The evolution of marsupial social organization

It is generally believed that marsupials are more primitive mammals than placentals and mainly solitary living, representing the ancestral form of social organization of all mammals. However, field studies have observed pair- and group-living in marsupial species, but no comparative study about their social evolution was ever done. Here we describe the results of primary literature research on marsupial social organization which indicate that most species can live in pairs or groups and many show intra-specific variation in social organization. Using Bayesian phylogenetic mixed-effects models with a moderate phylogenetic signal of 0.18 we found that solitary living is the most likely ancestral form (35% posterior probability), but has high uncertainty, and the combined probability of partly sociable marsupial ancestors (65%) should not be overlooked. For Australian marsupials, group-living species were less likely to be found in climates representing tropical rainforest, and species with a variable social organization were associated with low and unpredictable precipitation representing deserts. Our results suggest that modern marsupials are more sociable than previously believed and that there is no strong support that their ancestral state was strictly solitary living, such that the assumption of a solitary ancestral state of all mammals may also need reconsideration.