Low-ranking “new girl” chimpanzees seek out other gal pals with similar status, finds a new study of social relationships in the wild apes.
Unlike most primates, female chimps are loners compared to males. “They spend about half their time alone or with dependent kids,” said Duke University research scientist Steffen Foerster, who co-authored the study.
“Chimpanzee females’ more solitary existence isn’t that surprising given their dispersal patterns,” said Duke evolutionary anthropology professor Anne Pusey. Male chimpanzees stay with the group they were born into their entire lives, forging strong social bonds with other males. Many females, on the other hand, leave their families behind and strike out on their own to join new groups.
To find out if some females forge stronger bonds than can be explained by chance encounters or range overlap alone, Foerster, Pusey and colleagues analyzed 38 years’ worth of daily records for 53 adult females in Gombe National Park, a 13.5-square-mile park in western Tanzania where Pusey began working with Jane Goodall more than 40 years ago.
Over the decades, the females were spotted in more than 600 female-female pairs. For each pair, the researchers measured how much their ranges overlapped, how much time they spent together and how often they groomed each other, a chimp’s way of making friends and cementing social bonds.
Not surprisingly, mothers, daughters and sisters formed the strongest bonds. But among unrelated females — which made up more than 95 percent of the twosomes they studied — low-ranking females were more likely to seek each other out than females from other social ranks.
“It doesn’t necessarily mean that they like each other,” Foerster said. “The lowest-ranking females are the newest to arrive. When a female migrates into a new group, she starts at the bottom of the social ladder. It may be that they’re not really that into each other, but that they need to tolerate being in the same space,” he said.
Or it could be that low-ranking females team up for support against harassment from higher-ranking females. “The resident females don’t make it easy for the new females — they chase them around,” Pusey said.
Females with juvenile sons were also more likely to seek each other out, the researchers found, presumably to give the young males a start on developing the social skills they’ll need to thrive as adults.
The team’s next step is to examine the stability of female-female bonds over time. “If they endure despite other changes, ‘friendship’ is a more likely interpretation,” Foerster said.