Causes For Sociality In Primates

The main difference between the promiscuity of primitive placentals and the dispersed multimale/multifemale pattern of some of the nocturnal strepsirhines is the presence of social networks in the latter. Within these social networks, social contacts occur throughout the year and are not restricted to the breeding season. This contrasts with many other "solitary" mammals that have no friendly contacts with conspecifics apart from mother-infant relationships and mating contacts (Müller and Thalmann, 2000). The question why primates have evolved sociality while many other mammals did not has yet to be resolved. Sociality per se can bring several benefits, such as a reduction of ectoparasite-born disease through allogrooming, alloparenting by older offspring, and the sharing of information on food resources and sleeping sites (Clark, 1985; Müller, 1999a). Among small animals, thermoregulatory advantages through nest-sharing are possible (Genoud et al., 1997; Perret, 1998; van Schaik and van Hooff, 1983). It remains unclear, however, why other mammals can do without these suggested advantages, whereas primates, seemingly, cannot.

Since all living primates are social, it is impossible to investigate the natural history of nonsocial primates to infer causes of sociality in primates. For that reason, Müller and Soligo (2002, 2005) used the order Rodentia as a model. Rodents occupy almost all potential ecological niches and exhibit a wide variety of patterns of social organization ranging from intolerant solitary species to eusocial forms. The aim of this approach was to explore possible determinants for the absence or presence of sociality in rodents and to apply the findings to the origin of sociality in primates. The following potential determinants were investigated: substrate utilization (terrestrial versus arboreal), activity cycles (diurnal versus nocturnal), dietary preferences (faunivore, frugi-omnivore, granivore, frugi-herbivore, or herbivore) and body size (<100 g, 100 g-1 kg, or >1 kg).

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