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Gold from Gombe

U researchers mine new information on chimp behavior from 38 years' worth of data

By Deane Morrison

In 1972 Anne Pusey arrived at Tanzania's Gombe Stream Reserve to study its wild chimpanzees, with a focus on young chimps, under the direction of Jane Goodall. Newly graduated from Oxford University, Pusey quickly took to the rough terrain of Gombe, where three geographically separate groups of chimps lived. But as she followed the chimps, a particular behavior struck her more than any other. And it wasn't what the youngsters were doing.

"What grabbed me was that the females were moving around," says Pusey, now a professor of ecology, evolution, and behavior at the University. "I saw females getting up and leaving. I saw them coming into new groups and being chased by other females."

Why a female would leave her birth group for another, especially when she faced such a hostile reception, intrigued Pusey. As she later showed, many female chimps migrate to new groups, while males stay put – an unusual pattern among mammals. But what Pusey found even more unusual was the fact that some females broke the pattern and didn't leave. Why some and not others?

For more than 25 years, Pusey has been tackling difficult questions like that one. Studying chimps isn't only fascinating, she says, but of crucial importance because they are humanity's closest living relative.

"By understanding their behavior and comparing it to that of other apes and of humans, we can reconstruct what the common ancestor was probably like," she says. "We can then identify the changes that occurred during human evolution. We can also study the context and reasons for such phenomena as cooperation between males in territoriality, tool use, hunting, and meat sharing, which are already present in chimps but became elaborated during human evolution."

Among the Gombe chimps, Pusey observed a complex social structure.

"In chimps, males form stable groups based on kinship," she says. "But females are less social." Besides noting the tendency of young females to leave their home groups, she documented how, at puberty, males would leave their mothers to join the adult males in their group. The males would compete for females, but they regularly put their differences aside to patrol the boundaries of their range and chase off interlopers. But after many years these interactions took a new turn.

"I was there when two groups of male chimps made war," Pusey says. "These were chimps that had been friendly in the past, but that had gradually split into two groups. Each group would patrol the edge of its range, and if they encountered a single male from the other group, the chimps would cooperate to hold him down and bite him until he was mortally wounded."

Many male chimps also regularly beat up females in what appear to be unprovoked attacks. One hypothesis, says Pusey, is that the males try to intimidate females so they'll be receptive to their attackers when they next come into estrus. Unfortunately, because females are not very social, they take their lumps alone, with no protective male or band of fellow females to fight off the assailant.

"It's really quite awful," says Pusey. "I wouldn't want to be a female chimp."

SEVERAL YEARS AGO, PUSEY PERSUADED Jane Goodall to ship her records of the Gombe chimps – all 38 years' worth -- to the University. The huge volume of records now resides in the Ecology Building, a decidedly less humid and more secure spot than Goodall's house in Dar es Salaam, Tanzania, where many of the records had been kept. The records are housed in the Jane Goodall Institute's Center for Primate Studies, jointly founded by Pusey and Goodall. Twenty years of data have already been transferred to a computer database; transfer of the remainder will keep Pusey and her students occupied for several more years.

The Gombe data have already borne fruit. In August 1997 Pusey's graduate student Jennifer Williams, along with Pusey and Goodall, discovered from the data that female chimps have dominance hierarchies and that the higher rankers enjoy greater reproductive success.

"We're the first to show an effect of rank on anything in the lives of female chimps," says Pusey. Rank in females had been hard to detect, mainly because females aren't very social. But by analyzing all the recorded pant-grunts -- a chimp sound signaling submission -- between females from 1970 to 1992, the three researchers were able to assign each chimp to high, middle, or low rank. By every measure, the top chimps did better at producing and rearing offspring.

The work raises plenty more questions, among them, Why do females have rank hierarchies in the first place? Unlike other social primates, female chimps feed alone most of the time -- but perhaps they compete for the best ranges, says Pusey. Or, high-ranking females may get first access to the best food when the females do meet. Also, more work is needed to find out how females achieve their rank, especially when they leave their mothers' group for a new one, as half the Gombe females do.

In other parts of Africa, about 90 percent of females migrate, Pusey says, so she wants to understand why so many Gombe females stay in their home groups. One factor may be that because Gombe is small and isolated, there are few new groups females can reach. Therefore, they may fail to find males that suit them. Yet, because males stay put, females must migrate or risk inbreeding.

A complicating factor is the long life span of chimps, which may stretch for 50 years. That makes long-term studies the only way to gather enough data to see clear patterns. At this point, says Pusey, there are data on 20 females that have grown up in the Gombe study community, so researchers can begin to look for patterns.

"We can show that females that leave take longer to produce their first surviving baby," she says. "We also know that if females stay and their mothers are still alive, they share their range, so there might be some advantage to cooperating with their mothers to keep a good core area. Several females that stayed had very high-ranking mothers, but other females have left despite having high-ranking mothers that were still alive. Probably several factors are involved, and we need data on more females."

Female competition, says Pusey, may explain an especially fascinating behavior: the habit of some females of snatching and eating other chimps' infants. The behavior cropped up recently, when three females tried to get twins born to a chimp named Gremlin. They failed, but in the 1970s a mother-daughter team, Passion and Pom, succeeded several times.

"Jane Goodall looked in the records and saw there had been unexplained infant disappearances even before that," says Pusey. She says one hypothesis to explain infanticide is that the killers may improve their access to food by removing a rival's infant.

Protecting access to food may also play a part in the tendency of males patrolling the borders of their territory to attack not only intruding males, but intruding females -- provided the females aren't in breeding condition.

"At first people were puzzled, because these females could be thought of as potential mates," says Pusey. "Now we think the males are defending a long-term feeding territory for their resident females, thus improving their reproductive success. When the community's range is larger, resident females reproduce faster."

PUSEY AND HER GRADUATE STUDENTS are using the Gombe records, along with fresh field data and DNA analysis, to tackle some of the most intriguing questions about chimp behavior. Examining DNA from intestinal cells present in feces, Julie Constable is beginning to sort out paternity of chimps, a task that has always been difficult because of the promiscuity of females. If paternity lineages can be traced, researchers can begin to answer such questions as whether the highest ranking males actually father the most children. Amanda Vinson is doing a similar study with baboon DNA.

Another graduate student, Ian Gilby, asks why male chimps share meat with other chimps when they kill a monkey or other small animal. Such sharing may be a way of trading favors or buying affection from females -- or maybe just a way to get competitors to leave them alone so they can eat in peace.

Tool use fascinates Elizabeth Vinson, who is studying how infants learn to fish for termites with sticks. Termite fishing is unique to chimps, and its discovery at Gombe helped topple the idea that only humans use tools. Further, patterns of tool use vary between wild chimps in different areas. This, says Pusey, suggests protocultural behavior.

For Pusey, much of the joy of science comes from watching the next generation of students make their own discoveries and become excited about their work.

Above all, Pusey's work illustrates a primary characteristic of first-rate science: It raises more questions than it answers. But while she continues to search for answers to the big conundrums, she remains firmly grounded in -- and enthralled by -- daily life among chimps.

"It's one continuous soap opera," she says. "Chimps are intrinsically interesting."

Although Pusey now concentrates on chimps, she also spent 15 years studying the lions of the Serengeti in collaboration with Craig Packer, now a fellow professor of ecology, evolution, and behavior at the University.

It's a Jungle Out There: Researchers in the Wild

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