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Group Living: For Baboons Intermediate Size is Optimal
Research led by Catherine Markham of Stony Brook University reveals that intermediate-sized groups of baboons (50 to 70 individuals) exhibit optimal ranging behavior and low stress levels. Pictured is a group of wild baboons in East Africa. Credit: Beth Archie

Research led by Stony Brook anthropologist and colleagues published in PNAS provides evidence and new insights into the benefits of group living in primates

Stony Brook, NY, October 27, 2015 – Living with others can offer tremendous benefits for social animals – including primates, but these benefits could come at a high cost. New research by a team of scientists led by Catherine Markham, PhD, a Stony Brook University anthropologist, reveals that intermediate-sized groups provide the most benefits to wild baboons. Their findings, published in PNAS, offer new insight into the costs and benefits of group living.

In the paper titled “Optimal group size in a highly social mammal,” the authors reveal that while wild baboon groups range in size from 20 to 100 members, groups consisting of about 50 to 70 individuals (intermediate size) exhibit optimal ranging behavior and low physiological stress levels in individual baboons, which translates to a social environment that fosters the health and well-being of individual members. The finding provides novel empirical support for an ongoing theory in the fields of evolutionary biology and anthropology that in living intermediate-sized groups has advantages for social mammals.

“Strikingly, we found evidence that intermediate-sized groups have energetically optimal space-use strategies and both large and small groups experience ranging disadvantages,” said Dr. Markham, lead author and an Assistant Professor in the Department of Anthropology at Stony Brook University. “It appears that large, socially dominant groups are constrained by within-group competition whereas small, socially subordinate groups are constrained by between-group competition and/or predation pressures.”

The researchers compiled their findings based on observing five social wild baboon groups in East Africa over 11 years. This population of wild baboons has been studied continuously for over 40 years by the Amboseli Baboon Research Project. They observed and examined the effects of group size and ranging patterns for all of the groups. To gauge stress levels of individuals, they measured the glucocorticoid (stress hormone) levels found in individual waste droppings. 

Dr. Markham explained that regarding optimal group sizes for highly social species the key to the analysis is  how are trade-offs balanced, and do these trade-offs actually result in an optimal group size for a social species.

She said that their findings provide a testable hypothesis for evaluating group-size constraints in other group-living species, in which the costs of intra- and intergroup competition vary as a function of group size. Additionally, their findings provide implications for new research and a broader understanding of both whysome animals live with others and how many neighbors will be best for various species and situations.

Research collaborators are affiliated with Princeton University, Duke University and the Institute for Primate Research, National Museums of Kenya. Co-authors on the paper are Laurence R. Gesquiere, Susan C. Alberts and Jeanne Altmann.

The research was supported in part by the National Science Foundation and the National Institute on Aging.  Additional support was provided by the American Society of Primatologists, the Animal Behavior Society, the International Primatological Society. Sigma Xi.

About Stony Brook University
Part of the State University of New York system, Stony Brook University encompasses 200 buildings on 1,450 acres. Since welcoming its first incoming class in 1957, the University has grown tremendously, now with more than 25,000 students and 2,500 faculty. Its membership in the prestigious Association of American Universities (AAU) places Stony Brook among the top 62 research institutions in North America. U.S. News & World Report ranks Stony Brook among the top 40 public universities in the nation and Kiplinger named it one of the 35 best values in public colleges. One of four University Center campuses in the SUNY system, Stony Brook co-manages Brookhaven National Laboratory, putting it in an elite group of universities that run federal research and development laboratories. Stony Brook is ranked in the top 2 percent of the world's higher education institutions by the Times Higher Education World University Rankings. It is one of only 10 universities nationwide recognized by the National Science Foundation for combining research withundergraduate education. As the largest single-site employer on Long Island, Stony Brook is a driving force of the regional economy, with an annual economic impact of $4.65 billion, generating nearly 60,000 jobs, and accounts for nearly 4 percent of all economic activity in Nassau and Suffolk counties, and roughly 7.5 percent of total jobs in Suffolk County. 

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