Temperatures inside natural winter burrows of alpine marmots (T(bur)) dropped exponentially throughout the hibernation season from about 12-degrees-C in autumn to almost 0-degrees-C in spring. Laboratory measurements of oxygen consumption (VO2) showed that torpid marmots have minimal energy expenditure at ambient temperatures (T(a)) above 5-degrees-C. Metabolism increased with decreasing T(a) below 5-degrees-C. At T(a) almost-equal-to 0-degrees-C, VO2 was already four times higher than the minimal value. T(bur) was below 5-degrees-C for about two thirds of the hibernation season. The more animals hibernated together in a burrow, the latter T(bur) fell below this critical threshold. Hence, overwintering conditions in the natural habitat are severe for the animals, but mitigated by social hibernation.