Structural and Cognitive Solutions to Prevent Group Fragmentation in Group-Living Species

Group-living is one of the six major evolutionary transitions. However, group-living creates stresses that naturally cause group fragmentation, and hence loss of the benefits that group-living provides. How species that live in large groups counteract these forces is not well understood. I analyse comparative data on grooming networks from a large sample of primate species and show that two different social grades can be differentiated in terms of network size and structure. I show that living in large, stable groups involves a combination of increased investment in bonding behaviours (made possible by a dietary adjustment) and the evolution of neuronally expensive cognitive skills of the kind known to underpin social relationships in humans. The first allows the stresses created by these relationships to be defused; the second allows large numbers of weak relationships to be managed, creating a form of multilevel sociality based on strong versus weak ties similar to that found in human social networks.