The "living fossil," Dromiciops gliroides (monito del monte), is an endemic marsupial inhabiting the temperate rain forests of South America. It is cavity-dependent and faces high energetic costs associated with thermoregulation during the austral winter. Although D. gliroides is well known for seed dispersal in temperate rain forests, its ecology, behavior, and long-term population dynamics have received little attention. We monitored a population of D. gliroides and studied variation in abundance and density and seasonal changes in body mass and body condition index (BCI). In addition, we monitored activity and communal nesting with camera traps and nest boxes, respectively. Over 4 years we documented a mean population density of 26 (95% confidence interval = 19-32) individuals/ha. We found significantly greater body mass and BCI for females than for males, suggesting different energetic strategies during the prehibernation period. Animals were nocturnal and active until dawn. Communal nesting occurred during summer and early fall, but torpor by single individuals and small groups was increasingly frequent during winter. Communal nesting could be a key behavioral strategy affecting survival. However, given the greater frequency in warm seasons and groups composed of postreproductive females and juveniles, communal nesting might be more related to parental care associated with kin selection than to thermoregulation.