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A SIMPLE rule governs a seemingly random phenomenon: the sizes of the groups in which primates live. It seems our closest living relatives opt for social groupings that aren’t as varied and flexible as you might think.
Susanne Shultz at the University of Manchester, UK, and her colleagues compared group sizes in 215 primate species. The average number in a group varied between species but was always clustered around five distinct sizes (Biology Letters, DOI: 10.1098/rsbl.2017.0490).
The preferred group sizes were, roughly: 2.5, 5, 15, 30 and 50. The smallest normally had two adults and some offspring. Bigger ones tended to be either a single male with many females, or multiple males and females.
Other patterns, such as lots of males and few females, were rare. “The other thing that seems to be hard for primates to do is male and female pairs