August 24, 2016
Despite Jane Goodall’s pioneering chimpanzee behavioral research, female chimpanzee sociality has historically been “defined by motherhood or their sexual attraction to males.”
Biological anthropologist Melissa Emery Thompson, assistant professor of anthropology at the University of New Mexico, said the lack of knowledge about their social interactions may be because female chimpanzees are simply more challenging to understand than their rambunctious male counterparts.
Years of studying chimpanzees in the Kanyawara community of South Western Uganda has afforded Thompson a glimpse into the subtle social cues that govern female chimpanzee relationships. In her talk “The Secret Lives of Female Chimpanzees,” hosted by Chicago Council on Science and Technology (C2ST) last Thursday, Thompson argued that female chimpanzees are not antisocial but simply misunderstood.
With support from the Leakey Foundation, a nonprofit based in San Francisco that funds research on human origins, she has been working as a senior researcher with the Kibale Chimpanzee Project since 2000 to monitor the health, social behavior, and reproductive function of wild primates, and apply this research to humans.
Chimpanzees share around 98.8 percent of their DNA with humans, according to the American Museum of Natural History. Chimpanzees can give anthropologists evolutionary insights into human behavior because “they exhibit some behaviors that were once thought to be unique to humans,” said Thompson.
While studying the Kanyawara community, consisting of 60 chimpanzees, her first question was which factors determine successful motherhood for female chimpanzees?
She knew that female chimpanzees formed neighborhoods; areas of 6 to 10 kilometers where they foraged for fruit. One neighborhood, located in the southern part of the Kanyawara community, is dense with fruit trees, while the other neighborhood in the north is sparsely populated by fruit trees.
In the southern neighborhood, she observed higher birthrates, higher levels of estrogen and higher levels of progesterone in the female chimpanzees. Females living in the southern neighborhood were also more likely to survive than those living in the less-plentiful neighborhood.
“If food matters that much to your reproductive system, why are females not fighting over it all the time?” asked Thompson. The answer, she said, may be because the chimpanzees have to negotiate such a large area, around 25 to 30 kilometers.
Thompson equated the chimpanzee’s foraging decisions to people choosing between which farmers market to attend. If you hear of a great farmers market, but it’s all the way across town, “it might not always be worth it for you to make that big trip out there,” she said, “particularly if you’re not sure if they have what you need, or if the locals will have bought all the good stuff already.”
She said that female chimpanzees typically don’t fight over food, because they save energy and time by “shopping locally” even if the fruit around them is not as good of quality or less abundant.
Thompson is continuing her research in Uganda because she knows that there is still much to be learned about, and from, female chimpanzees.
“I think you mentioned that young chimpanzees will ride on their mother’s backs for years. So after they get kicked off of mommas back, do their levels of glucocorticoid stress hormones increase?” asked Aaron Freeman, science commentator and artist in residence at C2ST.
“Good question. I don’t know,” she said smiling. Thompson and her team assess chimpanzee hormone levels by collecting their pee with long nets. It’s difficult to get as good information about their stress levels when the young chimpanzees begin weaning, she said, because they are not drinking as much of their mother’s milk, and consequently don’t pee as much.
Once female chimpanzees reach reproductive maturity, they may emigrate from their home to find mates. To do so, they must try integrating themselves into a new group with an established hierarchy of female chimpanzees, Thompson said.
Their anal swellings, a sign that the female has reached sexual maturity, become a “passport” of sorts, making them more attractive to males and allowing them to move, more or less, freely between groups.
Other females, however, are not as thrilled by the arrival of new female travelers, seeing them as a potential threat or resource drain. Thompson observed that when no immigrants were present in a group, there were fewer instances of aggression among the females. Conversely, she noted elevated aggression among females when immigrants lived in that group.
Females living in “good” neighborhoods, or areas with plentiful resources, tended to attack immigrant females more frequently. In all her observations, Thompson found that females acted on extremes. Some avoided rivals, while others “aggressively excluded them.”
Like human females, female chimpanzees are making sophisticated decisions every day to protect their young, and to deal with the pressures of belonging to a species where males are often aggressive and older females are territorial.
“[Female chimpanzees] do their best to coexist and minimize their costs of living in their social world,” said Thompson.