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A new American research has found the first known evidence of 'missing social knowledge' in non-human primates. For the study, Thore Bergman, an assistant professor of psychology and ecology and evolutionary biology at the University of Michigan, looked at individual recognition in male geladas (Theropithecus gelada, close relatives of baboons) that live in fluid, multilevel groups that can number more than 1,200 individuals - 10 times larger than baboon groups. Most primates live in stable groups of individuals that are always together. But geladas society is more complicated. Just as humans have varying levels of associations with different people, geladas have different degrees of overlap with other geladas. At one end is the harem, a cohesive group of 10-20 geladas that is always together. At the other end is the herd, extremely large aggregations of harems that may only rarely come together. In between, are bands, groups of three to 20 harems, that are together the majority of the time. Bergman wanted to find "the limits of recognition in this fluid social system." Among humans, social scientists have developed an axiom known as "the Rule of 150" arguing that it is difficult for the average human to retain a great deal of information on more than 150 people, leading to a tendency for large organizations to subdivide into smaller and smaller sub-groups. But studying the evolution of this phenomenon has been difficult because most of our primate relatives live in relatively small, stable groups (well under 150 individuals) where everyone knows each other. This is where geladas come in. By observing how geladas responded to playbacks of different calls, Bergman found recognition only among geladas with the very highest levels of overlap. This finding "suggests that geladas are either unable or unmotivated to keep track of the individual identities of other males in their multi-level society - even males with whom they have a high degree of social overlap,'' Bergman writes. He said: "This finding is potentially significant because if sociality drives cognitive evolution, then we would expect to see evidence that sociality is cognitively challenging." Bergman's findings have appeared in the journal Proceedings of the Royal Society B.

Evidence of 'missing social knowledge' in non-human primates found