The potential benefits of social aggregation and communal grooming to the foraging ecology of muskrats were investigated. We hypothesized that muskrats in groups would be more resistant to hypothermia, would spend more time foraging in cold water, and would rewarm more quickly following emergence than solitary muskrats. Despite social grooming and aggregation behavior following episodes of aquatic activity, the post-immersion re-warming rates of muskrats increased only slightly in the presence of nestmates. The maximum rate of rewarming averaged 0.26°C/min when each animal was tested alone, and 0.30°C/min when it was tested in the presence of four nest-mates ( P = 0.096). Providing muskrats with the option to huddle between foraging and exploratory dives in cold water had little impact on the precision of control of body temperature. Although muskrats in groups tended to spend more time in water and expend less energy than solitary animals, these trends were not significant. The greatest differences were in intake of food, with grouped animals in one experiment ingesting nearly three times as much forage as animals feeding alone ( P = 0.015). We attribute the higher intake of food of grouped animals to social facilitation rather than to any gain in foraging efficiency arising from aggregation behavior out of water.