Postnatal Development of the Gray Short-Tailed Opossum ( <i>Monodelphis domestica</i> ): Implications for Metatherian Decline at the K–Pg

Marsupials give birth to extremely altricial offspring which must be reared externally for an extended period, often in a pouch. Despite often being considered a defining feature of marsupials, around a third of living species lack a pouch. Here, we describe the postnatal development of the gray short-tailed opossum, <i>Monodelphis domestica</i>, a small pouchless South American didelphid and consider its implications for life history evolution within Metatheria. We find that at birth, ossification and chondrogenesis in neonatal <i>M. domestica</i> is more extensive than in basal pouched Australidelphian marsupials like the dunnart. Key precocial milestones such as tarsal ossification, eye opening, growth of body fur, and chewing tooth eruption occur earlier and more rapidly. Principal component analysis of life history and reproductive traits reveals a pronounced r- to K-selected gradient across living marsupial species. Stochastic character-mapping based ancestral state reconstruction suggests that absence of the pouch, and by inference possession of an r-selected life history strategy characterised by large litters, short attachment phases, and accelerated weaning was likely ancestral amongst crown-group marsupials. The more K-selected reproductive strategy of pouched marsupials wherein there is a prolonged postnatal development window, and relative few young are produced, likely evolved during the early Cenozoic, and separately amongst australidelphian and 'ameridelphian' marsupials. Rather than making early marsupials more sensitive to environmental disturbances, we hypothesis that their possession of an 'r-selected' life history strategy may have been a key factor in their persistence through the K-Pg extinction.