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鈥檛 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, ).
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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. 鈥淭he other thing that seems to be hard for primates to do is male and female pairs combined in a group,鈥� says Shultz, even though this is common in birds.
Primates reuse these strategies because they keep facing the same challenges, Shultz says. 鈥淓cology and social relationships are tightly interconnected.鈥�
For instance, species occupying open ground form the largest groups, perhaps to defend against predators. Those that live in trees in dense forests prefer medium groups, as big groups would be impossible to coordinate.
In 2011, Shultz showed that primate group sizes also evolved in leaps (Nature, ).
This article appeared in print under the headline 鈥淲hen you鈥檙e just hanging around, size matters鈥�
