Senior author Anne Pusey, also from Duke, has spent the last 25 years archiving and digitizing Goodall's copious hand-written notes and check-sheets. Pusey was a doctoral student at Gombe from 1970-1975 and witnessed many of the key events firsthand.
"I remember an occasion in 1972 when I was following a group of mostly northern chimps towards the south," Pusey told Seeker. "There was a big group ahead, pant-hooting loudly, and Humphrey and another mature male touched hands and headed back north, while the others, including young northern males, went on and eventually joined the group that included most of the southern males, with much excitement and calling."
Later, at the beginning of 1975, when the two groups had completely separated and some of the southern males had already been killed, Pusey was with some northern chimps as they traveled south, rested silently on the top of the ridge at the edge of their territory and suddenly spotted Sniff — an adult male — and some of the other Kahama chimps in the trees in the valley below.
"Sniff and the others climbed down and disappeared," she said, "and the chimps I was with started stalking them, moving very stealthily and sniffing the vegetation as they went, but they didn’t find them and eventually turned back to their territory, and pant-hooted loudly after they had got back over the ridge."
Curious about the shifting alliances among these and the other male chimps, 19 in total, Pusey and her colleagues mapped the chimps' social networks at different periods between 1967 and 1972 to pinpoint when relations began to fray.
The analysis went back into the 60s in part because, beginning in 1963, initial observations of the original study chimpanzee community were made at a "feeding station," where other researchers used bananas to habituate the chimps and to facilitate observations.
By 1966, these researchers recognized that, of the 19 mature and adolescent males, some spent more time to the north of the feeding station and others spent more time to the south. These are the once loosely defined subgroups that later became known as Kahama to the south and Kasekela to the north.
In more recent years, some scientists have suggested that the friction between the groups was sparked by the feeding station. As Ian Morris writes in his book War! What Is It Good For? (Farrar, Straus and Giroux, 2014): "Goodall, they insisted, had caused the war. In her efforts to get the chimpanzees comfortable around humans, she had fed them bananas, and competition over this rich food, the critics suggested, had corrupted the chimps' naturally peaceful society and turned it violent."
Researchers ended provisioning in Gombe in 2000.
RELATED: Chimps and Six-Year-Olds Want to Mete Out Punishment — Even If It Comes at a Cost
While Feldblum, Pusey, and co-authors Sofia Manfedi and Ian Gilby cannot rule out the possibility that provisioning had some effect on the chimps' community cohesion, the new network analysis paints a different picture of the causes than do Goodall's critics.
For the new analysis, Feldblum and his team charted all of the 19 males and considered that, if two were spotted arriving together at a feeding station together more often than other pairs, they had a social bond.
As for what leads to such bonds, relatedness and social status appear to be factors.
"Maternal brothers seem to cooperate and associate with each other more than unrelated individuals, and males in our population tend to be more gregarious in general than females," Feldblum said. "There's some evidence also that males of similar ranks might be more likely to form social bonds."
Next, the researchers identified the most tightly knit groups in each network and determined how much their members changed over time. The authors show that, from 1967-1970, males in the original group intermingled.
Following Leakey's death, the clusters of males grew more distinct, with some spending more time in the north, and others in the south. By 1971, the northern and southern males met less and less often. When they did encounter each other, as Pusey later witnessed, they would hurl branches, hoot and charge through the forest as a show of strength.