The evolution of sociality and its differential expression in primate social organizations have been a major focus of primatology for several decades. The processes shaping specific mating systems and social relationships (such as sexual selection and life history strategies) and the underlying selective pressures (predation pressure, feeding competition, etc.) have been investigated for a growing number of primate species. We know much more about diurnal than nocturnal species, however. Recent attempts at synthetic overviews of prosimian social organization generally relegate nocturnal species to single, all-encompassing categories such as "solitary" [Kappeler, 1997], despite evidence of complexity within and variability between their social organizations [Gursky, 2000]. Several factors contribute to this over-generalization, not the least of which are observational challenges and a dearth of long-term studies on nocturnal prosimians. Fortunately, research on nocturnal primates has dramatically increased over the last decade in particular [Alterman et al., 1995]. Several volumes have been produced that focus on the ecology, behavior, genetics, and conservation of nocturnal primates [Charles-Dominique et al., 1980;Alterman et al., 1995]. We still have a long way to go toward a comprehensive view of the patterns and processes of sociality in the order Primates, but the case studies contained in this volume take us one step closer.
Most of the papers in this issue were part of a symposium on Sociality in Nocturnal Prosimians held at the XVIIth International Primatological Society meeting in Antananarivo, Madagascar, in 1998. They address the levels, units, and patterns of sociality within a broad sample of nocturnal species from four different families that have recently been intensively studied. The data from this compendium clearly show that intraspecific relationships and mating systems are more complex than previously thought.
Gursky [2000] shows that spectral tarsiers (Tarsius spectrum) appear to be far more gregarious (within and outside their sleeping sites) than their sister taxa, the Bornean and Philippine tarsiers. The results from Radespiel's [2000] study suggest the existence of an individualized social network within the dispersed social organization of Microcebus murinus. On the other hand, Schwab