Baboons choose sleeping sites in the following descending order of preference: (1) steep cliff faces and caves, (2) taller emerging trees in continuous forests, (3) the canopy of contiguous forest without emerging trees, and (4) open woodland trees. Choice of sleeping sites in an order appearing to agree with degree of inaccessibility to most predators suggests the hypothesis that predation avoidance is the major basis for use and choice of particular sleeping sites. If this preference order for kinds of sleeping sites is applicable to other large primates, it suggests that spacing of adequate sleeping sites relative to the distribution and density of food resources is one factor contributing to group size and possibly other features of primate social structure. The relatively even distribution of numerous adequate sleeping sites in tropical forests may be one factor permitting evolution of small social units. By contrast, sparse distribution of sleeping sites relative to resource field may permit the development of large social groups.