Sarah C. P. Williams
QnAs with Kelly Stewart
QnAs with Kelly Stewart

In the Virunga mountains of East Africa, hundreds of mountain gorillas roam through high-altitude cloud forests. The endangered great apes live in complex groups, collaborating and communicating as they forage, migrate, and defend themselves. For nearly four decades, primatologist Kelly Stewart and her husband Alexander “Sandy” Harcourt closely observed the animals’ behaviors, life histories, social systems, and ecology. The pair’s work, which has implications for conservation, socio-ecology, and evolutionary biology, was influenced by prominent researchers such as Jane Goodall, Biruté Galdikas, and Dian Fossey, under whom they studied in the 1970s. Among their contemporaries, perhaps none was closer to Stewart and Harcourt than another renowned couple of primatologists: Robert Seyfarth and Dorothy Cheney. Cheney died in 2018. Stewart recently authored a Retrospective on the indelible impact of Cheney’s career on primatology (1) and PNAS recently spoke with her about Cheney’s contributions to primatology. PNAS: How did you and Dorothy meet? Stewart: Dorothy and her husband Robert [Seyfarth] as well as my future husband Sandy [Harcourt] and I were all graduate students with Robert Hinde at Cambridge [University]. Dorothy and Robert graduated before Sandy and I, and by that time we had all become very close. We ended up staying in touch over many years in both professional and personal contexts. Later, when Sandy and I were at the Karisoke Research Center [in Rwanda], they came and did some experiments there with us. Because we had all worked under Robert Hinde, we shared a lot of interests and were all very influenced by his idea that the relationships among a group of animals is more than just the sum of its parts. More recently, after we all retired, the four of us had a lot of adventures all around the world together. PNAS: Can you describe the field of primatology in the 1970s, when you and Dorothy launched your careers? Stewart: It was the best of all times. There was still so much to find out about the natural history of wild animals, and there were big new ideas in the field. In 1975, E.O. Wilson came out with Sociobiology (2), which provided a synthesis of the behavioral sciences [masterfully reviewed in Hinde’s classic work “Animal Behavior” (3)] with new developments in evolutionary biology. This book provided this exciting new evolutionary framework in which to view the data we collected on animal behavior. We were no longer just going out and writing down who did what to whom and how often. We did that, but then we came home and figured out how these behaviors could fit into evolutionary theory. Primates were one of the best groups of animals to work with because there was so much data on different species; we could do a lot of comparative studies. PNAS: What were Cheney’s and Seyfarth’s biggest contributions to primatology? Stewart: Dorothy had this way of looking at relatively complex data on all these animals doing different things and distilling it together into a beautiful, elegant, simple model. Their work had a big impact on the field’s understanding of primate vocalizations. Before they came along, these experiments had been done in the lab; nobody thought you could collect data in the wild with that level of control and precision. But Dorothy and Robert always pointed out that monkeys didn’t evolve in laboratory cages, so they pushed to carry out these experiments in the wild. They were able to do vocal playback experiments with vervets, where they played recordings of the monkey’s alarm calls and observed how the monkeys reacted. It was seminal work and let them discover the meanings of these vocalizations (4). PNAS: How did their work influence your own research? Stewart: Dorothy and Robert really inspired us to look more closely at gorilla vocalizations. Dian [Fossey] had written one paper on vocalizations in 1972 (5). We were able to show that while gorilla vocalizations were not as complex as vervet vocalizations, which have different acoustic sounds for different predators, they did carry meaning. We studied grunts and showed that these sounds were very context-dependent; when a dominant animal gives a grunt as they are approaching a subordinate animal, it means something different than when an animal gives a grunt to a group as they walk away. But, in general, we found that grunts were signals of intent to change activity, as well as indications of the status quo; saying “everything is okay.” PNAS: You mention in the Retrospective that Dorothy’s work had an impact on a wide variety of fields, including cognitive psychology, linguistics, and philosophy. Can you elaborate? Stewart: I think it’s because we’re so closely related to primates; they’re one of our closest relatives. In many ways, primatology can tell us even more about ourselves than physiological lab studies on humans. When you study primates in the field, there isn’t all this other cultural stuff that gets in the way of behaviors that have been evolutionarily selected for. We are a social species, and research by primatologists on how monkeys and great apes behave has helped us understand…that we’re a species that could never have afforded to not be in a group. PNAS: What can scientists in other fields learn from how Dorothy approached her work? Stewart: Dorothy was an incredible scientist; she happened to be studying primatology, but the way she approached research and tried to always see the big picture rather than focus on minutiae is, I think, something any scientist can learn from. She also had this incredible ability to see patterns in data that really served her well. She wrote amazing models explaining the relationships between immature vervet monkeys and their mothers, and how those relationships meshed with the group as a whole (6). I actually took some of her papers into the field with me; I carried around these dog-eared copies because they illustrated this elegant way of thinking. PNAS: Has primatology evolved since the days you and Dorothy spent in the field? Stewart: Today, the study of wild animals is much more interdisciplinary—there are fewer studies where just one or two people go out and do it all. Now, you integrate genetics and physiology and all these other fields. In general, research projects now are much more focused as opposed to broad, exploratory projects, where you just go out and observe. There are also many new technologies changing what researchers can do. Tracking devices are letting people answer questions about animals’ sense of their habitat and how they move around. The exciting thing is that…some of these technical advances are getting us closer to really understanding how animals perceive things.