Body size and litter size as predictors of pouch presence in marsupials

The marsupial pouch is a key adaptation for offspring protection and development, yet its evolutionary drivers remain unclear. While body size matters, the role of litter size is less understood. Using phylogenetic comparative methods, we investigated the evolutionary relationship between pouch presence, body mass, and litter size across 195 marsupial species. Our results reveal that pouch presence is strongly phylogenetically conserved and positively correlated with body size, with all large-bodied species possessing a pouch. By contrast, pouch presence is negatively associated with litter size, with species with larger litters typically lacking a pouch, while those with smaller litters retain one. We found that body mass evolves faster in pouched lineages. Ancestral state reconstructions suggest multiple independent origins of the pouch, although the ancestral marsupial condition remains uncertain, but most likely corresponding to pouch absence. These findings support the hypothesis that the pouch evolves in response to trade-offs between offspring quantity and maternal investment, aligning with broader patterns of parental care strategies. Our work provides a new vision for the evolutionary trajectory of one of the most conspicuous marsupial features.