Primate social organization evolved from a flexible pair-living ancestor.

Explaining the evolution of primate social organization has been fundamental to understand human sociality and social evolution more broadly. It has often been suggested that the ancestor of all primates was solitary and that other forms of social organization evolved later, with transitions being driven by various life history traits and ecological factors. However, recent research showed that many understudied primate species previously assumed to be solitary actually live in pairs, and intraspecific variation in social organization is common. We built a detailed database from primary field studies quantifying the number of social units expressing different social organizations in each population. We used Bayesian phylogenetic models to infer the probability of each social organization, conditional on several socioecological and life history predictors. Here, we show that when intraspecific variation is accounted for, the ancestral social organization of primates was inferred to be variable, with the most common social organization being pair-living but with approximately 10 to 20% of social units of the ancestral population deviating from this pattern by being solitary living. Body size and activity patterns had large effects on transitions between types of social organizations. As in other mammalian clades, pair-living is closely linked to small body size and likely more common in ancestral species. Our results challenge the assumption that ancestral primates were solitary and that pair-living evolved afterward emphasizing the importance of focusing on field data and accounting for intraspecific variation, providing a flexible statistical framework for doing so.

Intra-specific variation in social organization of Strepsirrhines.

Strepsirrhines, that is, lemurs, galagos, and lorises, are considered basal primates, making them important to understand the evolution of primate sociality. Apart from some lemurs, they are nocturnal and solitary living, though the view of their sociality nature has changed with field studies being completed. We conducted a review of the primary literature about the social organization (group composition) of strepsirrhines, with the aim to determine whether intra-specific variation in social organization (IVSO) occurs and to determine how many species are pair-living, group-living, or solitary living. We found data in 83 peer-reviewed studies for 43 of the 132 strepsirrhine species and compared our results using two databases on social systems of mammals published in 2011 and 2013. While it is often assumed that primates show relatively fixed social organizations, we found that 60.5% of species for which data exist have IVSO. We found only 7% of the species to be truly solitary living (with 34.9% additional species to be sometimes solitary living), which is in contrast to the other databases, which had reported 60.9% and 37.7% of species to be solitary. We further explored group compositions by designating "functional groups" (e.g., foraging, breeding, and infant care groups). While functional groups might explain IVSO within a single species, this was not consistent over species with IVSO, such that IVSO was poorly explained by functional groups. Our study supports the view that most strepsirrhines are social (58.1% of species with another 34.9% of species sometimes living in pairs or groups) and show complex and often variable social organizations; reinforcing the assumption that the ancestor of all primates was social and not solitary.