Primate Social Organization Evolved from a Flexible Pair-Living Ancestor

Abstract

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. However, previous research included the assumption that many understudied primate species were solitary, then finding transitions to more complex social systems being driven by various life history traits and ecological factors. Here we show that when intra-specific variation is accounted for, the ancestral social organization of primates was variable, with the most common social organization being pair-living but with approximatively 15-20% of social units of the ancestral population deviating from this pattern by being solitary living. We built a detailed database from primary field studies quantifying the number of individuals (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 socio-ecological predictors, in ancestral populations. Body size and activity patterns had large effects on transitions between types of social organizations. Our results challenge the assumption that ancestral primates were solitary and that pair-living evolved afterwards. Moreover, our results emphasize the importance of focusing on field data and accounting for intra-specific variation. Pair-living is evolutionary ancient, likely caused by reproductive benefits such as access to partners and reduced intra-sexual competition, with more complex social structure (pair-bonding) and care systems (biparental and allo-parental care) evolving later.

Data-driven analyses of social complexity in bees reveal phenotypic diversification following a major evolutionary transition

“…Our findings shed light on an important macroevolutionary processtraversal of a substantial phenotypic distance and subsequent diversification within corbiculate bees. Our framework can be expanded to additional animal groups, including primates and birds [6][7][8] , where research on the evolution of social complexity suffers from the same limitations as in social insects. Our approach is especially promising for studies on the molecular underpinnings of social complexity because it is more likely to capture the relationship between proxies of social complexity and genomic, epigenetic, or proteomic data, as demonstrated in the genomic analysis we conducted here.…”

“…Sociality has evolved multiple times across independent taxonomic groups and has played a key role in the evolution of animals [1][2][3][4] . Despite remarkable variation among animal societies 4,5 , comparative studies have typically classified levels of social complexity based on few qualitative traits, such as group composition, reproductive skew, and parental care (e.g., in primates [6][7][8][9] ; in birds 10,11 ; in insects [12][13][14][15] ). This approach was crucial for the development of influential theories such as kin selection 16 , major transitions in evolution 17 , and developmental plasticity 18 .…”

Advancing cooperative breeding research with a peer-reviewed and “live” Cooperative-Breeding Database (Co-BreeD)

“…cooperative vs. non-cooperative breeders) towards analysing cooperative breeding as a continuous trait. This approach significantly increases the explanatory power of statistical analyses (Clutton-Brock, 2021; Olivier et al ., 2024).…”

Costs and benefits of solitary living in mammals

“…Therefore, one can differentiate between species that are obligately solitary living (all individuals are solitary), mainly solitary living (most individuals are solitary living) and facultatively solitary living (most individuals live in pairs or groups and less than 50% solitary). Considering such variation is important in comparative studies about social evolution and can challenge previous studies that ignored variation (Dalerum, 2007;Griesser & Suzuki, 2016;Olivier et al, 2024;Qiu et al, 2022).…”