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Rebecca Stumpf: What Makes Us Human?

Understanding Similarities, Differences Between Human and Non-human Primates

Rebecca Stumpf became a biological anthropologist because she was curious about the relationship between humans and other primates.

"Biological anthropology is a very diverse, and at the same time unified, field in the sense that it is trying to get at that important question of what makes us human and how are we different from and similar to the other primates," she says.

Stumpf has used a wide range of approaches, including morphology, behavioral studies of apes, and microbial and hormonal studies to pursue this question. Her main research focus has been sexual selection. If females are inclined to be choosy about mates and males are less so, as Darwin and others have postulated, this leads to conflict. How are those conflicting strategies resolved? And how and when do these strategies emerge?

"Across mammalian species we have this little collision," says Stumpf, with a grin. "My interests are in understanding how males and females solve these issues or attempt to solve these issues in a behavioral, morphological and microbial sense."

One of Stumpf's projects is based at a chimpanzee site in Uganda's Kibale National Park where she has been tracking 10 juvenile male and 10 juvenile female chimpanzees as they transition into adulthood.

"It’s such an important phase, not only for each individual, but also evolutionarily," notes Stumpf.

Unlike many primate species, it is the juvenile female chimpanzees who leave their home territory, typically around the age of 11 or 12. Stumpf would like to understand the trigger. Is it hormonal? Is it stress from harassment, whether from adult females or from related males trying to mate with them? Is it a function of the habitat?

Female chimpanzees are of interest to Stumpf for another behavior, as well. They tend to be very promiscuous (copulating hundreds of times with different partners) during the 10-12 days during which they have a highly visible estrous swelling.

That seemed to Stumpf to be an expense of time and of energy that was difficult to understand from an evolutionary perspective, so she explored this behavior. In recently published papers, Stumpf suggests that female chimpanzees are using a mixed strategy of trying to select the best males during the 2-3 day ovulatory window, while mating with others outside that window (but while still exhibiting estrous swelling) to minimize or prevent infanticide. Other primates, such as orangutans, also appear to use a mixed mating strategy, albeit through different means, in accordance with differences in their mating systems and ecology.

Given her interest in sexual selection, Stumpf’s work with IGB Host-Microbe theme leader and microbiologist Brenda Wilson, microbiologist Abigail Salyers and anthropologist Steve Leigh is an ideal collaboration. The researchers will use molecular genomic techniques to grasp the full complexity of the vaginal microbiome and see which microbes are harmful, which are beneficial and which are simply benign (commensal).

There is little data for humans and virtually none for primates like apes, monkeys and lemurs. Stumpf and her colleagues are curious to find out whether the patterns of vaginal microbe communities reflect evolutionary relationships among primates or something more immediate, like environmental factors.

"Studying these microbial populations is such a good cross-disciplinary partnership," says Stumpf. "Anthropologists benefit because it opens the door to this transformational way of understanding primate evolution and microbiologists benefit from gaining a whole new perspective on human microbes by comparing them to other primates."

Stumpf expects that the diversity of microbial communities will correspond to differences in mating systems, so that the more promiscuous animals may have a greater diversity of microbes.

She also predicts that females may have co-evolved with microbes so some sperm are more viable at different times in an ovulatory cycle. Perhaps when it is not a good time for a female to reproduce microbes make the biome hostile, Stumpf postulates. Then, perhaps during the ovulatory period, changes in the microbial community composition make the biome a more "welcoming" environment.

"Testing some of these hypotheses with factors such as mating systems and sexual strategies may provide some insight into whether females did co-evolve with microbes in some evolutionarily advantageous way," says Stumpf. "I expect there may be some interesting results there."

Understanding the vaginal microbial populations also could help wildlife conservationists increase reproductive success among threatened species. It also can help the medical community understand and prevent pre-term births among women, which in some cases has been linked to microbial imbalance.

"We are really just at the tip of the iceberg. This approach opens up a whole field of research," says Stumpf.