Notable paper: Gogarten, J. F., Bonnell, T. R., Brown, L. M., Campenni, M., Wasserman, M. D., & Chapman, C. A. (2014). Increasing group size alters behavior of a folivorous primate. International Journal of Primatology, 35(2), 590-608. doi:10.1007/s10764-014-9770-8
Summary: Jan Gogarten and colleagues studied the behavior of a group of red colobus monkeys in Kibale National Park, Uganda over 6 years, as the group grew in numbers from 57 to 98 members. They were able to document the increase in foraging effort as group size increased, individuals spent more time traveling, less time socializing, and ate a wider variety of foods.
Interesting because: Robin Dunbar (1998, and in many other publications) proposed that the time invested in the maintenance of social relationships must increase with group size, so that group size in primates is constrained by the cognitive ability available to track these relationships. Group size does correlate with the time spent grooming when compared across primate species (Lehmann et al. 2007). What is interesting in the within-population study by Gogarten and colleagues is that the time spent on social interactions actually declined as the group got larger. Instead, individuals spent more time traveling within their range, because food is harder to find for a larger group. Individuals were less able to find the foods that they prefer, and therefore relied on a broader range of different foods that they could find. The reduction of foraging efficiency for individuals might be a more important mechanism for constraining group size, causing them to reduce the time spent on social interactions even as the number of potential social partners increases.
Dunbar, R., & Dunbar, R. I. M. (1998). Grooming, gossip, and the evolution of language. Harvard University Press.
Lehmann, J., Korstjens, A. H., & Dunbar, R. I. M. (2007). Group size, grooming and social cohesion in primates. Animal Behaviour, 74(6), 1617-1629. doi:10.1016/j.anbehav.2006.10.025