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.