brain associated with sociability. The skills that enable baboons to process and interpret social information may have laid the cognitive precursors for language, Dunbar added.
Another recent study, published in the same issue of Science, has already demonstrated that highly social female baboons with rich social networks are more successful raising young.
"Whether the cognitive skills that enhance a baboon's ability to process all this social data also increase its chance of survival is the next experiment," said Alan Kamil, a biologist at the University of Nebraska-Lincoln. Kamil studies how cognitive abilities evolved and how birds use these skills in nature.
More immediately, Kamil conceded, "Bergman's study provides a significant advance in showing us what animals know about themselves and their environment."
For more on baboons, tune in to this week's Be the Creature. The TV series airs Sundays at 8 p.m. ET/PT in the United States and is available only on the National Geographic Channel.
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University of Pennsylvania Department of Psychology