Physiological ecology explains why some physiological designs are so intimately associated with a given environment. Here we present the case of the monito del monte (Dromiciops gliroides), an arboreal marsupial adapted to the southernmost relict fragment of the broadleaf forest biome in South America. Dromiciops is considered the last living representative genus of the order Microbiotheria, whose ancestors are known to have colonized Australia through an Antarctic bridge in the Cretaceous (65 mya). These marsupials survive almost exclusively in well-grown, undisturbed broadleaf forests, where densities are high (10–20 ind per ha), compared with other sympatric mammals (e.g. rodents). However, monitos show the typical long breeding period of Australidelphians (∼3.5 months), small litter size (1–4 pups), long lactation (∼70 days), extended parental care (1 year), and long generation time (2 years). Here we posit that the ecological success of monitos is explained by Microbiotheriid autoecological adaptations to the ecological niche provided by the broadleaf forest biome, retained until today by Dromiciops. These marsupial adaptations maximized survival, rather than reproductive output; and are characterized by omnivory-frugivory, nocturnal-arboreal habits, and sociality with nestling behavior. In addition, the unique opportunistic hibernation capacity of this marsupial allows it to shut down its metabolism in times of low trophic supply, promoting the efficient use of energy during periods of scarcity. In summary, Dromiciops (and probably, Microbiotherians) exhibits an investment strategy in viability and survival, which promoted longevity in the arboreal, cold, and seasonal niche of the forest.