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.

Endothermy, understood as the maintenance of continuous and high body temperatures (TB) due to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms of cold environments. As a response, social clustering, and nest use (collectively, communal nesting) are common strategies for heat conservation in small mammals and birds. To quantify the actual amount of energy that is saved by communal nesting, we studied the social marsupial Dromiciops gliroides (monito del monte), a relict marsupial species from the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by the cold, for which we calculated the energetic benefits that communal nesting confers. Using biophysical models and experimental coolings, we simulated heat exchanges experienced by grouped or solitary individuals, and also individuals within nests, collected from the field. Assuming a model of passive cooling, we calculated the net energy cost of euthermic maintenance (Ecost: the total energy needed to maintain euthermia). We adjusted 50 cooling curves, to exponential decay models, and found in all cases that the strategy minimizing heat loss is to be clustered within a nest, for which the Ecost was the minimum. This was significantly lower than the clustered condition, outside the nest, a reduction that represents almost half of energy consumption per day in a resting, thermoneutral condition for this marsupial. Overall, our results suggest that the strategy that significantly maximized heat conservation, compared with alternative strategies, was communal nesting. These findings support the idea that, in this social mammal, sociality is driven by bioenergetic benefits.

Endothermy, understood as the maintenance of continuous and high body temperatures (TB) due to the combination of metabolic heat production and an insulative cover, is severely challenged in small endotherms inhabiting cold environments. As a response, social clustering combined with nest use (=communal nesting) is a common strategy for heat conservation. To quantify the actual amount of energy that is saved by this strategy, we studied the social marsupial Dromiciops gliroides (monito del monte), an endemic species of the cold forests of southern South America. It is hypothesized that sociability in this marsupial was driven by cold conditions, but evidence supporting this hypothesis is unclear. Here we used taxidermic models ("mannequins") to experimentally test the energetic benefits of clustering combined with nest use. To do this, we fitted and compared cooling curves of solitary and grouped mannequins, within and outside of a nest, at the typical winter ambient temperatures of their habitat (5°C). We found that the strategy that minimized euthermic cost of maintenance, was the combination of nest use and clustering, thus supporting communal nesting as a social adaptation to cope with the cold. Considering the basal metabolic rate of monitos, our estimates suggest that the savings represents almost half of energy consumption per day (in resting conditions). This study shows how simple biophysical models could help to evaluate bioenergetic hypotheses for social behavior in cold-adapted endotherms.