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

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... 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.