We compared patterns of core body temperature (T_b) change, including inter-individual synchrony, in 2 free-living arctic hibernators that differ in size and sociality, the Alaska marmot (Marmota broweri) and the arctic ground squirrel (Urocitellus parryii). We report overwinter T_b changes from 3 to 4 marmots from the same hibernaculum in each of 3 years and from 7 ground squirrels that hibernated at 2 nearby burrow sites in 1 year. Very close synchrony in the timing of torpor and arousal cycles in Alaska marmots indicates social hibernation and thermoregulation, while lack of synchrony in arctic ground squirrels further confirms solitary hibernation. The mean duration between the first and last marmot measured within the group to initiate an arousal was 3.7 ± 2.5 h and to recool to 30°C during torpor entrance was 5.7 ± 3.7 h. The minimum T_b recorded in marmots was 0.6°C and in ground squirrels was -2.0°C. Marmots entering torpor displayed an interrupted pattern of T_b change defined by 2 distinct rates of cooling, early and late during entry, that differed by 21-fold. Ground squirrels cooled in a continuous pattern, initially 3-fold slower than marmots during rapid cooling but 4-fold faster during slow cooling. Both species must minimize energy expenditure to survive long arctic winters; our results suggest that Alaska marmots do this through social thermoregulation, while arctic ground squirrels decrease T_b below freezing to minimize the difference between body and ambient temperatures.