Living in a group can offer social animals, including primates, tremendous benefits. But those benefits may come at a high cost if there are too many—or too few—members in the neighborhood.
A new study shows baboons that live in intermediate-sized groups with about 50 to 80 members exhibit optimal ranging behavior and low physiological stress levels.
“Strikingly, we found evidence that intermediate-sized groups have energetically optimal space-use strategies and both large and small groups experience ranging disadvantages,” says Catherine Markham, assistant professor 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.”
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For a study published in the Proceedings of the National Academy of Sciences, researchers observed 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.
The key to analyzing optimal group sizes for highly social species is how trade-offs balance—and do these trade-offs actually result in an optimal group size for a social species.
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The findings provide a testable hypothesis for evaluating group-size constraints in other group-living species, in which the costs of intra- and inter-group competition vary as a function of group size, Markham says. Additionally, the findings provide a broader understanding of why some animals live with others and how many neighbors are best for various species and situations.
Researchers from Duke University, Princeton University, and the Institute for Primate Research, National Museums of Kenya collaborated on the study.
The National Science Foundation and the National Institute on Aging funded the work.
Source: Stony Brook University
