Baboons Excel at Assessing Rank and Family, Study Says

Bijal P. Trivedi
National Geographic Channel
March 5, 2004
Humans have an inherent ability to size up other members of their
species. In a matter of minutes they assess personal attributes, like
social status, and categorize one another into particular
groups—family, race, or caste. Baboons, as it happens, can do much
the same thing.

A recent report from researchers at the University of Pennsylvania suggests that baboons classify individuals based on rank and kinship and use this information to evaluate social interactions. The cognitive skills that allow both humans and baboons to think about social relationships could be useful for survival.

Baboons live in large social groups, and rank determines many aspects of their behavior.

A higher rank, for instance, ensures better access to resources like food, which in turn improves reproductive success. Low-ranking female baboons are known to suffer more harassment than high-ranking females; the constant irritation and bullying has been shown to cause low-ranking females stress that can eventually disrupt menstrual endocrinology, leading to infertility.

The new research suggests that being able to assess the rank of other members in the groups is important.

"You need to make judgments about when to get out of someone's way," said Robin Dunbar, an evolutionary psychologist at the University of Liverpool in the U.K. "This is particularly important for primates, because they form these little alliances, or coalitions, which can help defend them against high-ranking bullies."

Fight Recordings

Previous research had shown that baboons could distinguish members of various families. It had also shown that they knew the rank of individuals within each family.

But scientists were unsure whether baboons could simultaneously put the two facts together.

Thore Bergman, an evolutionary biologist from the University of Pennsylvania in Philadelphia, and his colleagues explored this question using a group of well-studied baboons in Botswana's Okavango Delta—the pedigree and social rank of every one of the 90 animals is known.

The researchers conducted a so-called playback experiment, in which they subjected 19 female baboons to a sequence of calls mimicking a squabble between two unrelated female members of the community. They discovered that each of the baboon's reactions depended on the rank and kinship of the baboons on the tape.

Baboon society is tremendously hierarchical. The communities are divided into several maternal lineages, or matrilines, all of which are ranked. In a community of three matrilines—A, B, and C, for example—all the females within A family outrank females in B family and C family. All females within C family are subordinate to those in A and B.

In addition, every female within each matriline has a rank.

The hierarchical structure is very stable within a community—rarely 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 neocortex—the 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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